i965: Use 64-byte offset alignment for shader storage buffers

This should be a cacheline (64 bytes) so that we can safely have the
CPU and GPU writing the same SSBO on non-cachecoherent systems (our
Atom CPUs). With UBOs, the GPU never writes, so there's no
problem. For an SSBO, the GPU and the CPU can be updating disjoint
regions of the buffer simultaneously and that will break if the
regions overlap the same cacheline.

v2:
- Use cacheline size (64 bytes) instead of 16 bytes (Kristian).
- Update commit log and add a comment in the code explaining
  why we use cacheline size (Ben).

Reviewed-by: Jordan Justen <[email protected]>
Reviewed-by: Kristian Høgsberg <[email protected]>

1 files changed, 9 insertions, 0 deletions

diff --git a/src/mesa/drivers/dri/i965/brw_context.c b/src/mesa/drivers/dri/i965/brw_context.c
index 7c1c13300dc..0cfc8435964 100644
--- a/src/mesa/drivers/dri/i965/brw_context.c
+++ b/src/mesa/drivers/dri/i965/brw_context.c
@@ -567,6 +567,15 @@ brw_initialize_context_constants(struct brw_context *brw)
     * However, unaligned accesses are slower, so enforce buffer alignment.
     */
    ctx->Const.UniformBufferOffsetAlignment = 16;
+
+   /* ShaderStorageBufferOffsetAlignment should be a cacheline (64 bytes) so
+    * that we can safely have the CPU and GPU writing the same SSBO on
+    * non-cachecoherent systems (our Atom CPUs). With UBOs, the GPU never
+    * writes, so there's no problem. For an SSBO, the GPU and the CPU can
+    * be updating disjoint regions of the buffer simultaneously and that will
+    * break if the regions overlap the same cacheline.
+    */
+   ctx->Const.ShaderStorageBufferOffsetAlignment = 64;
    ctx->Const.TextureBufferOffsetAlignment = 16;
    ctx->Const.MaxTextureBufferSize = 128 * 1024 * 1024;