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

[Thomas Hellstrom]

Hi,

On 04/04/2018 08:58 AM, Daniel Vetter wrote:
> On Wed, Apr 4, 2018 at 12:42 AM, Rob Clark <robdclark@xxxxxxxxx> 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@xxxxxxxxx>
> > ---
> > 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.

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.

/Thomas