[Mesa-dev] [PATCH] i965: Correct constant buffer MOCS
kenneth at whitecape.org
Tue Jun 2 20:32:13 PDT 2015
On Tuesday, June 02, 2015 08:07:50 PM Ben Widawsky wrote:
> I'm very confused here. It seems pretty clear that since the command has been
> introduced with support for MOCS, MOCS lives at bit 8 of dword 0 for all
> constant buffers. The error has existed since forever AFAICT.
> No piglit regressions or fixes:
> I suspect the low bits are discarded by the hardware when using the buffer
> offset (we were setting 1). I suppose it could potentially impact perf, though
> I'd imagine the kernel is already doing the right thing by default anyway. This
> would explain why the patch doesn't really do anything behaviorally.
> NOTE: gen8+ should have no change at all since MOCS was always 0 anyway.
> Signed-off-by: Ben Widawsky <ben at bwidawsk.net>
> src/mesa/drivers/dri/i965/gen7_vs_state.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
> diff --git a/src/mesa/drivers/dri/i965/gen7_vs_state.c b/src/mesa/drivers/dri/i965/gen7_vs_state.c
> index 278b3ec..f60dcb1 100644
> --- a/src/mesa/drivers/dri/i965/gen7_vs_state.c
> +++ b/src/mesa/drivers/dri/i965/gen7_vs_state.c
> @@ -42,13 +42,13 @@ gen7_upload_constant_state(struct brw_context *brw,
> int dwords = brw->gen >= 8 ? 11 : 7;
> - OUT_BATCH(opcode << 16 | (dwords - 2));
> + OUT_BATCH(opcode << 16 | (mocs << 8) | (dwords - 2));
> OUT_BATCH(active ? stage_state->push_const_size : 0);
> /* Pointer to the constant buffer. Covered by the set of state flags
> * from gen6_prepare_wm_contants
> - OUT_BATCH(active ? (stage_state->push_const_offset | mocs) : 0);
> + OUT_BATCH(active ? stage_state->push_const_offset : 0);
Now I'm confused.
On BDW, DW0 bits 14:8 are MOCS, but "must always be programmed to zero."
On pre-BDW, DW0 bits 14:8 are clearly marked as "Reserved MBZ".
The documentation for 3DSTATE_CONSTANT(Body) lists DWord 2 (of the body,
aka DWord 3 of the 3DSTATE_CONSTANT_VS packet as a whole) bits 4:0 as
"Constant Buffer Object Control State", for all pre-DevBDW platforms,
which is MOCS for all four buffers.
So the code seems correct as it was, based on the versions of
documentation I've read. You must be looking at something different
than I am...
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