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

[Emil Velikov]

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?

-Emil