linux-kernel - Re: [PATCH v4 1/6] drm: allow DRM_MODE_PAGE_FLIP

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Message-ID: <p-vKlt6LWsnO1-Ent-Y3sCt_NhITc2WrYQarRLJ2fIjxYFenrM9zCg6_YZxyh0gAaCXeBWGBnS5ww_aeT-EG95IsDTm5J-YegFrbINCC98w=@emersion.fr>
Date:   Tue, 04 Jul 2023 17:57:06 +0000
From:   Simon Ser <contact@...rsion.fr>
To:     Sebastian Wick <sebastian.wick@...hat.com>
Cc:     André Almeida <andrealmeid@...lia.com>,
        pierre-eric.pelloux-prayer@....com,
        Marek Olšák <maraeo@...il.com>,
        Michel Dänzer <michel.daenzer@...lbox.org>,
        Italo Nicola <italonicola@...labora.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        linux-kernel@...r.kernel.org, amd-gfx@...ts.freedesktop.org,
        wayland-devel@...ts.freedesktop.org,
        Pekka Paalanen <ppaalanen@...il.com>,
        dri-devel@...ts.freedesktop.org, kernel-dev@...lia.com,
        alexander.deucher@....com, hwentlan@....com,
        christian.koenig@....com, joshua@...ggi.es
Subject: Re: [PATCH v4 1/6] drm: allow DRM_MODE_PAGE_FLIP_ASYNC for atomic commits

On Tuesday, July 4th, 2023 at 19:06, Sebastian Wick <sebastian.wick@...hat.com> wrote:

> > + * When used with atomic uAPI, the driver will return an error if the hardware
> > + * doesn't support performing an asynchronous page-flip for this update.
> > + * User-space should handle this, e.g. by falling back to a regular page-flip.
> > + *
> > + * Note, some hardware might need to perform one last synchronous page-flip
> > + * before being able to switch to asynchronous page-flips. As an exception,
> > + * the driver will return success even though that first page-flip is not
> > + * asynchronous.
> 
> What would happen if one commits another async KMS update before the
> first page flip? Does one receive EAGAIN, does it amend the previous
> commit? What happens to the timing feedback?
> 
> This seems really risky to include tbh. I would prefer if we would not
> add such special cases for now.

This is not a new case, i915 already does this with the legacy API to
address some hw issues. Sadly I don't think we can do anything about
it.