gallium: document dual source blending restrictions on gallium

As per Brian's suggestion, document the restrictions on dual src blending.

Signed-off-by: Dave Airlie <airlied@redhat.com>

diff --git a/src/gallium/docs/source/cso/blend.rst b/src/gallium/docs/source/cso/blend.rst
index 7bde10c124597e6b428cd76c5b5594b8e4c3c5c3..dce999c8327975c0f1aa328cafbce83d85bb25cc 100644 (file)
--- a/src/gallium/docs/source/cso/blend.rst
+++ b/src/gallium/docs/source/cso/blend.rst
@@ -12,7 +12,11 @@ Blend Factors
 The blend factors largely follow the same pattern as their counterparts
 in other modern and legacy drawing APIs.
 
-XXX blurb about dual-source blends
+Dual source blend factors are supported for up to 1 MRT, although
+you can advertise > 1 MRT, the stack cannot handle them for a few reasons.
+There is no definition on how the 1D array of shader outputs should be mapped
+to something that would be a 2D array (location, index). No current hardware
+exposes > 1 MRT, and we should revisit this issue if anyone ever does.
 
 Logical Operations
 ------------------