This driver implements the point of a DRM pipeline where a connector allows
removal of all the following bridges up to the panel.
The DRM subsystem currently allows hotplug of the monitor but not preceding
components. However there are embedded devices where the "tail" of the DRM
pipeline, including one or more bridges, can be physically removed:
.------------------------.
| DISPLAY CONTROLLER |
| .---------. .------. |
| | ENCODER |<--| CRTC | |
| '---------' '------' |
'------|-----------------'
|
| HOTPLUG
V CONNECTOR
.---------. .--. .-. .---------. .-------.
| 0 to N | | _| _| | | 1 to N | | |
| BRIDGES |--DSI-->||_ |_ |--DSI-->| BRIDGES |--LVDS-->| PANEL |
| | | | | | | | | |
'---------' '--' '-' '---------' '-------'
[--- fixed components --] [----------- removable add-on -----------]
This driver supports such devices, where the final segment of a MIPI DSI
bus, including one or more bridges, can be physically disconnected and
reconnected at runtime, possibly with a different model.
This implementation supports a MIPI DSI bus only, but it is designed to be
as far as possible generic and extendable to other busses that have no
native hotplug and model ID discovery.
This driver does not provide facilities to add and remove the hot-pluggable
components from the kernel: this needs to be done by other means
(e.g. device tree overlay runtime insertion and removal). The
hotplug-bridge gets notified of hot-plugging by the DRM bridge notifier
callbacks after they get added or before they get removed.
The hotplug-bridge role is to implement the "hot-pluggable connector" in
the bridge chain. In this position, what the hotplug-bridge should ideally
do is:
* communicate with the previous component (bridge or encoder) so that it
believes it always has a connected bridge following it and the DRM card
is always present
* be notified of the addition and removal of the following bridge and
attach/detach to/from it
* communicate with the following bridge so that it will attach and detach
using the normal procedure (as if the entire pipeline were being created
or destroyed, not only the tail)
* expose the "add-on connected/disconnected" status via the DRM connector
connected/disconnected status, so that users of the DRM pipeline know
when they can render output on the display
However some aspects make it a bit more complex than that. Most notably:
* the next bridge can be probed and removed at any moment and all probing
sequences need to be handled
* the DSI host/device registration process, which adds to the DRM bridge
attach process, makes the initial card registration tricky
* the need to register and deregister the following bridges at runtime
without tearing down the whole DRM card prevents using the functions
that are normally recommended
* the automatic mechanism to call the appropriate .get_modes operation
(typically provided by the panel bridge) cannot work as the panel can
disappear and reappear as a different model, so an ad-hoc lookup is
needed
The code handling these and other tricky aspects is accurately documented
by comments in the code.
Co-developed-by: Paul Kocialkowski <[email protected]>
Signed-off-by: Paul Kocialkowski <[email protected]>
Signed-off-by: Luca Ceresoli <[email protected]>
---
MAINTAINERS | 1 +
drivers/gpu/drm/bridge/Kconfig | 15 +
drivers/gpu/drm/bridge/Makefile | 1 +
drivers/gpu/drm/bridge/hotplug-bridge.c | 561 ++++++++++++++++++++++++++++++++
4 files changed, 578 insertions(+)
diff --git a/MAINTAINERS b/MAINTAINERS
index e1affd13e30b..b3fe36ed35a0 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -6720,6 +6720,7 @@ DRM DRIVER FOR HOTPLUG VIDEO CONNECTOR BRIDGE
M: Luca Ceresoli <[email protected]>
S: Maintained
F: Documentation/devicetree/bindings/display/bridge/hotplug-video-connector-dsi.yaml
+F: drivers/gpu/drm/bridge/hotplug-bridge.c
DRM DRIVER FOR HX8357D PANELS
S: Orphan
diff --git a/drivers/gpu/drm/bridge/Kconfig b/drivers/gpu/drm/bridge/Kconfig
index efd996f6c138..409d090ee94d 100644
--- a/drivers/gpu/drm/bridge/Kconfig
+++ b/drivers/gpu/drm/bridge/Kconfig
@@ -90,6 +90,21 @@ config DRM_FSL_LDB
help
Support for i.MX8MP DPI-to-LVDS on-SoC encoder.
+config DRM_HOTPLUG_BRIDGE
+ tristate "Hotplug DRM bridge support"
+ depends on OF
+ select DRM_PANEL_BRIDGE
+ select DRM_MIPI_DSI
+ select DRM_KMS_HELPER
+ help
+ Driver for a DRM bridge representing a physical connector that
+ splits a DRM pipeline into a fixed part and a physically
+ removable part. The fixed part includes up to the encoder and
+ zero or more bridges. The removable part includes any following
+ bridges up to the connector and panel and can be physically
+ removed and connected at runtime, possibly with different
+ components.
+
config DRM_ITE_IT6505
tristate "ITE IT6505 DisplayPort bridge"
depends on OF
diff --git a/drivers/gpu/drm/bridge/Makefile b/drivers/gpu/drm/bridge/Makefile
index 017b5832733b..278f20729c6c 100644
--- a/drivers/gpu/drm/bridge/Makefile
+++ b/drivers/gpu/drm/bridge/Makefile
@@ -6,6 +6,7 @@ obj-$(CONFIG_DRM_CHRONTEL_CH7033) += chrontel-ch7033.o
obj-$(CONFIG_DRM_CROS_EC_ANX7688) += cros-ec-anx7688.o
obj-$(CONFIG_DRM_DISPLAY_CONNECTOR) += display-connector.o
obj-$(CONFIG_DRM_FSL_LDB) += fsl-ldb.o
+obj-$(CONFIG_DRM_HOTPLUG_BRIDGE) += hotplug-bridge.o
obj-$(CONFIG_DRM_ITE_IT6505) += ite-it6505.o
obj-$(CONFIG_DRM_LONTIUM_LT8912B) += lontium-lt8912b.o
obj-$(CONFIG_DRM_LONTIUM_LT9211) += lontium-lt9211.o
diff --git a/drivers/gpu/drm/bridge/hotplug-bridge.c b/drivers/gpu/drm/bridge/hotplug-bridge.c
new file mode 100644
index 000000000000..8af01be80191
--- /dev/null
+++ b/drivers/gpu/drm/bridge/hotplug-bridge.c
@@ -0,0 +1,561 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * A DRM bridge representing the split point between a fixed part of the
+ * DRM pipeline and a physically removable part. The fixed part includes up
+ * to the encoder and zero or more bridges. Insertion and removal of the
+ * "downstream" components happens via device driver probe/removal.
+ *
+ * Copyright (C) 2024, GE HealthCare
+ *
+ * Authors:
+ * Luca Ceresoli <[email protected]>
+ * Paul Kocialkowski <[email protected]>
+ */
+
+#include <linux/mutex.h>
+#include <linux/of.h>
+#include <linux/of_graph.h>
+#include <linux/platform_device.h>
+
+#include <drm/drm_atomic_helper.h>
+#include <drm/drm_bridge.h>
+#include <drm/drm_bridge_connector.h>
+#include <drm/drm_mipi_dsi.h>
+#include <drm/drm_of.h>
+#include <drm/drm_probe_helper.h>
+
+struct hotplug_bridge {
+ struct device *dev;
+
+ /* Local bridge */
+ struct drm_bridge bridge;
+ /* Local connector for the bridge chain */
+ struct drm_connector *connector;
+ /* Downstream bridge (next in the chain) */
+ struct drm_bridge *next_bridge;
+ struct mutex next_bridge_mutex;
+
+ struct work_struct hpd_work;
+ struct notifier_block notifier_block;
+
+ /* Local DSI host, for the downstream DSI device to attach to */
+ struct mipi_dsi_host dsi_host;
+ /* Local DSI device, attached to the upstream DSI host */
+ struct mipi_dsi_device *dsi_dev;
+ /* Upstream DSI host (the actual DSI controller) */
+ struct mipi_dsi_host *prev_dsi_host;
+};
+
+static struct hotplug_bridge *hotplug_bridge_from_drm_bridge(struct drm_bridge *bridge)
+{
+ return container_of(bridge, struct hotplug_bridge, bridge);
+}
+
+/*
+ * Attach the remote bridge to the encoder and to the next bridge in the
+ * chain, if possible. For this to succeed, we need to know:
+ *
+ * - the encoder, which is set at the first drm_bridge_attach() time
+ * - the next bridge, which is obtained via a notifier whenever the next
+ * bridge is (re)probed, or at probe time in case it was probed before us
+ *
+ * In order to handle different execution sequences, this function can be
+ * called from multiple places and needs to check all the prerequisites
+ * every time, and it will act only if both are met.
+ *
+ * Must be called with hpb->next_bridge_mutex held.
+ *
+ * Returns 0 if the encoder was attached successfully, -ENODEV if any of
+ * the two prerequisites above is not met (no encoder or no next bridge),
+ * the error returned by drm_bridge_attach() otherwise.
+ */
+static int hotplug_bridge_attach_to_encoder_chain(struct hotplug_bridge *hpb)
+{
+ int ret;
+
+ if (!hpb->next_bridge || !hpb->bridge.encoder)
+ return -ENODEV;
+
+ ret = drm_bridge_attach(hpb->bridge.encoder, hpb->next_bridge, &hpb->bridge,
+ DRM_BRIDGE_ATTACH_NO_CONNECTOR);
+ if (ret)
+ return dev_err_probe(hpb->dev, ret, "drm_bridge_attach failed\n");
+
+ dev_dbg(hpb->dev, "attached to encoder chain\n");
+
+ return 0;
+}
+
+/*
+ * Stop the video pipeline and detach next_bridge.
+ *
+ * Must be called with hpb->next_bridge_mutex held.
+ */
+static void hotplug_bridge_detach_from_encoder_chain(struct hotplug_bridge *hpb)
+{
+ WARN_ON_ONCE(!hpb->next_bridge);
+
+ dev_dbg(hpb->dev, "detaching from encoder chain\n");
+
+ drm_atomic_helper_shutdown(hpb->bridge.dev);
+
+ drm_encoder_cleanup_from(hpb->bridge.encoder, hpb->next_bridge);
+}
+
+static void hotplug_bridge_grab(struct hotplug_bridge *hpb)
+{
+ struct device *dev = hpb->dev;
+ struct drm_bridge *bridge;
+ struct drm_panel *panel;
+ int err;
+
+ mutex_lock(&hpb->next_bridge_mutex);
+
+ if (hpb->next_bridge)
+ goto out;
+
+ /*
+ * This is supposed to be replaced by devm_drm_of_get_bridge(), but
+ * that is a devm_, and we need to remove the panel bridge also on
+ * next_bridge disconnect.
+ */
+ err = drm_of_find_panel_or_bridge(dev->of_node, 1, 0, &panel, &bridge);
+ if (err)
+ goto out;
+
+ /* Convert the remote panel to a bridge */
+ if (panel)
+ bridge = drm_panel_bridge_add(panel);
+
+ hpb->next_bridge = bridge;
+
+ dev_dbg(dev, "grabbed next bridge (%pOFn)\n", hpb->next_bridge->of_node);
+
+ hpb->bridge.pre_enable_prev_first = hpb->next_bridge->pre_enable_prev_first;
+
+ hotplug_bridge_attach_to_encoder_chain(hpb);
+
+ queue_work(system_wq, &hpb->hpd_work);
+
+out:
+ mutex_unlock(&hpb->next_bridge_mutex);
+}
+
+/*
+ * Detach from the next bridge and remove the panel bridge, either on
+ * release or when the downstream bridge is being removed.
+ *
+ * Can be called in these ways:
+ *
+ * - bridge_being_removed is NULL: detach unconditionally
+ * (this is useful on .remove() to teardown everything)
+ * - bridge_being_removed == hpb->next_bridge: detach
+ * (the downstream bridge is being removed)
+ * - bridge_being_removed != hpb->next_bridge: do nothing
+ * (the bridge being removed is not the downstream bridge)
+ *
+ * In all cases, does nothing when there is no downstream bridge.
+ */
+static void hotplug_bridge_release(struct hotplug_bridge *hpb,
+ struct drm_bridge *bridge_being_removed)
+{
+ mutex_lock(&hpb->next_bridge_mutex);
+
+ if (!hpb->next_bridge)
+ goto out;
+
+ if (bridge_being_removed && bridge_being_removed != hpb->next_bridge)
+ goto out;
+
+ dev_dbg(hpb->dev, "releasing next bridge (%pOFn)\n", hpb->next_bridge->of_node);
+
+ hotplug_bridge_detach_from_encoder_chain(hpb);
+
+ /*
+ * This will check that the bridge actually belongs to panel-bridge
+ * before doing anything with it, so we can safely always call it.
+ */
+ drm_panel_bridge_remove(hpb->next_bridge);
+ hpb->next_bridge = NULL;
+
+ queue_work(system_wq, &hpb->hpd_work);
+
+out:
+ mutex_unlock(&hpb->next_bridge_mutex);
+}
+
+static int hotplug_bridge_notifier_call(struct notifier_block *block,
+ unsigned long event, void *private)
+{
+ struct hotplug_bridge *hpb = container_of(block, struct hotplug_bridge, notifier_block);
+ struct drm_bridge *bridge = private;
+
+ switch (event) {
+ case DRM_BRIDGE_NOTIFY_ADD:
+ hotplug_bridge_grab(hpb);
+ break;
+ case DRM_BRIDGE_NOTIFY_REMOVE:
+ hotplug_bridge_release(hpb, bridge);
+ break;
+ }
+
+ return NOTIFY_DONE;
+}
+
+static int hotplug_bridge_attach(struct drm_bridge *bridge,
+ enum drm_bridge_attach_flags flags)
+{
+ struct hotplug_bridge *hpb = hotplug_bridge_from_drm_bridge(bridge);
+ struct device *dev = hpb->dev;
+ struct drm_connector *connector;
+ struct drm_encoder *encoder = hpb->bridge.encoder;
+ int err;
+
+ /* Encoder was not yet provided to our bridge */
+ if (!encoder)
+ return -ENODEV;
+
+ /* Connector was already created */
+ if (hpb->connector)
+ return dev_err_probe(dev, -EBUSY, "connector already created\n");
+
+ connector = drm_bridge_connector_init(bridge->dev, encoder);
+ if (IS_ERR(connector))
+ return dev_err_probe(dev, PTR_ERR(connector), "failed to initialize connector\n");
+
+ drm_connector_attach_encoder(connector, encoder);
+
+ hpb->connector = connector;
+
+ drm_connector_register(connector);
+
+ mutex_lock(&hpb->next_bridge_mutex);
+ err = hotplug_bridge_attach_to_encoder_chain(hpb);
+ mutex_unlock(&hpb->next_bridge_mutex);
+
+ /* -ENODEV is acceptable, in case next_bridge is not yet known */
+ if (err == -ENODEV)
+ err = 0;
+
+ return err;
+}
+
+static void hotplug_bridge_detach(struct drm_bridge *bridge)
+{
+ struct hotplug_bridge *hpb = hotplug_bridge_from_drm_bridge(bridge);
+
+ mutex_lock(&hpb->next_bridge_mutex);
+ hotplug_bridge_detach_from_encoder_chain(hpb);
+ mutex_unlock(&hpb->next_bridge_mutex);
+
+ if (hpb->connector) {
+ drm_connector_unregister(hpb->connector);
+ drm_connector_cleanup(hpb->connector);
+ hpb->connector = NULL;
+ }
+}
+
+static enum drm_connector_status hotplug_bridge_detect(struct drm_bridge *bridge)
+{
+ struct hotplug_bridge *hpb = hotplug_bridge_from_drm_bridge(bridge);
+
+ return hpb->next_bridge ? connector_status_connected : connector_status_disconnected;
+}
+
+static void hotplug_bridge_hpd_work_func(struct work_struct *work)
+{
+ struct hotplug_bridge *hpd = container_of(work, struct hotplug_bridge, hpd_work);
+
+ if (hpd->bridge.dev)
+ drm_helper_hpd_irq_event(hpd->bridge.dev);
+}
+
+static int hotplug_bridge_get_modes(struct drm_bridge *bridge, struct drm_connector *connector)
+{
+ struct hotplug_bridge *hpb = hotplug_bridge_from_drm_bridge(bridge);
+ struct drm_bridge *br;
+ int nmodes = 0;
+
+ mutex_lock(&hpb->next_bridge_mutex);
+
+ if (!hpb->next_bridge || !hpb->bridge.encoder)
+ goto out;
+
+ /*
+ * In non-removable pipelines, drm_bridge_connector_init() stores
+ * in the bridge_connector a pointer to the first bridge having
+ * OP_MODES (typically the panel bridge), so the .get_modes op will
+ * be automatically be called on that bridge. In our case the
+ * connector is created once when attaching to the encoder, but the
+ * panel bridge will appear later and can disappear and change at
+ * runtime, so we need to look for it every time dynamically.
+ */
+ br = hpb->next_bridge;
+ list_for_each_entry_from(br, &hpb->bridge.encoder->bridge_chain, chain_node) {
+ if (br->ops & DRM_BRIDGE_OP_MODES) {
+ nmodes = br->funcs->get_modes(br, connector);
+ break;
+ }
+ }
+
+out:
+ mutex_unlock(&hpb->next_bridge_mutex);
+
+ return nmodes;
+}
+
+static const struct drm_bridge_funcs hotplug_bridge_funcs = {
+ .attach = hotplug_bridge_attach,
+ .detach = hotplug_bridge_detach,
+ .detect = hotplug_bridge_detect,
+ .get_modes = hotplug_bridge_get_modes,
+};
+
+static int hotplug_bridge_dsi_detach(struct mipi_dsi_host *host,
+ struct mipi_dsi_device *device_remote)
+{
+ struct hotplug_bridge *hpb = dev_get_drvdata(host->dev);
+
+ if (!hpb->dsi_dev)
+ return -ENODEV;
+
+ mipi_dsi_detach(hpb->dsi_dev);
+ mipi_dsi_device_unregister(hpb->dsi_dev);
+ hpb->dsi_dev = NULL;
+
+ return 0;
+}
+
+/*
+ * Attach the local DSI device to the upstream DSI host, possibly with a
+ * "null" format.
+ *
+ * In "normal" bridges this function should be _only_ used as the .attach
+ * callback of hotplug_bridge_dsi_ops. But "normal" bridges have their
+ * downstream DSI device always connected, which we don't. When booting
+ * without anything connected downstream, our upstream bridge could be not
+ * even calling drm_bridge_add() until we do attach ourselves as a DSI
+ * device, preventing the whole DRM card from being instantiated.
+ *
+ * In order to always have a DRM card after boot, we do call this same
+ * function while probing in order to attach as a DSI device to the DSI
+ * master. However during probe we don't know the bus format yet. It would
+ * be nice to be able to update the format afterwards when a downstream DSI
+ * device is attaching to our local host, but there is no callback for
+ * that. To overcome this limitation, this function can be called in two
+ * ways:
+ *
+ * - during probe, to make the upstream bridge happy, when there is no
+ * next_dsi_dev yet and thus the lanes/format/etc are unknown
+ * - as the mipi_dsi_host_ops.attach callback proper, as soon as the
+ * next_dsi_dev is known
+ *
+ * The resulting call sequence is:
+ *
+ * 1. hotplug_bridge_dsi_attach() called by hotplug_bridge_probe() with
+ * next_dsi_dev == NULL: we attach to the host but with a fake format
+ * so the DRM card can be populated. hpb->dsi_dev becomes non-NULL.
+ * 2. hotplug_bridge_dsi_attach() called as .attach callback from a
+ * downstream device when it becomes available: we need to detach in
+ * order to re-attach with the format of the device. hpb->dsi_dev
+ * is found non-NULL, then reused so it will be non-NULL again.
+ * 3. hotplug_bridge_dsi_detach() called as the .detach callback by a
+ * downstream device: cleans up everything normally. hpb->dsi_dev goes
+ * from non-NULL to NULL.
+ * 4. hotplug_bridge_dsi_attach() called by a downstream device: attaches
+ * normally to the upstream DSI host. hpb->dsi_dev goes from NULL to
+ * non-NULL.
+ *
+ * Steps 3 and 4 are the "normal" attach/detach steps as on "normal"
+ * bridges.
+ *
+ * Steps 1 and 2 happen only the first time, steps 3 and 4 will happen
+ * every time the downstream bridge disconnects and reconnects.
+ */
+static int hotplug_bridge_dsi_attach(struct mipi_dsi_host *host,
+ struct mipi_dsi_device *next_dsi_dev)
+{
+ struct device *dev = host->dev;
+ struct hotplug_bridge *hpb = dev_get_drvdata(dev);
+ struct mipi_dsi_device *dsi_dev;
+ const struct mipi_dsi_device_info dsi_info = {
+ .type = "hotplug-bridge",
+ .channel = 0,
+ .node = NULL,
+ };
+ int err;
+
+ /*
+ * Step 2 only (first time we are called for an actual device
+ * attaching): clean up the fake attach done at step 1
+ */
+ if (hpb->dsi_dev)
+ hotplug_bridge_dsi_detach(&hpb->dsi_host, NULL);
+
+ /* Register a local DSI device with the remote DSI host */
+ dsi_dev = mipi_dsi_device_register_full(hpb->prev_dsi_host,
+ &dsi_info);
+ if (IS_ERR(dsi_dev))
+ return PTR_ERR(dsi_dev);
+
+ /* At step 1 we have no downstream device to get the format from */
+ if (next_dsi_dev) {
+ dsi_dev->channel = next_dsi_dev->channel;
+ dsi_dev->lanes = next_dsi_dev->lanes;
+ dsi_dev->format = next_dsi_dev->format;
+ dsi_dev->mode_flags = next_dsi_dev->mode_flags;
+ }
+
+ /* Attach our local DSI device to the remote DSI host */
+ err = mipi_dsi_attach(dsi_dev);
+ if (err) {
+ mipi_dsi_device_unregister(dsi_dev);
+ return dev_err_probe(dev, err, "failed to attach hotplug dsi device to host\n");
+ }
+
+ hpb->dsi_dev = dsi_dev;
+
+ return 0;
+}
+
+/*
+ * Propagate mipi_dsi_device_transfer() to the upstream DSI host.
+ *
+ * Reimplements identically the minimal needed part of
+ * mipi_dsi_device_transfer(), including the -ENOSYS return value.
+ */
+static ssize_t hotplug_bridge_dsi_transfer(struct mipi_dsi_host *host,
+ const struct mipi_dsi_msg *msg)
+{
+ struct hotplug_bridge *hpb = dev_get_drvdata(host->dev);
+ const struct mipi_dsi_host_ops *ops;
+
+ if (!hpb->dsi_dev)
+ return -ENODEV;
+
+ ops = hpb->dsi_dev->host->ops;
+
+ if (!ops || !ops->transfer)
+ return -ENOSYS;
+
+ return ops->transfer(hpb->dsi_dev->host, msg);
+}
+
+static const struct mipi_dsi_host_ops hotplug_bridge_dsi_ops = {
+ .attach = hotplug_bridge_dsi_attach,
+ .detach = hotplug_bridge_dsi_detach,
+ .transfer = hotplug_bridge_dsi_transfer,
+};
+
+/*
+ * Find the upstream DSI host and register our downstream-facing DSI host.
+ */
+static int hotplug_bridge_dsi_setup(struct hotplug_bridge *hpb)
+{
+ struct device *dev = hpb->dev;
+ struct device_node *endpoint;
+ struct device_node *node;
+
+ endpoint = of_graph_get_endpoint_by_regs(dev->of_node, 0, -1);
+ node = of_graph_get_remote_port_parent(endpoint);
+
+ hpb->prev_dsi_host = of_find_mipi_dsi_host_by_node(node);
+
+ of_node_put(node);
+ of_node_put(endpoint);
+
+ if (!hpb->prev_dsi_host)
+ return -EPROBE_DEFER;
+
+ hpb->dsi_host.dev = dev;
+ hpb->dsi_host.ops = &hotplug_bridge_dsi_ops;
+
+ return mipi_dsi_host_register(&hpb->dsi_host);
+}
+
+static void hotplug_bridge_dsi_cleanup(struct hotplug_bridge *hpb)
+{
+ mipi_dsi_host_unregister(&hpb->dsi_host);
+}
+
+static int hotplug_bridge_probe(struct platform_device *platform_dev)
+{
+ struct device *dev = &platform_dev->dev;
+ struct notifier_block *notifier_block;
+ struct hotplug_bridge *hpb;
+ struct drm_bridge *bridge;
+ int ret;
+
+ hpb = devm_kzalloc(dev, sizeof(*hpb), GFP_KERNEL);
+ if (!hpb)
+ return -ENOMEM;
+
+ hpb->dev = dev;
+
+ mutex_init(&hpb->next_bridge_mutex);
+ INIT_WORK(&hpb->hpd_work, hotplug_bridge_hpd_work_func);
+
+ notifier_block = &hpb->notifier_block;
+ notifier_block->notifier_call = hotplug_bridge_notifier_call;
+
+ ret = hotplug_bridge_dsi_setup(hpb);
+ if (ret)
+ return dev_err_probe(dev, ret, "failed to setup DSI\n");
+
+ bridge = &hpb->bridge;
+ bridge->of_node = dev->of_node;
+ bridge->funcs = &hotplug_bridge_funcs;
+ bridge->type = DRM_MODE_CONNECTOR_DSI;
+ bridge->ops |= DRM_BRIDGE_OP_DETECT | DRM_BRIDGE_OP_HPD | DRM_BRIDGE_OP_MODES;
+
+ platform_set_drvdata(platform_dev, hpb);
+
+ devm_drm_bridge_add(dev, bridge);
+
+ ret = hotplug_bridge_dsi_attach(&hpb->dsi_host, NULL);
+ if (ret)
+ return dev_err_probe(dev, ret, "failed first attach to upstream DSI host\n");
+
+ /* To be notified when the next bridge appears... */
+ drm_bridge_notifier_register(notifier_block);
+
+ /* ...but also check now, in case the next bridge was probed earlier */
+ hotplug_bridge_grab(hpb);
+
+ return 0;
+}
+
+static void hotplug_bridge_remove(struct platform_device *platform_dev)
+{
+ struct hotplug_bridge *hpb = platform_get_drvdata(platform_dev);
+
+ cancel_work_sync(&hpb->hpd_work);
+
+ drm_bridge_notifier_unregister(&hpb->notifier_block);
+
+ hotplug_bridge_release(hpb, NULL);
+
+ hotplug_bridge_dsi_cleanup(hpb);
+}
+
+static const struct of_device_id hotplug_bridge_of_match[] = {
+ { .compatible = "hotplug-video-connector-dsi" },
+ {},
+};
+MODULE_DEVICE_TABLE(of, hotplug_bridge_of_match);
+
+static struct platform_driver hotplug_bridge_driver = {
+ .probe = hotplug_bridge_probe,
+ .remove_new = hotplug_bridge_remove,
+ .driver = {
+ .name = "hotplug-drm-bridge",
+ .of_match_table = hotplug_bridge_of_match,
+ },
+};
+
+module_platform_driver(hotplug_bridge_driver);
+
+MODULE_AUTHOR("Luca Ceresoli <[email protected]>");
+MODULE_AUTHOR("Paul Kocialkowski <[email protected]>");
+MODULE_DESCRIPTION("Hotplug DRM Bridge");
+MODULE_LICENSE("GPL");
--
2.34.1
On Tue, Mar 26, 2024 at 05:28:14PM +0100, Luca Ceresoli wrote:
> This driver implements the point of a DRM pipeline where a connector allows
> removal of all the following bridges up to the panel.
>
> The DRM subsystem currently allows hotplug of the monitor but not preceding
> components. However there are embedded devices where the "tail" of the DRM
> pipeline, including one or more bridges, can be physically removed:
>
> .------------------------.
> | DISPLAY CONTROLLER |
> | .---------. .------. |
> | | ENCODER |<--| CRTC | |
> | '---------' '------' |
> '------|-----------------'
> |
> | HOTPLUG
> V CONNECTOR
> .---------. .--. .-. .---------. .-------.
> | 0 to N | | _| _| | | 1 to N | | |
> | BRIDGES |--DSI-->||_ |_ |--DSI-->| BRIDGES |--LVDS-->| PANEL |
> | | | | | | | | | |
> '---------' '--' '-' '---------' '-------'
>
> [--- fixed components --] [----------- removable add-on -----------]
>
> This driver supports such devices, where the final segment of a MIPI DSI
> bus, including one or more bridges, can be physically disconnected and
> reconnected at runtime, possibly with a different model.
>
> This implementation supports a MIPI DSI bus only, but it is designed to be
> as far as possible generic and extendable to other busses that have no
> native hotplug and model ID discovery.
>
> This driver does not provide facilities to add and remove the hot-pluggable
> components from the kernel: this needs to be done by other means
> (e.g. device tree overlay runtime insertion and removal). The
> hotplug-bridge gets notified of hot-plugging by the DRM bridge notifier
> callbacks after they get added or before they get removed.
>
> The hotplug-bridge role is to implement the "hot-pluggable connector" in
> the bridge chain. In this position, what the hotplug-bridge should ideally
> do is:
>
> * communicate with the previous component (bridge or encoder) so that it
> believes it always has a connected bridge following it and the DRM card
> is always present
> * be notified of the addition and removal of the following bridge and
> attach/detach to/from it
> * communicate with the following bridge so that it will attach and detach
> using the normal procedure (as if the entire pipeline were being created
> or destroyed, not only the tail)
> * expose the "add-on connected/disconnected" status via the DRM connector
> connected/disconnected status, so that users of the DRM pipeline know
> when they can render output on the display
>
> However some aspects make it a bit more complex than that. Most notably:
>
> * the next bridge can be probed and removed at any moment and all probing
> sequences need to be handled
> * the DSI host/device registration process, which adds to the DRM bridge
> attach process, makes the initial card registration tricky
> * the need to register and deregister the following bridges at runtime
> without tearing down the whole DRM card prevents using the functions
> that are normally recommended
> * the automatic mechanism to call the appropriate .get_modes operation
> (typically provided by the panel bridge) cannot work as the panel can
> disappear and reappear as a different model, so an ad-hoc lookup is
> needed
There's several additional hurdles there:
- You mentioned the connector in your ideal scenario. But as soon as
you remove the last bridge, the connector will probably go away too.
There's two scenarii here then:
- The driver is ok, and it will stay there until the last user its to
the main DRM device. Which means that if you create a new one,
you'll have the old one and the new one together, but you can't
tell which one you're supposed to use.
- If the driver isn't ok, the connector will be freed immediately.
There's plenty of lingering pointers in the framework, and
especially the states though, leading to use-after-free errors.
- So far, we told everyone that the graphics pipeline wasn't going to
change. How do you expect applications to deal with a connector going
away without any regression? I guess the natural thing here would be
to emit a uevent just like we do when the connection status change,
but the thing is: we're doing that for the connector, and the
connector is gone.
Between the userspace expectations and the memory-safety issue plaguing
way too many drivers, I'm not sure this approach can work.
I guess one way to somewhat achieve what you're trying to do would be to
introduce the connection status at the bridge level, reflect the
aggregate connection status of all bridges on the connector, and make
each bridge driver probe its device in the connect hook through DCS or
I2C.
We wouldn't be able to change the bridge halfway through the system's
life, but like I said, KMS cannot hotplug connectors at the moment and
doing so requires far more work.
Maxime
Hi Maxime,
On Wed, 27 Mar 2024 13:42:40 +0100
Maxime Ripard <[email protected]> wrote:
> On Tue, Mar 26, 2024 at 05:28:14PM +0100, Luca Ceresoli wrote:
> > This driver implements the point of a DRM pipeline where a connector allows
> > removal of all the following bridges up to the panel.
> >
> > The DRM subsystem currently allows hotplug of the monitor but not preceding
> > components. However there are embedded devices where the "tail" of the DRM
> > pipeline, including one or more bridges, can be physically removed:
> >
> > .------------------------.
> > | DISPLAY CONTROLLER |
> > | .---------. .------. |
> > | | ENCODER |<--| CRTC | |
> > | '---------' '------' |
> > '------|-----------------'
> > |
> > | HOTPLUG
> > V CONNECTOR
> > .---------. .--. .-. .---------. .-------.
> > | 0 to N | | _| _| | | 1 to N | | |
> > | BRIDGES |--DSI-->||_ |_ |--DSI-->| BRIDGES |--LVDS-->| PANEL |
> > | | | | | | | | | |
> > '---------' '--' '-' '---------' '-------'
> >
> > [--- fixed components --] [----------- removable add-on -----------]
> >
> > This driver supports such devices, where the final segment of a MIPI DSI
> > bus, including one or more bridges, can be physically disconnected and
> > reconnected at runtime, possibly with a different model.
> >
> > This implementation supports a MIPI DSI bus only, but it is designed to be
> > as far as possible generic and extendable to other busses that have no
> > native hotplug and model ID discovery.
> >
> > This driver does not provide facilities to add and remove the hot-pluggable
> > components from the kernel: this needs to be done by other means
> > (e.g. device tree overlay runtime insertion and removal). The
> > hotplug-bridge gets notified of hot-plugging by the DRM bridge notifier
> > callbacks after they get added or before they get removed.
> >
> > The hotplug-bridge role is to implement the "hot-pluggable connector" in
> > the bridge chain. In this position, what the hotplug-bridge should ideally
> > do is:
> >
> > * communicate with the previous component (bridge or encoder) so that it
> > believes it always has a connected bridge following it and the DRM card
> > is always present
> > * be notified of the addition and removal of the following bridge and
> > attach/detach to/from it
> > * communicate with the following bridge so that it will attach and detach
> > using the normal procedure (as if the entire pipeline were being created
> > or destroyed, not only the tail)
> > * expose the "add-on connected/disconnected" status via the DRM connector
> > connected/disconnected status, so that users of the DRM pipeline know
> > when they can render output on the display
> >
> > However some aspects make it a bit more complex than that. Most notably:
> >
> > * the next bridge can be probed and removed at any moment and all probing
> > sequences need to be handled
> > * the DSI host/device registration process, which adds to the DRM bridge
> > attach process, makes the initial card registration tricky
> > * the need to register and deregister the following bridges at runtime
> > without tearing down the whole DRM card prevents using the functions
> > that are normally recommended
> > * the automatic mechanism to call the appropriate .get_modes operation
> > (typically provided by the panel bridge) cannot work as the panel can
> > disappear and reappear as a different model, so an ad-hoc lookup is
> > needed
>
> There's several additional hurdles there:
>
> - You mentioned the connector in your ideal scenario. But as soon as
> you remove the last bridge, the connector will probably go away too.
> There's two scenarii here then:
>
> - The driver is ok, and it will stay there until the last user its to
> the main DRM device. Which means that if you create a new one,
> you'll have the old one and the new one together, but you can't
> tell which one you're supposed to use.
>
> - If the driver isn't ok, the connector will be freed immediately.
> There's plenty of lingering pointers in the framework, and
> especially the states though, leading to use-after-free errors.
>
> - So far, we told everyone that the graphics pipeline wasn't going to
> change. How do you expect applications to deal with a connector going
> away without any regression? I guess the natural thing here would be
> to emit a uevent just like we do when the connection status change,
> but the thing is: we're doing that for the connector, and the
> connector is gone.
Thanks for your feedback. I probably should have discussed this aspect
in my cover letter, sorry about that, let me amend now.
I think there are two possible approaches.
The first approach is based on removing the drm_connector. My laptop
uses the i915 driver, and I have observed that attaching/removing a
USB-C dock with an HDMI connector connected to a monitor, a new
drm_connector appears/disappears for the card. User space gets notified
and the external monitor is enabled/disabled, just the way a desktop
user would expect, so this is possible. I had a look at the driver but
how this magic happens was not clear to me honestly.
The second approach is simpler and based on keeping the drm_connector
always instantiated, and it is what this driver does. The drm_connector
is added by the hotplug-bridge driver in the drm_bridge_funcs.attach op,
which happens initially, and only removed by drm_bridge_funcs.detach,
so it is never removed when detaching the _following_ part of the
pipeline (which the card is unaware of). So the encoder always has a
drm_connector.
Note when attaching to the downstream bridge we pass the
DRM_BRIDGE_ATTACH_NO_CONNECTOR flag, which _should_ prevent creation of a
second connector. I'd expect some drivers to not honour that flag, but
they can be fixed if needed.
When the tail of the pipeline is connected/removed, the
hpb->next_bridge pointer becomes valid/NULL. And
hotplug_bridge_detect() looks at exactly that pointer to return a
connected or disconnected status.
The result is that when the add-on is connected, 'modetest -c' shows:
Connectors:
id encoder status name size (mm) modes encoders
37 0 connected DSI-1 293x165 1 36
modes:
index name refresh (Hz) hdisp hss hse htot vdisp vss vse vtot
#0 1920x1080 60.00 1920 1978 2020 2108 1080 1088 1102 1116 141140 flags: ; type: preferred, driver
props:
...
and when it is disconnected, it shows:
Connectors:
id encoder status name size (mm) modes encoders
37 0 disconnected DSI-1 0x0 0 36
props:
...
weston detects the HPD events from the connector and starts/stops using
the removable display correctly.
Does this clarify the approach?
I could be missing some aspects of course, especially in case of more
complex hardware setups than the one I have. However the code in this
series has been tested for a long time and no memory-safety issue has
appeared.
> Between the userspace expectations and the memory-safety issue plaguing
> way too many drivers, I'm not sure this approach can work.
>
> I guess one way to somewhat achieve what you're trying to do would be to
> introduce the connection status at the bridge level, reflect the
> aggregate connection status of all bridges on the connector, and make
> each bridge driver probe its device in the connect hook through DCS or
> I2C.
I think you mean: keeping all the bridge drivers instantiated, even
when the physical chip is removed.
This is of course another possible approach. However it would be more
invasive, forcing bridge drivers to change their current behaviour. And
it would violate the design that a driver is probed when a device is
there, and removed when the hardware goes away.
The approach I took firstly allows to have zero modifications to
existing bridge drivers -- not necessarily the right thing to do, but I
didn't find any good reason to require that.
Additionally, it is designed to allow removing an add-on having bridge
XYZ and then plugging in another add-on with bridge ABC, having a
different driver. Keeping alive the XYZ driver on unplug would not make
sense in such a case. This is not a tested scenario as I have no
hardware allowing that, but it is part of the design goals and I see no
obvious reason it wouldn't work with this patch as is, since the
downstream bridge driver is removed on disconnect and probed on connect
for whatever bridge will be connected.
Best regards,
Luca
--
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com
Hi Maxime,
On Wed, 27 Mar 2024 17:08:49 +0100
Luca Ceresoli <[email protected]> wrote:
[...]
> > There's several additional hurdles there:
> >
> > - You mentioned the connector in your ideal scenario. But as soon as
> > you remove the last bridge, the connector will probably go away too.
> > There's two scenarii here then:
> >
> > - The driver is ok, and it will stay there until the last user its to
> > the main DRM device. Which means that if you create a new one,
> > you'll have the old one and the new one together, but you can't
> > tell which one you're supposed to use.
> >
> > - If the driver isn't ok, the connector will be freed immediately.
> > There's plenty of lingering pointers in the framework, and
> > especially the states though, leading to use-after-free errors.
> >
> > - So far, we told everyone that the graphics pipeline wasn't going to
> > change. How do you expect applications to deal with a connector going
> > away without any regression? I guess the natural thing here would be
> > to emit a uevent just like we do when the connection status change,
> > but the thing is: we're doing that for the connector, and the
> > connector is gone.
>
> Thanks for your feedback. I probably should have discussed this aspect
> in my cover letter, sorry about that, let me amend now.
>
> I think there are two possible approaches.
>
> The first approach is based on removing the drm_connector. My laptop
> uses the i915 driver, and I have observed that attaching/removing a
> USB-C dock with an HDMI connector connected to a monitor, a new
> drm_connector appears/disappears for the card. User space gets notified
> and the external monitor is enabled/disabled, just the way a desktop
> user would expect, so this is possible. I had a look at the driver but
> how this magic happens was not clear to me honestly.
>
> The second approach is simpler and based on keeping the drm_connector
> always instantiated, and it is what this driver does. The drm_connector
> is added by the hotplug-bridge driver in the drm_bridge_funcs.attach op,
> which happens initially, and only removed by drm_bridge_funcs.detach,
> so it is never removed when detaching the _following_ part of the
> pipeline (which the card is unaware of). So the encoder always has a
> drm_connector.
>
> Note when attaching to the downstream bridge we pass the
> DRM_BRIDGE_ATTACH_NO_CONNECTOR flag, which _should_ prevent creation of a
> second connector. I'd expect some drivers to not honour that flag, but
> they can be fixed if needed.
>
> When the tail of the pipeline is connected/removed, the
> hpb->next_bridge pointer becomes valid/NULL. And
> hotplug_bridge_detect() looks at exactly that pointer to return a
> connected or disconnected status.
>
> The result is that when the add-on is connected, 'modetest -c' shows:
>
> Connectors:
> id encoder status name size (mm) modes encoders
> 37 0 connected DSI-1 293x165 1 36
> modes:
> index name refresh (Hz) hdisp hss hse htot vdisp vss vse vtot
> #0 1920x1080 60.00 1920 1978 2020 2108 1080 1088 1102 1116 141140 flags: ; type: preferred, driver
> props:
> ...
>
> and when it is disconnected, it shows:
>
> Connectors:
> id encoder status name size (mm) modes encoders
> 37 0 disconnected DSI-1 0x0 0 36
> props:
> ...
>
> weston detects the HPD events from the connector and starts/stops using
> the removable display correctly.
>
> Does this clarify the approach?
>
> I could be missing some aspects of course, especially in case of more
> complex hardware setups than the one I have. However the code in this
> series has been tested for a long time and no memory-safety issue has
> appeared.
>
> > Between the userspace expectations and the memory-safety issue plaguing
> > way too many drivers, I'm not sure this approach can work.
> >
> > I guess one way to somewhat achieve what you're trying to do would be to
> > introduce the connection status at the bridge level, reflect the
> > aggregate connection status of all bridges on the connector, and make
> > each bridge driver probe its device in the connect hook through DCS or
> > I2C.
>
> I think you mean: keeping all the bridge drivers instantiated, even
> when the physical chip is removed.
>
> This is of course another possible approach. However it would be more
> invasive, forcing bridge drivers to change their current behaviour. And
> it would violate the design that a driver is probed when a device is
> there, and removed when the hardware goes away.
>
> The approach I took firstly allows to have zero modifications to
> existing bridge drivers -- not necessarily the right thing to do, but I
> didn't find any good reason to require that.
>
> Additionally, it is designed to allow removing an add-on having bridge
> XYZ and then plugging in another add-on with bridge ABC, having a
> different driver. Keeping alive the XYZ driver on unplug would not make
> sense in such a case. This is not a tested scenario as I have no
> hardware allowing that, but it is part of the design goals and I see no
> obvious reason it wouldn't work with this patch as is, since the
> downstream bridge driver is removed on disconnect and probed on connect
> for whatever bridge will be connected.
Did you have a chance to think about this? I was wondering whether my
comments addresses some of your concerns, and whether these additional
clarifications make my approach look reasonable to you.
Best regards,
Luca
--
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com