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

[Tomasz Figa]

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. :)

Thanks,
Tomasz