On 04/04/2018 11:56 AM, Daniel Vetter wrote:
On Wed, Apr 04, 2018 at 11:10:08AM +0200, Thomas Hellstrom wrote:
On 04/04/2018 10:43 AM, Daniel Vetter wrote:
On Wed, Apr 04, 2018 at 10:22:21AM +0200, Thomas Hellstrom wrote:

On 04/04/2018 08:58 AM, Daniel Vetter wrote:
On Wed, Apr 4, 2018 at 12:42 AM, Rob Clark <robdcl...@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 <robdcl...@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.
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.
OK, let me rephrase:

 From the driver's point-of-view, in the atomic world, new content and the
need for manual upload is indicated by a change in fb attached to the plane.

With Rob's patch here, (correct me if I'm wrong) in addition new content and
the need for manual upload is identified by the dirty flag, (since the fb
stays the same and the driver thus never identifies a page-flip).
Hm, I'm not entirely sure Rob's approach is correct. Imo a pageflip
(atomic or not) should result in the entire buffer getting uploaded. The
dirty flag is kinda redundant, a flip with the same buffer works the same
way as a dirtyfb with the entire buffer as the dirty rectangle.

In both these situations, clip rects can provide a hint to restrict the
dirty region.

Now I was thinking about the preferred way for user-space to communicate
front buffer rendering through the atomic ioctl:

1) Expose a dirty (or content_age property)
2) Attach a clip-rect blob property.
3) Fake a page-flip by ping-ponging two frame-buffers pointing to the same
underlying buffer object.

Are you saying that people are already using 3) and we should keep using
I'm saying they're using 3b), flip the same buffer wrapped in the same
drm_framebuffer, and expect it to work.

The only advantage dirtyfb has is that it allows you to supply the
optimized upload rectangles, but at the cost of a funny api (it's working
on the fb, not the plane/crtc you want to upload) and lack of drm_event to
confirm when exactly you uploaded your stuff. But imo they should be the
same underlying operation.

Also note that atomic helpers don't optimize out plane flips for same
buffers. We only optimize out some of the waiting, in a failed attempt at
making cursors stall less, but that's not fixed with the async plane
update stuff. And we can obviously optimize out the prepare/cleanup hooks,
because the buffer should be pinned already.

I'm a bit confused.

Apparently we have different opinions in when an uploading driver should assume altered plane content and the need for re-upload. vmwgfx is from what I know currently assuming that this happens only on changed fb attachment (what we call a page-flip) whereas if I understand you correctly it should happen on each atomic state commit?

If we should assume the latter, then it has odd implications, let's say you have 8 screens up, and you pan one of them on the large fb, why would you upload the contents of the other 7?

Likewise for cursors,  why would you want to upload the cursor image on each cursor move?

So in my POW, option 1) is the option that aligns with the current vmwgfx implementation and from what I can tell, what Rob has implemented in his patch.


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