[Mesa-dev] [PATCH RFC 1/6] i965: make BRW_COMPRESSION_2NDHALF valid for brw_SAMPLE

[Chia-I Wu]

From: Chia-I Wu <olv at lunarg.com>

SIMD8 sampler messages are allowed in SIMD16 mode, and they could not work
without BRW_COMPRESSION_2NDHALF.  Later PRMs (gen5 and later) do not
explicitly state whether BRW_COMPRESSION_2NDHALF is allowed, but they do have
examples using send with SecHalf.  It should be safe to assume SecHalf is
valid.

Signed-off-by: Chia-I Wu <olv at lunarg.com>
---
 src/mesa/drivers/dri/i965/brw_eu_emit.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/src/mesa/drivers/dri/i965/brw_eu_emit.c b/src/mesa/drivers/dri/i965/brw_eu_emit.c
index 7ed3df0..12515ec 100644
--- a/src/mesa/drivers/dri/i965/brw_eu_emit.c
+++ b/src/mesa/drivers/dri/i965/brw_eu_emit.c
@@ -2195,7 +2195,22 @@ void brw_SAMPLE(struct brw_compile *p,
 
    insn = next_insn(p, BRW_OPCODE_SEND);
    insn->header.predicate_control = 0; /* XXX */
-   insn->header.compression_control = BRW_COMPRESSION_NONE;
+
+   /* From the 965 PRM (volume 4, part 1, section 14.2.41):
+    *
+    *    "Instruction compression is not allowed for this instruction (that
+    *     is, send). The hardware behavior is undefined if this instruction is
+    *     set as compressed. However, compress control can be set to "SecHalf"
+    *     to affect the EMask generation."
+    *
+    * No similar wording is found in later PRMs, but there are examples
+    * utilizing send with SecHalf.  More importantly, SIMD8 sampler messages
+    * are allowed in SIMD16 mode and they could not work without SecHalf.  For
+    * these reasons, we allow BRW_COMPRESSION_2NDHALF here.
+    */
+   if (insn->header.compression_control != BRW_COMPRESSION_2NDHALF)
+      insn->header.compression_control = BRW_COMPRESSION_NONE;
+
    if (brw->gen < 6)
       insn->header.destreg__conditionalmod = msg_reg_nr;
 
-- 
1.8.3.1