[PATCH 3/4] drm: introduce DRM_CAP_ATOMIC_ASYNC_PAGE_FLIP

[Simon Ser]

On Friday, August 26th, 2022 at 10:19, Ville Syrjälä <ville.syrjala at linux.intel.com> wrote:

> On Wed, Aug 24, 2022 at 03:08:55PM +0000, Simon Ser wrote:
> > This new kernel capability indicates whether async page-flips are
> > supported via the atomic uAPI. DRM clients can use it to check
> > for support before feeding DRM_MODE_PAGE_FLIP_ASYNC to the kernel.
> 
> I think we'd need to clarify the semantics of the async flag
> for atomic commits.
> 
> Eg. on Intel hw only pure page flips are possible async, if you do
> anything else (change plane size/pos/scaling/etc.) you will need
> to do a sync update. Technically not even all page flips (from the
> uapi POV) might be possible as the exact scanout source address
> is specified via two registers, only one of which can be update
> async. So technically the two framebuffers might be laid out
> just slightly differently which could prevent an async flip.
> Also only some subset of planes actually support async flips.

Also IIRC some format modifiers don't support async flip at all (CCS)?

> And on hw where multiple planes support it on the same crtc, only one
> plane can do it at a time. Well, more accurately we can only select
> one plane at a time to give us the "flip done" interrupt. I guess
> if the user wants to async flip multiple planes at the same time
> we could do them serially as opposed to in parallel to make sure
> all the flips actually happened before we signal completion of the
> entire commit. Async flips of multiple planes probably won't
> happen atomically anyway so doing them serially seems fine.
> 
> ATM in i915 we probably don't have sufficient state checks in
> place to catch all the restrictions, and instead in part we rely
> on the limited scope of the legacy async flip ioctl to make sure
> the operation doesn't attempt something the hw can't do.

Yeah, that makes sense.

In the documentation patch discussion [1], it appears it's not clear what
drivers should do when async flip isn't possible with the legacy uAPI.

For the atomic uAPI, we need to pick on of these two approaches:

1. Let the kernel fall back to a sync flip if async isn't possible. This
   simplifies user-space, but then user-space has no reliable way to figure out
   what really happened (sync or async?). That could be fixed with a new
   read-only CRTC prop indicating whether the last flip was async or not.
   However, maybe someone will come up in the future with user-space which
   needs to only apply an update if async flip is possible, in which case this
   approach falls short.
2. Make the kernel return EINVAL if async flip isn't possible. This adds more
   complexity to user-space, but enables a more reliable and deterministic
   uAPI. This is also more consistent with the rest of the existing atomic
   uAPI.

Note, current user-space would only need to opportunistically enable async
flip. IOW, I think that for current user-space plans "async if possible,
otherwise sync" is good enough. That behavior maps well to the Vulkan present
modes as well (which says that "this mode *may* result in visible tearing", but
doesn't guarantee it).

Another possible shortcoming of the proposed new uAPI here is that user-space
cannot submit a single atomic commit which updates multiple CRTCs, and
individually select which CRTC does an async flip. This could be fixed with
a "ASYNC_FLIP" CRTC prop which the kernel always resets to 0 on commit. I'm not
sure we want/need to cross that bridge right now, it would be easy enough to
add as a second step if some user-space would benefit from it.

What do you think?

[1]: https://lore.kernel.org/dri-devel/ASSNOUe9wtsXskZjVlf1X4pl53T7pVE0MfEzkQ_h4cX0tjnF7e3cxpwGpRNPudmIHoRuW4kz_v1AeTpXgouLpTYcI8q-lPTzc1YMLR8JiJM=@emersion.fr/