mesa: Make single-buffered GLES representation internally consistent

There are a few places in the code where clearing and reading are done on
incorrect buffers for GLES contexts.  See comments for details.  This
fixes 75 GLES3 dEQP tests on the surfaceless platform with no regressions.

v2: Corrected unclear comment
v3: Make the change in context.c instead of get.c
v4: Removed whitespace

Reviewed-by: Stéphane Marchesin <[email protected]>
Reviewed-by: Chad Versace <[email protected]>

3 files changed, 30 insertions, 2 deletions

diff --git a/src/mesa/main/buffers.c b/src/mesa/main/buffers.c
index e8aedde8777..86696b80c87 100644
--- a/src/mesa/main/buffers.c
+++ b/src/mesa/main/buffers.c
@@ -173,12 +173,22 @@ draw_buffer_enum_to_bitmask(const struct gl_context *ctx, GLenum buffer)
  * return -1 for an invalid buffer.
  */
 static gl_buffer_index
-read_buffer_enum_to_index(GLenum buffer)
+read_buffer_enum_to_index(const struct gl_context *ctx, GLenum buffer)
 {
    switch (buffer) {
       case GL_FRONT:
          return BUFFER_FRONT_LEFT;
       case GL_BACK:
+         if (_mesa_is_gles(ctx)) {
+            /* In draw_buffer_enum_to_bitmask, when GLES contexts draw to
+             * GL_BACK with a single-buffered configuration, we actually end
+             * up drawing to the sole front buffer in our internal
+             * representation.  For consistency, we must read from that
+             * front left buffer too.
+             */
+            if (!ctx->DrawBuffer->Visual.doubleBufferMode)
+               return BUFFER_FRONT_LEFT;
+         }
          return BUFFER_BACK_LEFT;
       case GL_RIGHT:
          return BUFFER_FRONT_RIGHT;
@@ -724,7 +734,7 @@ read_buffer(struct gl_context *ctx, struct gl_framebuffer *fb,
       if (_mesa_is_gles3(ctx) && !is_legal_es3_readbuffer_enum(buffer))
          srcBuffer = -1;
       else
-         srcBuffer = read_buffer_enum_to_index(buffer);
+         srcBuffer = read_buffer_enum_to_index(ctx, buffer);
 
       if (srcBuffer == -1) {
          _mesa_error(ctx, GL_INVALID_ENUM,
diff --git a/src/mesa/main/clear.c b/src/mesa/main/clear.c
index 35b912cbf10..a1bb36efe24 100644
--- a/src/mesa/main/clear.c
+++ b/src/mesa/main/clear.c
@@ -267,6 +267,14 @@ make_color_buffer_mask(struct gl_context *ctx, GLint drawbuffer)
          mask |= BUFFER_BIT_FRONT_RIGHT;
       break;
    case GL_BACK:
+      /* For GLES contexts with a single buffered configuration, we actually
+       * only have a front renderbuffer, so any clear calls to GL_BACK should
+       * affect that buffer. See draw_buffer_enum_to_bitmask for details.
+       */
+      if (_mesa_is_gles(ctx))
+         if (!ctx->DrawBuffer->Visual.doubleBufferMode)
+            if (att[BUFFER_FRONT_LEFT].Renderbuffer)
+               mask |= BUFFER_BIT_FRONT_LEFT;
       if (att[BUFFER_BACK_LEFT].Renderbuffer)
          mask |= BUFFER_BIT_BACK_LEFT;
       if (att[BUFFER_BACK_RIGHT].Renderbuffer)
diff --git a/src/mesa/main/context.c b/src/mesa/main/context.c
index c30031eced9..574c0fb7cec 100644
--- a/src/mesa/main/context.c
+++ b/src/mesa/main/context.c
@@ -1667,6 +1667,16 @@ _mesa_make_current( struct gl_context *newCtx,
          }
          if (!newCtx->ReadBuffer || _mesa_is_winsys_fbo(newCtx->ReadBuffer)) {
             _mesa_reference_framebuffer(&newCtx->ReadBuffer, readBuffer);
+            /* In _mesa_initialize_window_framebuffer, for single-buffered
+             * visuals, the ColorReadBuffer is set to be GL_FRONT, even with
+             * GLES contexts. When calling read_buffer, we verify we are reading
+             * from GL_BACK in is_legal_es3_readbuffer_enum.  But the default is
+             * incorrect, and certain dEQP tests check this.  So fix it here.
+             */
+            if (_mesa_is_gles(newCtx) &&
+               !newCtx->ReadBuffer->Visual.doubleBufferMode)
+               if (newCtx->ReadBuffer->ColorReadBuffer == GL_FRONT)
+                  newCtx->ReadBuffer->ColorReadBuffer = GL_BACK;
          }
 
          /* XXX only set this flag if we're really changing the draw/read