[Mesa-stable] [PATCH] egl/dri2: Implement swapInterval fallback in a conformant way (v2)

[Juan A. Suarez Romero]

On Mon, 2017-09-25 at 16:25 +0900, Tomasz Figa wrote:
> On Fri, Aug 11, 2017 at 1:31 PM, Tomasz Figa <tfiga at chromium.org> wrote:
> > On Fri, Aug 11, 2017 at 2:29 AM, Emil Velikov <emil.l.velikov at gmail.com> wrote:
> > > On 10 August 2017 at 14:59, Tomasz Figa <tfiga at chromium.org> wrote:
> > > > dri2_fallback_swap_interval() currently used to stub out swap interval
> > > > support in Android backend does nothing besides returning EGL_FALSE.
> > > > This causes at least one known application (Android Snapchat) to fail
> > > > due to an unexpected error and my loose interpretation of the EGL 1.5
> > > > specification justifies it. Relevant quote below:
> > > > 
> > > >     The function
> > > > 
> > > >         EGLBoolean eglSwapInterval(EGLDisplay dpy, EGLint interval);
> > > > 
> > > >     specifies the minimum number of video frame periods per buffer swap
> > > >     for the draw surface of the current context, for the current rendering
> > > >     API. [...]
> > > > 
> > > >     The parameter interval specifies the minimum number of video frames
> > > >     that are displayed before a buffer swap will occur. The interval
> > > >     specified by the function applies to the draw surface bound to the
> > > >     context that is current on the calling thread. [...] interval is
> > > >     silently clamped to minimum and maximum implementation dependent
> > > >     values before being stored; these values are defined by EGLConfig
> > > >     attributes EGL_MIN_SWAP_INTERVAL and EGL_MAX_SWAP_INTERVAL
> > > >     respectively.
> > > > 
> > > >     The default swap interval is 1.
> > > > 
> > > > Even though it does not specify the exact behavior if the platform does
> > > > not support changing the swap interval, the default assumed state is the
> > > > swap interval of 1, which I interpret as a value that eglSwapInterval()
> > > > should succeed if called with, even if there is no ability to change the
> > > > interval (but there is no change requested). Moreover, since the
> > > > behavior is defined to clamp the requested value to minimum and maximum
> > > > and at least the default value of 1 must be present in the range, the
> > > > implementation might be expected to have a valid range, which in case of
> > > > the feature being unsupported, would correspond to {1} and any request
> > > > might be expected to be clamped to this value.
> > > > 
> > > > Fix this by defaulting dri2_dpy's min_swap_interval, max_swap_interval
> > > > and default_swap_interval to 1 in dri2_setup_screen() and let platforms,
> > > > which support this functionality set their own values after this
> > > > function returns. Thanks to patches merged earlier, we can also remove
> > > > the dri2_fallback_swap_interval() completely, as with a singular range
> > > > it would not be called anyway.
> > > > 
> > > > v2: Remove dri2_fallback_swap_interval() completely thanks to higher
> > > >     layer already clamping the requested interval and not calling the
> > > >     driver layer if the clamped value is the same as current.
> > > > 
> > > 
> > > Not every driver is EGL 1.5, although the 1.4 spec has exact same text.
> > > 
> > > Reviewed-by: Emil Velikov <emil.velikov at collabora.com>
> > > 
> > > We could have this in stable, but will need a clamp like v1 since
> > > Eric's rework did not land there.
> > > Gents, how do you feel on the topic?
> > 
> > FWIW, Eric's patches applied cleanly onto our chromium branch, which
> > was based on something not very far away from what 17.2 branched at.
> > If for some reasons we prefer to avoid further cherry picks, we can go
> > with my v1 for stable, which doesn't depend on them.
> 
> Gentle ping. :)
> 

Tomasz, didn't Chad and Tapani granted a R-b?

	J.A.

> Thanks,
> Tomasz
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