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

[Tomasz Figa]

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;
-- 
2.14.0.434.g98096fd7a8-goog