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

[Tapani Pälli]

I agree to the interpretation, it is stated that EGL_FALSE happens 'on 
failure' and there are 2 error cases specified for eglSwapInterval. It 
seems right to assume those are the only cases where it can fail (no 
current context or surface bound).

Reviewed-by: Tapani Pälli <tapani.palli at intel.com>

On 08/10/2017 08:43 AM, Tomasz Figa 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.
> 
> This is further confirmed by the code in _eglSwapInterval() in
> src/egl/main/eglsurface.c, which is the default fallback implementation
> for EGL drivers not implementing its own. The problem with it is that
> the DRI2 EGL driver provides its own implementation that calls into
> platform backends, so we cannot just simply fall back to it.
> 
> Signed-off-by: Tomasz Figa <tfiga at chromium.org>
> ---
>   src/egl/drivers/dri2/egl_dri2.c           | 12 ++++++++++++
>   src/egl/drivers/dri2/egl_dri2_fallbacks.h |  9 ++++++++-
>   src/egl/drivers/dri2/platform_x11.c       |  1 +
>   3 files changed, 21 insertions(+), 1 deletion(-)
> 
> diff --git a/src/egl/drivers/dri2/egl_dri2.c b/src/egl/drivers/dri2/egl_dri2.c
> index f0d1ded408..686dd68d29 100644
> --- a/src/egl/drivers/dri2/egl_dri2.c
> +++ b/src/egl/drivers/dri2/egl_dri2.c
> @@ -630,6 +630,18 @@ dri2_setup_screen(_EGLDisplay *disp)
>      struct dri2_egl_display *dri2_dpy = dri2_egl_display(disp);
>      unsigned int api_mask;
>   
> +   /*
> +    * EGL 1.5 specification defines the default value to 1. Moreover,
> +    * eglSwapInterval() is required to clamp requested value to the supported
> +    * range. Since the default value is implicitly assumed to be supported,
> +    * use it as both minimum and maximum for the platforms that do not allow
> +    * changing the interval. Platforms, which allow it (e.g. x11, wayland)
> +    * override these values already.
> +    */
> +   dri2_dpy->min_swap_interval = 1;
> +   dri2_dpy->max_swap_interval = 1;
> +   dri2_dpy->default_swap_interval = 1;
> +
>      if (dri2_dpy->image_driver) {
>         api_mask = dri2_dpy->image_driver->getAPIMask(dri2_dpy->dri_screen);
>      } else if (dri2_dpy->dri2) {
> diff --git a/src/egl/drivers/dri2/egl_dri2_fallbacks.h b/src/egl/drivers/dri2/egl_dri2_fallbacks.h
> index 604db881a8..c70c686f8d 100644
> --- a/src/egl/drivers/dri2/egl_dri2_fallbacks.h
> +++ b/src/egl/drivers/dri2/egl_dri2_fallbacks.h
> @@ -59,7 +59,14 @@ static inline EGLBoolean
>   dri2_fallback_swap_interval(_EGLDriver *drv, _EGLDisplay *dpy,
>                               _EGLSurface *surf, EGLint interval)
>   {
> -   return EGL_FALSE;
> +   if (interval > surf->Config->MaxSwapInterval)
> +      interval = surf->Config->MaxSwapInterval;
> +   else if (interval < surf->Config->MinSwapInterval)
> +      interval = surf->Config->MinSwapInterval;
> +
> +   surf->SwapInterval = interval;
> +
> +   return EGL_TRUE;
>   }
>   
>   static inline EGLBoolean
> diff --git a/src/egl/drivers/dri2/platform_x11.c b/src/egl/drivers/dri2/platform_x11.c
> index 4610ec579f..97cdd09078 100644
> --- a/src/egl/drivers/dri2/platform_x11.c
> +++ b/src/egl/drivers/dri2/platform_x11.c
> @@ -1283,6 +1283,7 @@ dri2_x11_setup_swap_interval(struct dri2_egl_display *dri2_dpy)
>       */
>      dri2_dpy->min_swap_interval = 0;
>      dri2_dpy->max_swap_interval = 0;
> +   dri2_dpy->default_swap_interval = 0;
>   
>      if (!dri2_dpy->swap_available)
>         return;
>