[Mesa-dev] [PATCH] st/mesa: update the blend before a full color clear

[gurchetansingh at chromium.org]

From: Gurchetan Singh <gurchetansingh at chromium.org>

Consider this series of events:

glColorMask(GL_FALSE, GL_TRUE, GL_TRUE, GL_TRUE);
glClearColor(0.125f, 0.25f, 0.5f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT);
glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
glClearColor(0.1f, 0.1f, 0.1f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT);

With virgl, this may render an incorrect result.  This is because
there our two paths for clears in Gallium -- one for full clears
(st->pipe->clear) and another for color-masked/scissored clears
(clear_with_quad).  We only encode the color mask in the
virgl_encode_blend_state in the clear_with_quad case.

We should set the colormask the case of full clears as well, since
we need to update the GL state on the host driver.

With this patch, the number of dEQP GLES2 failures on Virgl with a
nvidia host drivers goes from 260 to 136 with no regressions.
Two representative test cases are:

dEQP-GLES2.functional.color_clear.masked_scissored_rgb
dEQP-GLES2.functional.color_clear.masked_scissored_rgba

Note: I tried a virgl specific solution (crrev.com/c/909961), however
that caused a regression in dEQP-GLES2.functional.depth_stencil_clear.depth
since the cso_cache was not updated.
---
 src/mesa/state_tracker/st_cb_clear.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/src/mesa/state_tracker/st_cb_clear.c b/src/mesa/state_tracker/st_cb_clear.c
index 68677182ab..d8ad2b3b4a 100644
--- a/src/mesa/state_tracker/st_cb_clear.c
+++ b/src/mesa/state_tracker/st_cb_clear.c
@@ -444,6 +444,13 @@ st_Clear(struct gl_context *ctx, GLbitfield mask)
       clear_with_quad(ctx, quad_buffers);
    }
    if (clear_buffers) {
+      /* The colormask may go from a masked value to being completely unmasked.
+       * In that case, we should notify the driver via st_update_blend. The cso
+       * cache should take care of not emitting old states.
+       */
+      if (clear_buffers & PIPE_CLEAR_COLOR)
+         st_update_blend(st);
+
       /* We can't translate the clear color to the colorbuffer format,
        * because different colorbuffers may have different formats.
        */
-- 
2.13.5