[Mesa-dev] [PATCH] Make single-buffered GLES representation internally consistent

[Gurchetan Singh]

There 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
---
 src/mesa/main/buffers.c | 14 ++++++++++++--
 src/mesa/main/clear.c   |  8 ++++++++
 src/mesa/main/get.c     |  9 +++++++++
 3 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/src/mesa/main/buffers.c b/src/mesa/main/buffers.c
index e8aedde..86696b8 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 35b912c..a1bb36e 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/get.c b/src/mesa/main/get.c
index 6ffa99c..ec6bc3e 100644
--- a/src/mesa/main/get.c
+++ b/src/mesa/main/get.c
@@ -627,6 +627,15 @@ find_custom_value(struct gl_context *ctx, const struct value_desc *d, union valu
       break;
 
    case GL_READ_BUFFER:
+      /* 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(ctx))
+         if (ctx->ReadBuffer->ColorReadBuffer == GL_FRONT)
+            ctx->ReadBuffer->ColorReadBuffer = GL_BACK;
       v->value_enum = ctx->ReadBuffer->ColorReadBuffer;
       break;
 
-- 
2.1.2