Display controller (DC) performs isochronous memory transfers, and thus,
has a requirement for a minimum memory bandwidth that shall be fulfilled,
otherwise framebuffer data can't be fetched fast enough and this results
in a DC's data-FIFO underflow that follows by a visual corruption.
The Memory Controller drivers provide facility for memory bandwidth
management via interconnect API. Let's wire up the interconnect API
support to the DC driver in order to fix the distorted display output
on T30 Ouya, T124 TK1 and other Tegra devices.
Tested-by: Peter Geis <[email protected]> # Ouya T30
Tested-by: Matt Merhar <[email protected]> # Ouya T30
Tested-by: Nicolas Chauvet <[email protected]> # PAZ00 T20 and TK1 T124
Signed-off-by: Dmitry Osipenko <[email protected]>
---
drivers/gpu/drm/tegra/Kconfig | 1 +
drivers/gpu/drm/tegra/dc.c | 349 ++++++++++++++++++++++++++++++++++
drivers/gpu/drm/tegra/dc.h | 14 ++
drivers/gpu/drm/tegra/drm.c | 14 ++
drivers/gpu/drm/tegra/hub.c | 3 +
drivers/gpu/drm/tegra/plane.c | 121 ++++++++++++
drivers/gpu/drm/tegra/plane.h | 15 ++
7 files changed, 517 insertions(+)
diff --git a/drivers/gpu/drm/tegra/Kconfig b/drivers/gpu/drm/tegra/Kconfig
index 5043dcaf1cf9..1650a448eabd 100644
--- a/drivers/gpu/drm/tegra/Kconfig
+++ b/drivers/gpu/drm/tegra/Kconfig
@@ -9,6 +9,7 @@ config DRM_TEGRA
select DRM_MIPI_DSI
select DRM_PANEL
select TEGRA_HOST1X
+ select INTERCONNECT
select IOMMU_IOVA
select CEC_CORE if CEC_NOTIFIER
help
diff --git a/drivers/gpu/drm/tegra/dc.c b/drivers/gpu/drm/tegra/dc.c
index 0ae3a025efe9..7c6079984906 100644
--- a/drivers/gpu/drm/tegra/dc.c
+++ b/drivers/gpu/drm/tegra/dc.c
@@ -8,6 +8,7 @@
#include <linux/debugfs.h>
#include <linux/delay.h>
#include <linux/iommu.h>
+#include <linux/interconnect.h>
#include <linux/module.h>
#include <linux/of_device.h>
#include <linux/pm_runtime.h>
@@ -616,6 +617,9 @@ static int tegra_plane_atomic_check(struct drm_plane *plane,
struct tegra_dc *dc = to_tegra_dc(state->crtc);
int err;
+ plane_state->peak_memory_bandwidth = 0;
+ plane_state->avg_memory_bandwidth = 0;
+
/* no need for further checks if the plane is being disabled */
if (!state->crtc)
return 0;
@@ -802,6 +806,12 @@ static struct drm_plane *tegra_primary_plane_create(struct drm_device *drm,
formats = dc->soc->primary_formats;
modifiers = dc->soc->modifiers;
+ err = tegra_plane_interconnect_init(plane);
+ if (err) {
+ kfree(plane);
+ return ERR_PTR(err);
+ }
+
err = drm_universal_plane_init(drm, &plane->base, possible_crtcs,
&tegra_plane_funcs, formats,
num_formats, modifiers, type, NULL);
@@ -833,9 +843,13 @@ static const u32 tegra_cursor_plane_formats[] = {
static int tegra_cursor_atomic_check(struct drm_plane *plane,
struct drm_plane_state *state)
{
+ struct tegra_plane_state *plane_state = to_tegra_plane_state(state);
struct tegra_plane *tegra = to_tegra_plane(plane);
int err;
+ plane_state->peak_memory_bandwidth = 0;
+ plane_state->avg_memory_bandwidth = 0;
+
/* no need for further checks if the plane is being disabled */
if (!state->crtc)
return 0;
@@ -973,6 +987,12 @@ static struct drm_plane *tegra_dc_cursor_plane_create(struct drm_device *drm,
num_formats = ARRAY_SIZE(tegra_cursor_plane_formats);
formats = tegra_cursor_plane_formats;
+ err = tegra_plane_interconnect_init(plane);
+ if (err) {
+ kfree(plane);
+ return ERR_PTR(err);
+ }
+
err = drm_universal_plane_init(drm, &plane->base, possible_crtcs,
&tegra_plane_funcs, formats,
num_formats, NULL,
@@ -1087,6 +1107,12 @@ static struct drm_plane *tegra_dc_overlay_plane_create(struct drm_device *drm,
num_formats = dc->soc->num_overlay_formats;
formats = dc->soc->overlay_formats;
+ err = tegra_plane_interconnect_init(plane);
+ if (err) {
+ kfree(plane);
+ return ERR_PTR(err);
+ }
+
if (!cursor)
type = DRM_PLANE_TYPE_OVERLAY;
else
@@ -1204,6 +1230,7 @@ tegra_crtc_atomic_duplicate_state(struct drm_crtc *crtc)
{
struct tegra_dc_state *state = to_dc_state(crtc->state);
struct tegra_dc_state *copy;
+ unsigned int i;
copy = kmalloc(sizeof(*copy), GFP_KERNEL);
if (!copy)
@@ -1215,6 +1242,9 @@ tegra_crtc_atomic_duplicate_state(struct drm_crtc *crtc)
copy->div = state->div;
copy->planes = state->planes;
+ for (i = 0; i < ARRAY_SIZE(state->plane_peak_bw); i++)
+ copy->plane_peak_bw[i] = state->plane_peak_bw[i];
+
return ©->base;
}
@@ -1741,6 +1771,106 @@ static int tegra_dc_wait_idle(struct tegra_dc *dc, unsigned long timeout)
return -ETIMEDOUT;
}
+static void
+tegra_crtc_update_memory_bandwidth(struct drm_crtc *crtc,
+ struct drm_atomic_state *state,
+ bool prepare_bandwidth_transition)
+{
+ const struct tegra_plane_state *old_tegra_state, *new_tegra_state;
+ const struct tegra_dc_state *old_dc_state, *new_dc_state;
+ u32 i, new_avg_bw, old_avg_bw, new_peak_bw, old_peak_bw;
+ const struct drm_plane_state *old_plane_state;
+ const struct drm_crtc_state *old_crtc_state;
+ struct tegra_dc_window window, old_window;
+ struct tegra_dc *dc = to_tegra_dc(crtc);
+ struct tegra_plane *tegra;
+ struct drm_plane *plane;
+
+ if (dc->soc->has_nvdisplay)
+ return;
+
+ old_crtc_state = drm_atomic_get_old_crtc_state(state, crtc);
+ old_dc_state = to_const_dc_state(old_crtc_state);
+ new_dc_state = to_const_dc_state(crtc->state);
+
+ if (!crtc->state->active) {
+ if (!old_crtc_state->active)
+ return;
+
+ /*
+ * When CRTC is disabled on DPMS, the state of attached planes
+ * is kept unchanged. Hence we need to enforce removal of the
+ * bandwidths from the ICC paths.
+ */
+ drm_atomic_crtc_for_each_plane(plane, crtc) {
+ tegra = to_tegra_plane(plane);
+
+ icc_set_bw(tegra->icc_mem, 0, 0);
+ icc_set_bw(tegra->icc_mem_vfilter, 0, 0);
+ }
+
+ return;
+ }
+
+ for_each_old_plane_in_state(old_crtc_state->state, plane,
+ old_plane_state, i) {
+ old_tegra_state = to_const_tegra_plane_state(old_plane_state);
+ new_tegra_state = to_const_tegra_plane_state(plane->state);
+ tegra = to_tegra_plane(plane);
+
+ /*
+ * We're iterating over the global atomic state and it contains
+ * planes from another CRTC, hence we need to filter out the
+ * planes unrelated to this CRTC.
+ */
+ if (tegra->dc != dc)
+ continue;
+
+ new_avg_bw = new_tegra_state->avg_memory_bandwidth;
+ old_avg_bw = old_tegra_state->avg_memory_bandwidth;
+
+ new_peak_bw = new_dc_state->plane_peak_bw[tegra->index];
+ old_peak_bw = old_dc_state->plane_peak_bw[tegra->index];
+
+ /*
+ * See the comment related to !crtc->state->active above,
+ * which explains why bandwidths need to be updated when
+ * CRTC is turning ON.
+ */
+ if (new_avg_bw == old_avg_bw && new_peak_bw == old_peak_bw &&
+ old_crtc_state->active)
+ continue;
+
+ window.src.h = drm_rect_height(&plane->state->src) >> 16;
+ window.dst.h = drm_rect_height(&plane->state->dst);
+
+ old_window.src.h = drm_rect_height(&old_plane_state->src) >> 16;
+ old_window.dst.h = drm_rect_height(&old_plane_state->dst);
+
+ /*
+ * During the preparation phase (atomic_begin), the memory
+ * freq should go high before the DC changes are committed
+ * if bandwidth requirement goes up, otherwise memory freq
+ * should to stay high if BW requirement goes down. The
+ * opposite applies to the completion phase (post_commit).
+ */
+ if (prepare_bandwidth_transition) {
+ new_avg_bw = max(old_avg_bw, new_avg_bw);
+ new_peak_bw = max(old_peak_bw, new_peak_bw);
+
+ if (tegra_plane_use_vertical_filtering(tegra, &old_window))
+ window = old_window;
+ }
+
+ icc_set_bw(tegra->icc_mem, new_avg_bw, new_peak_bw);
+
+ if (tegra_plane_use_vertical_filtering(tegra, &window))
+ icc_set_bw(tegra->icc_mem_vfilter, new_avg_bw, new_peak_bw);
+ else
+ icc_set_bw(tegra->icc_mem_vfilter, 0, 0);
+ }
+}
+
static void tegra_crtc_atomic_disable(struct drm_crtc *crtc,
struct drm_atomic_state *state)
{
@@ -1922,6 +2052,8 @@ static void tegra_crtc_atomic_begin(struct drm_crtc *crtc,
{
unsigned long flags;
+ tegra_crtc_update_memory_bandwidth(crtc, state, true);
+
if (crtc->state->event) {
spin_lock_irqsave(&crtc->dev->event_lock, flags);
@@ -1954,7 +2086,212 @@ static void tegra_crtc_atomic_flush(struct drm_crtc *crtc,
value = tegra_dc_readl(dc, DC_CMD_STATE_CONTROL);
}
+static bool tegra_plane_is_cursor(const struct drm_plane_state *state)
+{
+ const struct tegra_dc_soc_info *soc = to_tegra_dc(state->crtc)->soc;
+ const struct drm_format_info *fmt = state->fb->format;
+ unsigned int src_w = drm_rect_width(&state->src) >> 16;
+ unsigned int dst_w = drm_rect_width(&state->dst);
+
+ if (state->plane->type != DRM_PLANE_TYPE_CURSOR)
+ return false;
+
+ if (soc->supports_cursor)
+ return true;
+
+ if (src_w != dst_w || fmt->num_planes != 1 || src_w * fmt->cpp[0] > 256)
+ return false;
+
+ return true;
+}
+
+static unsigned long
+tegra_plane_overlap_mask(struct drm_crtc_state *state,
+ const struct drm_plane_state *plane_state)
+{
+ const struct drm_plane_state *other_state;
+ const struct tegra_plane *tegra;
+ unsigned long overlap_mask = 0;
+ struct drm_plane *plane;
+ struct drm_rect rect;
+
+ if (!plane_state->visible || !plane_state->fb)
+ return 0;
+
+ drm_atomic_crtc_state_for_each_plane_state(plane, other_state, state) {
+ rect = plane_state->dst;
+
+ tegra = to_tegra_plane(other_state->plane);
+
+ if (!other_state->visible || !other_state->fb)
+ continue;
+
+ /*
+ * Ignore cursor plane overlaps because it's not practical to
+ * assume that it contributes to the bandwidth in overlapping
+ * area if window width is small.
+ */
+ if (tegra_plane_is_cursor(other_state))
+ continue;
+
+ if (drm_rect_intersect(&rect, &other_state->dst))
+ overlap_mask |= BIT(tegra->index);
+ }
+
+ /*
+ * Data prefetch FIFO will easily help to overcome temporal memory
+ * pressure if other plane overlaps with the cursor plane.
+ */
+ if (tegra_plane_is_cursor(plane_state) && overlap_mask)
+ return 0;
+
+ return overlap_mask;
+}
+
+static struct drm_plane *
+tegra_crtc_get_plane_by_index(struct drm_crtc *crtc, unsigned int index)
+{
+ struct drm_plane *plane;
+
+ drm_atomic_crtc_for_each_plane(plane, crtc) {
+ if (to_tegra_plane(plane)->index == index)
+ return plane;
+ }
+
+ return NULL;
+}
+
+static int tegra_crtc_check_bandwidth_state(struct drm_crtc *crtc,
+ struct drm_atomic_state *state)
+{
+ ulong overlap_mask[TEGRA_DC_LEGACY_PLANES_NUM] = {}, mask;
+ u32 plane_peak_bw[TEGRA_DC_LEGACY_PLANES_NUM] = {};
+ bool all_planes_overlap_simultaneously = true;
+ const struct tegra_plane_state *tegra_state;
+ const struct drm_plane_state *plane_state;
+ const struct tegra_dc_state *old_dc_state;
+ struct tegra_dc *dc = to_tegra_dc(crtc);
+ const struct drm_crtc_state *old_state;
+ struct tegra_dc_state *new_dc_state;
+ struct drm_crtc_state *new_state;
+ struct tegra_plane *tegra;
+ struct drm_plane *plane;
+ u32 i, k, overlap_bw;
+
+ /*
+ * The nv-display uses shared planes. The algorithm below assumes
+ * maximum 3 planes per-CRTC, this assumption isn't applicable to
+ * the nv-display. Note that T124 support has additional windows,
+ * but currently they aren't supported by the driver.
+ */
+ if (dc->soc->has_nvdisplay)
+ return 0;
+
+ new_state = drm_atomic_get_new_crtc_state(state, crtc);
+ new_dc_state = to_dc_state(new_state);
+
+ /*
+ * For overlapping planes pixel's data is fetched for each plane at
+ * the same time, hence bandwidths are accumulated in this case.
+ * This needs to be taken into account for calculating total bandwidth
+ * consumed by all planes.
+ *
+ * Here we get the overlapping state of each plane, which is a
+ * bitmask of plane indices telling with what planes there is an
+ * overlap. Note that bitmask[plane] includes BIT(plane) in order
+ * to make further code nicer and simpler.
+ */
+ drm_atomic_crtc_state_for_each_plane_state(plane, plane_state, new_state) {
+ tegra_state = to_const_tegra_plane_state(plane_state);
+ tegra = to_tegra_plane(plane);
+
+ plane_peak_bw[tegra->index] = tegra_state->peak_memory_bandwidth;
+ mask = tegra_plane_overlap_mask(new_state, plane_state);
+ overlap_mask[tegra->index] = mask;
+
+ if (hweight_long(mask) != 3)
+ all_planes_overlap_simultaneously = false;
+ }
+
+ old_state = drm_atomic_get_old_crtc_state(state, crtc);
+ old_dc_state = to_const_dc_state(old_state);
+
+ /*
+ * Then we calculate maximum bandwidth of each plane state.
+ * The bandwidth includes the plane BW + BW of the "simultaneously"
+ * overlapping planes, where "simultaneously" means areas where DC
+ * fetches from the planes simultaneously during of scan-out process.
+ *
+ * For example, if plane A overlaps with planes B and C, but B and C
+ * don't overlap, then the peak bandwidth will be either in area where
+ * A-and-B or A-and-C planes overlap.
+ *
+ * The plane_peak_bw[] contains peak memory bandwidth values of
+ * each plane, this information is needed by interconnect provider
+ * in order to set up latency allowness based on the peak BW, see
+ * tegra_crtc_update_memory_bandwidth().
+ */
+ for (i = 0; i < ARRAY_SIZE(plane_peak_bw); i++) {
+ overlap_bw = 0;
+
+ for_each_set_bit(k, &overlap_mask[i], 3) {
+ if (k == i)
+ continue;
+
+ if (all_planes_overlap_simultaneously)
+ overlap_bw += plane_peak_bw[k];
+ else
+ overlap_bw = max(overlap_bw, plane_peak_bw[k]);
+ }
+
+ new_dc_state->plane_peak_bw[i] = plane_peak_bw[i] + overlap_bw;
+
+ /*
+ * If plane's peak bandwidth changed (for example plane isn't
+ * overlapped anymore) and plane isn't in the atomic state,
+ * then add plane to the state in order to have the bandwidth
+ * updated.
+ */
+ if (old_dc_state->plane_peak_bw[i] !=
+ new_dc_state->plane_peak_bw[i]) {
+ plane = tegra_crtc_get_plane_by_index(crtc, i);
+ if (!plane)
+ continue;
+
+ plane_state = drm_atomic_get_plane_state(state, plane);
+ if (IS_ERR(plane_state))
+ return PTR_ERR(plane_state);
+ }
+ }
+
+ return 0;
+}
+
+static int tegra_crtc_atomic_check(struct drm_crtc *crtc,
+ struct drm_atomic_state *state)
+{
+ int err;
+
+ err = tegra_crtc_check_bandwidth_state(crtc, state);
+ if (err)
+ return err;
+
+ return 0;
+}
+
+void tegra_crtc_atomic_post_commit(struct drm_crtc *crtc,
+ struct drm_atomic_state *state)
+{
+ /*
+ * Display bandwidth is allowed to go down only once hardware state
+ * is known to be armed, i.e. state was committed and VBLANK event
+ * received.
+ */
+ tegra_crtc_update_memory_bandwidth(crtc, state, false);
+}
+
static const struct drm_crtc_helper_funcs tegra_crtc_helper_funcs = {
+ .atomic_check = tegra_crtc_atomic_check,
.atomic_begin = tegra_crtc_atomic_begin,
.atomic_flush = tegra_crtc_atomic_flush,
.atomic_enable = tegra_crtc_atomic_enable,
@@ -2245,7 +2582,9 @@ static const struct tegra_dc_soc_info tegra20_dc_soc_info = {
.overlay_formats = tegra20_overlay_formats,
.modifiers = tegra20_modifiers,
.has_win_a_without_filters = true,
+ .has_win_b_vfilter_mem_client = true,
.has_win_c_without_vert_filter = true,
+ .plane_tiled_memory_bandwidth_x2 = false,
};
static const struct tegra_dc_soc_info tegra30_dc_soc_info = {
@@ -2264,7 +2603,9 @@ static const struct tegra_dc_soc_info tegra30_dc_soc_info = {
.overlay_formats = tegra20_overlay_formats,
.modifiers = tegra20_modifiers,
.has_win_a_without_filters = false,
+ .has_win_b_vfilter_mem_client = true,
.has_win_c_without_vert_filter = false,
+ .plane_tiled_memory_bandwidth_x2 = true,
};
static const struct tegra_dc_soc_info tegra114_dc_soc_info = {
@@ -2283,7 +2624,9 @@ static const struct tegra_dc_soc_info tegra114_dc_soc_info = {
.overlay_formats = tegra114_overlay_formats,
.modifiers = tegra20_modifiers,
.has_win_a_without_filters = false,
+ .has_win_b_vfilter_mem_client = false,
.has_win_c_without_vert_filter = false,
+ .plane_tiled_memory_bandwidth_x2 = true,
};
static const struct tegra_dc_soc_info tegra124_dc_soc_info = {
@@ -2302,7 +2645,9 @@ static const struct tegra_dc_soc_info tegra124_dc_soc_info = {
.overlay_formats = tegra124_overlay_formats,
.modifiers = tegra124_modifiers,
.has_win_a_without_filters = false,
+ .has_win_b_vfilter_mem_client = false,
.has_win_c_without_vert_filter = false,
+ .plane_tiled_memory_bandwidth_x2 = false,
};
static const struct tegra_dc_soc_info tegra210_dc_soc_info = {
@@ -2321,7 +2666,9 @@ static const struct tegra_dc_soc_info tegra210_dc_soc_info = {
.overlay_formats = tegra114_overlay_formats,
.modifiers = tegra124_modifiers,
.has_win_a_without_filters = false,
+ .has_win_b_vfilter_mem_client = false,
.has_win_c_without_vert_filter = false,
+ .plane_tiled_memory_bandwidth_x2 = false,
};
static const struct tegra_windowgroup_soc tegra186_dc_wgrps[] = {
@@ -2370,6 +2717,7 @@ static const struct tegra_dc_soc_info tegra186_dc_soc_info = {
.has_nvdisplay = true,
.wgrps = tegra186_dc_wgrps,
.num_wgrps = ARRAY_SIZE(tegra186_dc_wgrps),
+ .plane_tiled_memory_bandwidth_x2 = false,
};
static const struct tegra_windowgroup_soc tegra194_dc_wgrps[] = {
@@ -2418,6 +2766,7 @@ static const struct tegra_dc_soc_info tegra194_dc_soc_info = {
.has_nvdisplay = true,
.wgrps = tegra194_dc_wgrps,
.num_wgrps = ARRAY_SIZE(tegra194_dc_wgrps),
+ .plane_tiled_memory_bandwidth_x2 = false,
};
static const struct of_device_id tegra_dc_of_match[] = {
diff --git a/drivers/gpu/drm/tegra/dc.h b/drivers/gpu/drm/tegra/dc.h
index 051d03dcb9b0..0d7bdf66a1ec 100644
--- a/drivers/gpu/drm/tegra/dc.h
+++ b/drivers/gpu/drm/tegra/dc.h
@@ -15,6 +15,8 @@
struct tegra_output;
+#define TEGRA_DC_LEGACY_PLANES_NUM 6
+
struct tegra_dc_state {
struct drm_crtc_state base;
@@ -23,6 +25,8 @@ struct tegra_dc_state {
unsigned int div;
u32 planes;
+
+ unsigned long plane_peak_bw[TEGRA_DC_LEGACY_PLANES_NUM];
};
static inline struct tegra_dc_state *to_dc_state(struct drm_crtc_state *state)
@@ -33,6 +37,12 @@ static inline struct tegra_dc_state *to_dc_state(struct drm_crtc_state *state)
return NULL;
}
+static inline const struct tegra_dc_state *
+to_const_dc_state(const struct drm_crtc_state *state)
+{
+ return to_dc_state((struct drm_crtc_state *)state);
+}
+
struct tegra_dc_stats {
unsigned long frames;
unsigned long vblank;
@@ -65,7 +75,9 @@ struct tegra_dc_soc_info {
unsigned int num_overlay_formats;
const u64 *modifiers;
bool has_win_a_without_filters;
+ bool has_win_b_vfilter_mem_client;
bool has_win_c_without_vert_filter;
+ unsigned int plane_tiled_memory_bandwidth_x2;
};
struct tegra_dc {
@@ -151,6 +163,8 @@ int tegra_dc_state_setup_clock(struct tegra_dc *dc,
struct drm_crtc_state *crtc_state,
struct clk *clk, unsigned long pclk,
unsigned int div);
+void tegra_crtc_atomic_post_commit(struct drm_crtc *crtc,
+ struct drm_atomic_state *state);
/* from rgb.c */
int tegra_dc_rgb_probe(struct tegra_dc *dc);
diff --git a/drivers/gpu/drm/tegra/drm.c b/drivers/gpu/drm/tegra/drm.c
index e9ce7d6992d2..47e7945dbc3b 100644
--- a/drivers/gpu/drm/tegra/drm.c
+++ b/drivers/gpu/drm/tegra/drm.c
@@ -20,6 +20,7 @@
#include <drm/drm_prime.h>
#include <drm/drm_vblank.h>
+#include "dc.h"
#include "drm.h"
#include "gem.h"
@@ -59,6 +60,17 @@ static const struct drm_mode_config_funcs tegra_drm_mode_config_funcs = {
.atomic_commit = drm_atomic_helper_commit,
};
+static void tegra_atomic_post_commit(struct drm_device *drm,
+ struct drm_atomic_state *old_state)
+{
+ struct drm_crtc_state *old_crtc_state __maybe_unused;
+ struct drm_crtc *crtc;
+ unsigned int i;
+
+ for_each_old_crtc_in_state(old_state, crtc, old_crtc_state, i)
+ tegra_crtc_atomic_post_commit(crtc, old_state);
+}
+
static void tegra_atomic_commit_tail(struct drm_atomic_state *old_state)
{
struct drm_device *drm = old_state->dev;
@@ -75,6 +87,8 @@ static void tegra_atomic_commit_tail(struct drm_atomic_state *old_state)
} else {
drm_atomic_helper_commit_tail_rpm(old_state);
}
+
+ tegra_atomic_post_commit(drm, old_state);
}
static const struct drm_mode_config_helper_funcs
diff --git a/drivers/gpu/drm/tegra/hub.c b/drivers/gpu/drm/tegra/hub.c
index 5ce771cba133..2a46289a3896 100644
--- a/drivers/gpu/drm/tegra/hub.c
+++ b/drivers/gpu/drm/tegra/hub.c
@@ -344,6 +344,9 @@ static int tegra_shared_plane_atomic_check(struct drm_plane *plane,
struct tegra_dc *dc = to_tegra_dc(state->crtc);
int err;
+ plane_state->peak_memory_bandwidth = 0;
+ plane_state->avg_memory_bandwidth = 0;
+
/* no need for further checks if the plane is being disabled */
if (!state->crtc || !state->fb)
return 0;
diff --git a/drivers/gpu/drm/tegra/plane.c b/drivers/gpu/drm/tegra/plane.c
index 539d14935728..1e589c5af143 100644
--- a/drivers/gpu/drm/tegra/plane.c
+++ b/drivers/gpu/drm/tegra/plane.c
@@ -4,6 +4,7 @@
*/
#include <linux/iommu.h>
+#include <linux/interconnect.h>
#include <drm/drm_atomic.h>
#include <drm/drm_atomic_helper.h>
@@ -64,6 +65,8 @@ tegra_plane_atomic_duplicate_state(struct drm_plane *plane)
copy->reflect_x = state->reflect_x;
copy->reflect_y = state->reflect_y;
copy->opaque = state->opaque;
+ copy->peak_memory_bandwidth = state->peak_memory_bandwidth;
+ copy->avg_memory_bandwidth = state->avg_memory_bandwidth;
for (i = 0; i < 2; i++)
copy->blending[i] = state->blending[i];
@@ -212,6 +215,87 @@ void tegra_plane_cleanup_fb(struct drm_plane *plane,
tegra_dc_unpin(dc, to_tegra_plane_state(state));
}
+static int tegra_plane_check_memory_bandwidth(struct drm_plane_state *state)
+{
+ struct tegra_plane_state *tegra_state = to_tegra_plane_state(state);
+ unsigned int i, bpp, bpp_plane, dst_w, src_w, src_h, mul;
+ const struct tegra_dc_soc_info *soc;
+ const struct drm_format_info *fmt;
+ struct drm_crtc_state *crtc_state;
+ u32 avg_bandwidth, peak_bandwidth;
+
+ if (!state->visible)
+ return 0;
+
+ crtc_state = drm_atomic_get_new_crtc_state(state->state, state->crtc);
+ if (!crtc_state)
+ return -EINVAL;
+
+ src_w = drm_rect_width(&state->src) >> 16;
+ src_h = drm_rect_height(&state->src) >> 16;
+ dst_w = drm_rect_width(&state->dst);
+
+ fmt = state->fb->format;
+ soc = to_tegra_dc(state->crtc)->soc;
+
+ /*
+ * Note that real memory bandwidth vary depending on format and
+ * memory layout, we are not taking that into account because small
+ * estimation error isn't important since bandwidth is rounded up
+ * anyway.
+ */
+ for (i = 0, bpp = 0; i < fmt->num_planes; i++) {
+ bpp_plane = fmt->cpp[i] * 8;
+
+ /*
+ * Sub-sampling is relevant for chroma planes only and vertical
+ * readouts are not cached, hence only horizontal sub-sampling
+ * matters.
+ */
+ if (i > 0)
+ bpp_plane /= fmt->hsub;
+
+ bpp += bpp_plane;
+ }
+
+ /*
+ * Horizontal downscale takes extra bandwidth which roughly depends
+ * on the scaled width.
+ */
+ if (src_w > dst_w)
+ mul = (src_w - dst_w) * bpp / 2048 + 1;
+ else
+ mul = 1;
+
+ /* average bandwidth in bytes/s */
+ avg_bandwidth = src_w * src_h * bpp / 8 * mul;
+ avg_bandwidth *= drm_mode_vrefresh(&crtc_state->mode);
+
+ /* mode.clock in kHz, peak bandwidth in kbit/s */
+ peak_bandwidth = crtc_state->mode.clock * bpp * mul;
+
+ /* ICC bandwidth in kbyte/s */
+ peak_bandwidth = kbps_to_icc(peak_bandwidth);
+ avg_bandwidth = Bps_to_icc(avg_bandwidth);
+
+ /*
+ * Tegra30/114 Memory Controller can't interleave DC memory requests
+ * and DC uses 16-bytes atom for the tiled windows, while DDR3 uses 32
+ * bytes atom. Hence there is x2 memory overfetch for tiled framebuffer
+ * and DDR3 on older SoCs.
+ */
+ if (soc->plane_tiled_memory_bandwidth_x2 &&
+ tegra_state->tiling.mode == TEGRA_BO_TILING_MODE_TILED) {
+ peak_bandwidth *= 2;
+ avg_bandwidth *= 2;
+ }
+
+ tegra_state->peak_memory_bandwidth = peak_bandwidth;
+ tegra_state->avg_memory_bandwidth = avg_bandwidth;
+
+ return 0;
+}
+
int tegra_plane_state_add(struct tegra_plane *plane,
struct drm_plane_state *state)
{
@@ -230,6 +314,10 @@ int tegra_plane_state_add(struct tegra_plane *plane,
if (err < 0)
return err;
+ err = tegra_plane_check_memory_bandwidth(state);
+ if (err < 0)
+ return err;
+
tegra = to_dc_state(crtc_state);
tegra->planes |= WIN_A_ACT_REQ << plane->index;
@@ -595,3 +683,36 @@ int tegra_plane_setup_legacy_state(struct tegra_plane *tegra,
return 0;
}
+
+static const char * const tegra_plane_icc_names[] = {
+ "wina", "winb", "winc", "", "", "", "cursor",
+};
+
+int tegra_plane_interconnect_init(struct tegra_plane *plane)
+{
+ const char *icc_name = tegra_plane_icc_names[plane->index];
+ struct device *dev = plane->dc->dev;
+ struct tegra_dc *dc = plane->dc;
+ int err;
+
+ plane->icc_mem = devm_of_icc_get(dev, icc_name);
+ err = PTR_ERR_OR_ZERO(plane->icc_mem);
+ if (err) {
+ dev_err_probe(dev, err, "failed to get %s interconnect\n",
+ icc_name);
+ return err;
+ }
+
+ /* plane B on T20/30 has a dedicated memory client for a 6-tap vertical filter */
+ if (plane->index == 1 && dc->soc->has_win_b_vfilter_mem_client) {
+ plane->icc_mem_vfilter = devm_of_icc_get(dev, "winb-vfilter");
+ err = PTR_ERR_OR_ZERO(plane->icc_mem_vfilter);
+ if (err) {
+ dev_err_probe(dev, err, "failed to get %s interconnect\n",
+ "winb-vfilter");
+ return err;
+ }
+ }
+
+ return 0;
+}
diff --git a/drivers/gpu/drm/tegra/plane.h b/drivers/gpu/drm/tegra/plane.h
index c691dd79b27b..f2731aae7d01 100644
--- a/drivers/gpu/drm/tegra/plane.h
+++ b/drivers/gpu/drm/tegra/plane.h
@@ -8,6 +8,7 @@
#include <drm/drm_plane.h>
+struct icc_path;
struct tegra_bo;
struct tegra_dc;
@@ -16,6 +17,9 @@ struct tegra_plane {
struct tegra_dc *dc;
unsigned int offset;
unsigned int index;
+
+ struct icc_path *icc_mem;
+ struct icc_path *icc_mem_vfilter;
};
struct tegra_cursor {
@@ -52,6 +56,10 @@ struct tegra_plane_state {
/* used for legacy blending support only */
struct tegra_plane_legacy_blending_state blending[2];
bool opaque;
+
+ /* bandwidths are in ICC units, i.e. kbytes/sec */
+ u32 peak_memory_bandwidth;
+ u32 avg_memory_bandwidth;
};
static inline struct tegra_plane_state *
@@ -63,6 +71,12 @@ to_tegra_plane_state(struct drm_plane_state *state)
return NULL;
}
+static inline const struct tegra_plane_state *
+to_const_tegra_plane_state(const struct drm_plane_state *state)
+{
+ return to_tegra_plane_state((struct drm_plane_state *)state);
+}
+
extern const struct drm_plane_funcs tegra_plane_funcs;
int tegra_plane_prepare_fb(struct drm_plane *plane,
@@ -77,5 +91,6 @@ int tegra_plane_format(u32 fourcc, u32 *format, u32 *swap);
bool tegra_plane_format_is_yuv(unsigned int format, bool *planar);
int tegra_plane_setup_legacy_state(struct tegra_plane *tegra,
struct tegra_plane_state *state);
+int tegra_plane_interconnect_init(struct tegra_plane *plane);
#endif /* TEGRA_PLANE_H */
--
2.29.2
On Tue, Mar 02, 2021 at 03:44:44PM +0300, Dmitry Osipenko wrote:
> Display controller (DC) performs isochronous memory transfers, and thus,
> has a requirement for a minimum memory bandwidth that shall be fulfilled,
> otherwise framebuffer data can't be fetched fast enough and this results
> in a DC's data-FIFO underflow that follows by a visual corruption.
>
> The Memory Controller drivers provide facility for memory bandwidth
> management via interconnect API. Let's wire up the interconnect API
> support to the DC driver in order to fix the distorted display output
> on T30 Ouya, T124 TK1 and other Tegra devices.
I did a read on the code. I have put some thoughts and nits inbetween the
diff, but here are more general questions about the patch:
Is there a reason why the bandwidth is allocated per plane instead of just
using one peak and average for the whole configuration? Or is there a need
to expose all the planes as interconnect channels and allocate their
bandwidth individually?
From algorithmic part I see that the plane overlaps are calculated twice
(A vs B and later B vs A). The cursor plane is ignored, but nevertheless
its overlap mask is calculated before being thrown away. The bandwidths
are also calculated twice: once for pre-commit and then again for
post-commit. Is setting bandwidth for an interconnect expensive when
re-setting a value that was already set? The code seems to avoid this
case, but maybe unnecessarily?
[...cut the big and interesting part...]
[...]
> @@ -65,7 +75,9 @@ struct tegra_dc_soc_info {
> unsigned int num_overlay_formats;
> const u64 *modifiers;
> bool has_win_a_without_filters;
> + bool has_win_b_vfilter_mem_client;
> bool has_win_c_without_vert_filter;
> + unsigned int plane_tiled_memory_bandwidth_x2;
This looks more like bool in the code using it.
[...]
> --- a/drivers/gpu/drm/tegra/plane.c
> +++ b/drivers/gpu/drm/tegra/plane.c
[...]
> +static int tegra_plane_check_memory_bandwidth(struct drm_plane_state *state)
The function's body looks more like 'update' or 'recalculate' rather
than 'check' the memory bandwidth.
> + struct tegra_plane_state *tegra_state = to_tegra_plane_state(state);
> + unsigned int i, bpp, bpp_plane, dst_w, src_w, src_h, mul;
> + const struct tegra_dc_soc_info *soc;
> + const struct drm_format_info *fmt;
> + struct drm_crtc_state *crtc_state;
> + u32 avg_bandwidth, peak_bandwidth;
> +
> + if (!state->visible)
> + return 0;
> +
> + crtc_state = drm_atomic_get_new_crtc_state(state->state, state->crtc);
> + if (!crtc_state)
> + return -EINVAL;
> +
> + src_w = drm_rect_width(&state->src) >> 16;
> + src_h = drm_rect_height(&state->src) >> 16;
> + dst_w = drm_rect_width(&state->dst);
> +
> + fmt = state->fb->format;
> + soc = to_tegra_dc(state->crtc)->soc;
> +
> + /*
> + * Note that real memory bandwidth vary depending on format and
> + * memory layout, we are not taking that into account because small
> + * estimation error isn't important since bandwidth is rounded up
> + * anyway.
> + */
> + for (i = 0, bpp = 0; i < fmt->num_planes; i++) {
> + bpp_plane = fmt->cpp[i] * 8;
Nit: you could declare the bpp_plane here.
> + /*
> + * Sub-sampling is relevant for chroma planes only and vertical
> + * readouts are not cached, hence only horizontal sub-sampling
> + * matters.
> + */
> + if (i > 0)
> + bpp_plane /= fmt->hsub;
> +
> + bpp += bpp_plane;
> + }
> +
> + /*
> + * Horizontal downscale takes extra bandwidth which roughly depends
> + * on the scaled width.
> + */
> + if (src_w > dst_w)
> + mul = (src_w - dst_w) * bpp / 2048 + 1;
> + else
> + mul = 1;
Does it really need more bandwidth to scale down? Does it read the same
data multiple times just to throw it away?
> + /* average bandwidth in bytes/s */
> + avg_bandwidth = src_w * src_h * bpp / 8 * mul;
> + avg_bandwidth *= drm_mode_vrefresh(&crtc_state->mode);
> +
> + /* mode.clock in kHz, peak bandwidth in kbit/s */
> + peak_bandwidth = crtc_state->mode.clock * bpp * mul;
I would guess that (src_w * bpp / 8) needs rounding up unless the plane
is continuous. Or you could just add the max rounding error here and
get a safe overestimated value. Maybe this would be better done when
summing per-plane widths.
> + /* ICC bandwidth in kbyte/s */
> + peak_bandwidth = kbps_to_icc(peak_bandwidth);
> + avg_bandwidth = Bps_to_icc(avg_bandwidth);
This could be merged with the assignments after the following 'if' block.
> + /*
> + * Tegra30/114 Memory Controller can't interleave DC memory requests
> + * and DC uses 16-bytes atom for the tiled windows, while DDR3 uses 32
> + * bytes atom. Hence there is x2 memory overfetch for tiled framebuffer
> + * and DDR3 on older SoCs.
> + */
> + if (soc->plane_tiled_memory_bandwidth_x2 &&
> + tegra_state->tiling.mode == TEGRA_BO_TILING_MODE_TILED) {
> + peak_bandwidth *= 2;
> + avg_bandwidth *= 2;
> + }
> +
> + tegra_state->peak_memory_bandwidth = peak_bandwidth;
> + tegra_state->avg_memory_bandwidth = avg_bandwidth;
> +
> + return 0;
> +}
[...]
> +static const char * const tegra_plane_icc_names[] = {
> + "wina", "winb", "winc", "", "", "", "cursor",
> +};
> +
> +int tegra_plane_interconnect_init(struct tegra_plane *plane)
> +{
> + const char *icc_name = tegra_plane_icc_names[plane->index];
Is plane->index guaranteed to be <= 6? I would guess so, but maybe
BUILD_BUG_ON(sizeof(icc_names)==TEGRA_DC_LEGACY_PLANES_NUM) or some
other check could document this?
[...]
Best Regards
Micha? Miros?aw
04.03.2021 02:08, Michał Mirosław пишет:
> On Tue, Mar 02, 2021 at 03:44:44PM +0300, Dmitry Osipenko wrote:
>> Display controller (DC) performs isochronous memory transfers, and thus,
>> has a requirement for a minimum memory bandwidth that shall be fulfilled,
>> otherwise framebuffer data can't be fetched fast enough and this results
>> in a DC's data-FIFO underflow that follows by a visual corruption.
>>
>> The Memory Controller drivers provide facility for memory bandwidth
>> management via interconnect API. Let's wire up the interconnect API
>> support to the DC driver in order to fix the distorted display output
>> on T30 Ouya, T124 TK1 and other Tegra devices.
>
> I did a read on the code. I have put some thoughts and nits inbetween the
> diff, but here are more general questions about the patch:
Hello, Michał! Thank you very much for taking a look at the patch!
> Is there a reason why the bandwidth is allocated per plane instead of just
> using one peak and average for the whole configuration? Or is there a need
> to expose all the planes as interconnect channels and allocate their
> bandwidth individually?
Each display plane has individual memory client on Tegra, memory ICC
paths are specified per memory client. This is how memory ICCs are
defined in the DT binding and that's what memory drivers are expecting
to deal with. It is also nice to see in ICC debugfs how much memory
bandwidth is consumed by each individual memory client.
> From algorithmic part I see that the plane overlaps are calculated twice
> (A vs B and later B vs A). The cursor plane is ignored, but nevertheless
> its overlap mask is calculated before being thrown away.
The algorithm assumes that we have a fixed number of planes to care
about and it's not executed in a hot code path, hence it's more optimal
to have a simpler and smaller code rather than try to optimize it
without gaining any benefits, IMO.
> The bandwidths
> are also calculated twice: once for pre-commit and then again for
> post-commit. Is setting bandwidth for an interconnect expensive when
> re-setting a value that was already set? The code seems to avoid this
> case, but maybe unnecessarily?
The CCF discards dummy rate-changes in the end of ICC code path.
Besides, we're not performing it in a hot code paths, hence performance
isn't a concern in this patch.
The tegra_crtc_update_memory_bandwidth() relies on being called before
and after the atomic commit. For example CRTC's "active" state is
updated only after the completion of commit-tail phase.
Earlier versions of this patch had checks that tried to avoid setting
bandwidth in both 'begin' and 'end' phases of a commit, but then I found
that it was buggy in regards to DPMS handling and it was much more
optimal to remove the unnecessary "optimizations" since code was blowing
up in complexity when I tried to fix it.
The tegra_crtc_update_memory_bandwidth() still checks whether old BW =
new BW, hence in practice actual ICC changes are only performed when
plane is turned on/off.
> [...cut the big and interesting part...]
>
> [...]
>> @@ -65,7 +75,9 @@ struct tegra_dc_soc_info {
>> unsigned int num_overlay_formats;
>> const u64 *modifiers;
>> bool has_win_a_without_filters;
>> + bool has_win_b_vfilter_mem_client;
>> bool has_win_c_without_vert_filter;
>> + unsigned int plane_tiled_memory_bandwidth_x2;
>
> This looks more like bool in the code using it.
Good catch, thank you!
> [...]
>> --- a/drivers/gpu/drm/tegra/plane.c
>> +++ b/drivers/gpu/drm/tegra/plane.c
> [...]
>> +static int tegra_plane_check_memory_bandwidth(struct drm_plane_state *state)
>
> The function's body looks more like 'update' or 'recalculate' rather
> than 'check' the memory bandwidth.
The 'check' in the name is intended to show that function belongs to
atomic-state checking.
But tegra_plane_calculate_memory_bandwidth_state() also is a good
variant. I'll consider the renaming, thanks!
>> + struct tegra_plane_state *tegra_state = to_tegra_plane_state(state);
>> + unsigned int i, bpp, bpp_plane, dst_w, src_w, src_h, mul;
>> + const struct tegra_dc_soc_info *soc;
>> + const struct drm_format_info *fmt;
>> + struct drm_crtc_state *crtc_state;
>> + u32 avg_bandwidth, peak_bandwidth;
>> +
>> + if (!state->visible)
>> + return 0;
>> +
>> + crtc_state = drm_atomic_get_new_crtc_state(state->state, state->crtc);
>> + if (!crtc_state)
>> + return -EINVAL;
>> +
>> + src_w = drm_rect_width(&state->src) >> 16;
>> + src_h = drm_rect_height(&state->src) >> 16;
>> + dst_w = drm_rect_width(&state->dst);
>> +
>> + fmt = state->fb->format;
>> + soc = to_tegra_dc(state->crtc)->soc;
>> +
>> + /*
>> + * Note that real memory bandwidth vary depending on format and
>> + * memory layout, we are not taking that into account because small
>> + * estimation error isn't important since bandwidth is rounded up
>> + * anyway.
>> + */
>> + for (i = 0, bpp = 0; i < fmt->num_planes; i++) {
>> + bpp_plane = fmt->cpp[i] * 8;
>
> Nit: you could declare the bpp_plane here.
>
>> + /*
>> + * Sub-sampling is relevant for chroma planes only and vertical
>> + * readouts are not cached, hence only horizontal sub-sampling
>> + * matters.
>> + */
>> + if (i > 0)
>> + bpp_plane /= fmt->hsub;
>> +
>> + bpp += bpp_plane;
>> + }
>> +
>> + /*
>> + * Horizontal downscale takes extra bandwidth which roughly depends
>> + * on the scaled width.
>> + */
>> + if (src_w > dst_w)
>> + mul = (src_w - dst_w) * bpp / 2048 + 1;
>> + else
>> + mul = 1;
>
> Does it really need more bandwidth to scale down? Does it read the same
> data multiple times just to throw it away?
The hardware isn't optimized for downscale, it indeed takes more
bandwidth. You'll witness a severe underflow of plane's memory FIFO
buffer on trying to downscale 1080p plane to 50x50. It's more optimal to
make a 2d blit to intermediate framebuffer for a large downscales.
>> + /* average bandwidth in bytes/s */
>> + avg_bandwidth = src_w * src_h * bpp / 8 * mul;
>> + avg_bandwidth *= drm_mode_vrefresh(&crtc_state->mode);
>> +
>> + /* mode.clock in kHz, peak bandwidth in kbit/s */
>> + peak_bandwidth = crtc_state->mode.clock * bpp * mul;
>
> I would guess that (src_w * bpp / 8) needs rounding up unless the plane
> is continuous. Or you could just add the max rounding error here and
> get a safe overestimated value. Maybe this would be better done when
> summing per-plane widths.
Looks like indeed it should be good to add a round-up, thanks.
>> + /* ICC bandwidth in kbyte/s */
>> + peak_bandwidth = kbps_to_icc(peak_bandwidth);
>> + avg_bandwidth = Bps_to_icc(avg_bandwidth);
>
> This could be merged with the assignments after the following 'if' block.
Seem so, thanks.
>> + /*
>> + * Tegra30/114 Memory Controller can't interleave DC memory requests
>> + * and DC uses 16-bytes atom for the tiled windows, while DDR3 uses 32
>> + * bytes atom. Hence there is x2 memory overfetch for tiled framebuffer
>> + * and DDR3 on older SoCs.
>> + */
>> + if (soc->plane_tiled_memory_bandwidth_x2 &&
>> + tegra_state->tiling.mode == TEGRA_BO_TILING_MODE_TILED) {
>> + peak_bandwidth *= 2;
>> + avg_bandwidth *= 2;
>> + }
>> +
>> + tegra_state->peak_memory_bandwidth = peak_bandwidth;
>> + tegra_state->avg_memory_bandwidth = avg_bandwidth;
>> +
>> + return 0;
>> +}
>
> [...]
>> +static const char * const tegra_plane_icc_names[] = {
>> + "wina", "winb", "winc", "", "", "", "cursor",
>> +};
>> +
>> +int tegra_plane_interconnect_init(struct tegra_plane *plane)
>> +{
>> + const char *icc_name = tegra_plane_icc_names[plane->index];
>
> Is plane->index guaranteed to be <= 6? I would guess so, but maybe
> BUILD_BUG_ON(sizeof(icc_names)==TEGRA_DC_LEGACY_PLANES_NUM) or some
> other check could document this?
The maximum index should always stay 6 for older h/w. It should be fine
to add extra sanity check to prevent an explicit misuse of the function,
just in a case. Thank you for the suggestion.
On Fri, Mar 05, 2021 at 12:45:51AM +0300, Dmitry Osipenko wrote:
> 04.03.2021 02:08, Michał Mirosław пишет:
> > On Tue, Mar 02, 2021 at 03:44:44PM +0300, Dmitry Osipenko wrote:
> >> Display controller (DC) performs isochronous memory transfers, and thus,
> >> has a requirement for a minimum memory bandwidth that shall be fulfilled,
> >> otherwise framebuffer data can't be fetched fast enough and this results
> >> in a DC's data-FIFO underflow that follows by a visual corruption.
[...]
> >> + /*
> >> + * Horizontal downscale takes extra bandwidth which roughly depends
> >> + * on the scaled width.
> >> + */
> >> + if (src_w > dst_w)
> >> + mul = (src_w - dst_w) * bpp / 2048 + 1;
> >> + else
> >> + mul = 1;
> >
> > Does it really need more bandwidth to scale down? Does it read the same
> > data multiple times just to throw it away?
> The hardware isn't optimized for downscale, it indeed takes more
> bandwidth. You'll witness a severe underflow of plane's memory FIFO
> buffer on trying to downscale 1080p plane to 50x50.
[...]
In your example, does it really need 16x the bandwidth compared to
no scaling case? The naive way to implement downscaling would be to read
all the pixels and only take every N-th. Maybe the problem is that in
downscaling mode the latency requirements are tighter? Why would bandwidth
required be proportional to a difference between the widths (instead e.g.
to src/dst or dst*cacheline_size)?
Best Regards
Michał Mirosław
06.03.2021 02:02, Michał Mirosław пишет:
> On Fri, Mar 05, 2021 at 12:45:51AM +0300, Dmitry Osipenko wrote:
>> 04.03.2021 02:08, Michał Mirosław пишет:
>>> On Tue, Mar 02, 2021 at 03:44:44PM +0300, Dmitry Osipenko wrote:
>>>> Display controller (DC) performs isochronous memory transfers, and thus,
>>>> has a requirement for a minimum memory bandwidth that shall be fulfilled,
>>>> otherwise framebuffer data can't be fetched fast enough and this results
>>>> in a DC's data-FIFO underflow that follows by a visual corruption.
> [...]
>>>> + /*
>>>> + * Horizontal downscale takes extra bandwidth which roughly depends
>>>> + * on the scaled width.
>>>> + */
>>>> + if (src_w > dst_w)
>>>> + mul = (src_w - dst_w) * bpp / 2048 + 1;
>>>> + else
>>>> + mul = 1;
>>>
>>> Does it really need more bandwidth to scale down? Does it read the same
>>> data multiple times just to throw it away?
>> The hardware isn't optimized for downscale, it indeed takes more
>> bandwidth. You'll witness a severe underflow of plane's memory FIFO
>> buffer on trying to downscale 1080p plane to 50x50.
> [...]
>
> In your example, does it really need 16x the bandwidth compared to
> no scaling case? The naive way to implement downscaling would be to read
> all the pixels and only take every N-th. Maybe the problem is that in
> downscaling mode the latency requirements are tighter? Why would bandwidth
> required be proportional to a difference between the widths (instead e.g.
> to src/dst or dst*cacheline_size)?
Seems you're right, it's actually not the bandwidth. Recently I added
memory client statistics gathering support to grate-kernel for Tegra20
and it shows that the consumed bandwidth is actually lower when plane is
downscaled.
So it should be the latency, which depends on memory frequency, and
thus, on bandwidth. I'll try to improve comment to the code in the next
version, thanks.