Re: [RFC] drm/atomic+msm: add helper to implement legacy dirtyfb

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Apr 04, 2018 at 10:22:21AM +0200, Thomas Hellstrom wrote:
> Hi,
> 
> On 04/04/2018 08:58 AM, Daniel Vetter wrote:
> > On Wed, Apr 4, 2018 at 12:42 AM, Rob Clark <robdclark@gmail.com> wrote:
> > > Add an atomic helper to implement dirtyfb support.  This is needed to
> > > support DSI command-mode panels with x11 userspace (ie. when we can't
> > > rely on pageflips to trigger a flush to the panel).
> > > 
> > > To signal to the driver that the async atomic update needs to
> > > synchronize with fences, even though the fb didn't change, the
> > > drm_atomic_state::dirty flag is added.
> > > 
> > > Signed-off-by: Rob Clark <robdclark@gmail.com>
> > > ---
> > > Background: there are a number of different folks working on getting
> > > upstream kernel working on various different phones/tablets with qcom
> > > SoC's.. many of them have command mode panels, so we kind of need a
> > > way to support the legacy dirtyfb ioctl for x11 support.
> > > 
> > > I know there is work on a proprer non-legacy atomic property for
> > > userspace to communicate dirty-rect(s) to the kernel, so this can
> > > be improved from triggering a full-frame flush once that is in
> > > place.  But we kinda needa a stop-gap solution.
> > > 
> > > I had considered an in-driver solution for this, but things get a
> > > bit tricky if userspace ands up combining dirtyfb ioctls with page-
> > > flips, because we need to synchronize setting various CTL.FLUSH bits
> > > with setting the CTL.START bit.  (ie. really all we need to do for
> > > cmd mode panels is bang CTL.START, but is this ends up racing with
> > > pageflips setting FLUSH bits, then bad things.)  The easiest soln
> > > is to wrap this up as an atomic commit and rely on the worker to
> > > serialize things.  Hence adding an atomic dirtyfb helper.
> > > 
> > > I guess at least the helper, with some small addition to translate
> > > and pass-thru the dirty rect(s) is useful to the final atomic dirty-
> > > rect property solution.  Depending on how far off that is, a stop-
> > > gap solution could be useful.
> > Adding Noralf, who iirc already posted the full dirty helpers already somewhere.
> > -Daniel
> 
> I've asked Deepak to RFC the core changes suggested for the full dirty blob
> on dri-devel. It builds on DisplayLink's suggestion, with a simple helper to
> get to the desired coordinates.
> 
> One thing to perhaps discuss is how we would like to fit this with
> front-buffer rendering and the dirty ioctl. In the page-flip context, the
> dirty rects, like egl's swapbuffer_with_damage is a hint to restrict the
> damage region that can be fully ignored by the driver, new content is
> indicated by a new framebuffer.
> 
> We could do the same for frontbuffer rendering: Either set a dirty flag like
> you do here, or provide a content_age state member. Since we clear the dirty
> flag on state copies, I guess that would be sufficient. The blob rectangles
> would then become a hint to restrict the damage region.

I'm not entirely following here - I thought for frontbuffer rendering the
dirty rects have always just been a hint, and that the driver was always
free to re-upload the entire buffer to the screen.

And through a helper like Rob's proposing here (and have floated around in
different versions already) we'd essentially map a frontbuffer dirtyfb
call to a fake flip with dirty rect. Manual upload drivers already need to
upload the entire screen if they get a flip, since some userspace uses
that to flush out frontbuffer rendering (instead of calling dirtyfb).

So from that pov the new dirty flag is kinda not necessary imo.

> Another approach would be to have the presence of dirty rects without
> framebuffer change to indicate frontbuffer rendering.
> 
> I think I like the first approach best, although it may be tempting for
> user-space apps to just set the dirty bit instead of providing the full
> damage region.

Or I'm not following you here, because I don't quite see the difference
between dirtyfb and a flip.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch