Date: Wed, 27 Apr 2016 13:14:20 +0200
From: Daniel Vetter <daniel@ffwll.ch>
To: Noralf =?iso-8859-1?Q?Tr=F8nnes?= <noralf@tronnes.org>
Cc: dri-devel@lists.freedesktop.org, linux-fbdev@vger.kernel.org,
        laurent.pinchart@ideasonboard.com, tomi.valkeinen@ti.com,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 4/8] drm/fb-helper: Add fb_deferred_io support
Message-ID: <20160427111420.GA2558@phenom.ffwll.local>
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On Wed, Apr 27, 2016 at 11:45:31AM +0200, Noralf Tr?nnes wrote:
> 
> Den 26.04.2016 19:19, skrev Daniel Vetter:
> >On Tue, Apr 26, 2016 at 06:24:54PM +0200, Noralf Tr?nnes wrote:
> >>Den 25.04.2016 11:09, skrev Daniel Vetter:
> >>>On Sun, Apr 24, 2016 at 10:48:58PM +0200, Noralf Tr?nnes wrote:
> >>>>This adds deferred io support if CONFIG_FB_DEFERRED_IO is enabled.
> >>>>The fbdev framebuffer changes are flushed using the callback
> >>>>(struct drm_framebuffer *)->funcs->dirty() by a dedicated worker
> >>>>ensuring that it always runs in process context.
> >>>>
> >>>>Signed-off-by: Noralf Tr?nnes <noralf@tronnes.org>
> >>>>---
> >>>>
> >>>>Changes since v1:
> >>>>- Use a dedicated worker to run the framebuffer flushing like qxl does
> >>>>- Add parameter descriptions to drm_fb_helper_deferred_io
> >>>>
> >>>>  drivers/gpu/drm/drm_fb_helper.c | 127 +++++++++++++++++++++++++++++++++++++++-
> >>>>  include/drm/drm_fb_helper.h     |  17 ++++++
> >>>>  2 files changed, 143 insertions(+), 1 deletion(-)
> >>>>
> >>>>diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> >>>>index 855108e..46ee6f8 100644
> >>>>--- a/drivers/gpu/drm/drm_fb_helper.c
> >>>>+++ b/drivers/gpu/drm/drm_fb_helper.c
> >>>>@@ -40,6 +40,7 @@
> >>>>  #include <drm/drm_crtc_helper.h>
> >>>>  #include <drm/drm_atomic.h>
> >>>>  #include <drm/drm_atomic_helper.h>
> >>>>+#include <drm/drm_rect.h>
> >>>>
> >>>>  static bool drm_fbdev_emulation = true;
> >>>>  module_param_named(fbdev_emulation, drm_fbdev_emulation, bool, 0600);
> >>>>@@ -48,6 +49,10 @@ MODULE_PARM_DESC(fbdev_emulation,
> >>>>
> >>>>  static LIST_HEAD(kernel_fb_helper_list);
> >>>>
> >>>>+static void drm_fb_helper_dirty_init(struct drm_fb_helper *helper);
> >>>>+static void drm_fb_helper_dirty(struct fb_info *info, u32 x, u32 y,
> >>>>+				u32 width, u32 height);
> >>>>+
> >>>>  /**
> >>>>   * DOC: fbdev helpers
> >>>>   *
> >>>>@@ -84,6 +89,16 @@ static LIST_HEAD(kernel_fb_helper_list);
> >>>>   * and set up an initial configuration using the detected hardware, drivers
> >>>>   * should call drm_fb_helper_single_add_all_connectors() followed by
> >>>>   * drm_fb_helper_initial_config().
> >>>>+ *
> >>>>+ * If CONFIG_FB_DEFERRED_IO is enabled and
> >>>>+ * (struct drm_framebuffer *)->funcs->dirty is set, the
> >>>>+ * drm_fb_helper_{cfb,sys}_{write,fillrect,copyarea,imageblit} functions
> >>>>+ * will accumulate changes and schedule (struct fb_helper).dirty_work to run
> >>>>+ * right away. This worker then calls the dirty() function ensuring that it
> >>>>+ * will always run in process context since the fb_*() function could be
> >>>>+ * running in atomic context. If drm_fb_helper_deferred_io() is used as the
> >>>>+ * deferred_io callback it will also schedule dirty_work with the damage
> >>>>+ * collected from the mmap page writes.
> >>>One thing to consider (and personally I don't care either way) is whether
> >>>we shouldn't just select CONFIG_FB_DEFERRED_IO if the fbdev helpers are
> >>>enabled. Pushing that out to drivers is imo a bit fragile.
> >>>
> >>>But like I said I'm ok with either way.
> >>My concern was adding code and data that only a few drivers would
> >>actually use. But of course there's the tradeoff with complexity.
> >>I use this to enable it:
> >>         select FB_DEFERRED_IO if DRM_KMS_FB_HELPER
> >>
> >>I guess the maintainer has to make this choice between size and complexity
> >>:-)
> >>I can enable it by default if you want, drm is both huge and complex so I
> >>don't know what's best.
> >>
> >>As a sidenote, I have also put all the fbdev code in a file of it's own to
> >>make it simple with regards to the DRM_FBDEV_EMULATION user option:
> >>tinydrm-$(CONFIG_DRM_KMS_FB_HELPER)     += tinydrm-fbdev.o
> >Ok, if you ask maintainers then please nuke the #ifdef from .c files. If
> >you select CONFIG_DRM_KMS_FB_HELPER, then you get hdmi, edid, dp aux, dp
> >mst and whatever else helpers, even if you don't need them. Adding 3
> >functions for defio when you select fbdev helpers and maybe don't need
> >them is totally harmless. And removing the #ifdef will look so much better
> >;-)
> 
> Will do :-)
> Kernel development is just my hobby so I'm not well versed in all of this.

You're doing great tbh!

> >>>>   */
> >>>>
> >>>>  /**
> >>>>@@ -401,11 +416,14 @@ backoff:
> >>>>  static int restore_fbdev_mode(struct drm_fb_helper *fb_helper)
> >>>>  {
> >>>>  	struct drm_device *dev = fb_helper->dev;
> >>>>+	struct fb_info *info = fb_helper->fbdev;
> >>>>  	struct drm_plane *plane;
> >>>>  	int i;
> >>>>
> >>>>  	drm_warn_on_modeset_not_all_locked(dev);
> >>>>
> >>>>+	drm_fb_helper_dirty(info, 0, 0, info->var.xres, info->var.yres);
> >>>Why is this needed? If you do a modeset (or pageflip or whatever) drivers
> >>>are supposed to re-upload the entire screen. We've talked about adding a
> >>>dirty rectangle to atomic to allow userspace to optimize this, but there
> >>>should _never_ be a need to do a dirtyfb call around a modeset. Probably
> >>>just a driver bug in your panel drm drivers?
> >>Ok, in tinydrm I now set a flag in &drm_simple_display_pipe_funcs
> >>->plane_update to indicate that the next dirty() should do the whole
> >>framebuffer which seems to work fine.
> >>Should I actually perform the update as well?
> >>If so I would need to add a worker in tinydrm to do that.
> >Yes, plane update should always do a full update. Not sure how you get
> >away with delaying that to ->dirty, maybe modesetting isn't
> >double-buffering when you don't have a GL that could do glamour.
> >
> >->dirty is _only_ for frontbuffer rendering, not for page-flipping to an
> >entirely new buffer. In short if someone calls ->dirty on a buffer which
> >is currently not being displayed than a) they're silly b) drivers should
> >treat it as a no-op. Maybe we need a helper to do that ...
> >-Daniel
> 
> drm_fb_helper will call dirty() as long as there's fbdev activity, so the
> driver needs to take that into account. For instance fbcon with a blinking
> cursor will trigger calls even if a buffer has been set up on the drm side.
> tinydrm checks the fb against the fb set on the plane and if it differs
> it's a no-op.

Was really just an idea to make drivers a bit simpler, since pretty much
all of them we need to do this check. But with a grand total of just 3 (4
with tinydrm) implementing a non-trivial dirty callback that's not really
worth it I think.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch