[Mesa-dev] [PATCH 01/13] i965: Factor out logic to build a send message instruction with indirect descriptor.

[Matt Turner]

On Wed, Mar 11, 2015 at 1:07 PM, Kenneth Graunke <kenneth at whitecape.org> wrote:
> On Wednesday, March 11, 2015 07:25:14 PM Francisco Jerez wrote:
>> "Pohjolainen, Topi" <topi.pohjolainen at intel.com> writes:
>> > On Fri, Feb 27, 2015 at 05:34:44PM +0200, Francisco Jerez wrote:
>> >> @@ -1218,17 +1198,6 @@ fs_generator::generate_varying_pull_constant_load_gen7(fs_inst *inst,
>> >>                                false /* header */,
>> >>                                simd_mode,
>> >>                                0);
>> >> -      brw_inst_set_exec_size(p->brw, insn_or, BRW_EXECUTE_1);
>> >> -      brw_inst_set_src1_reg_type(p->brw, insn_or, BRW_REGISTER_TYPE_UD);
>> >> -      brw_set_src0(p, insn_or, addr);
>> >> -      brw_set_dest(p, insn_or, addr);
>> >> -
>> >> -
>> >> -      /* dst = send(offset, a0.0) */
>> >> -      brw_inst *insn_send = brw_next_insn(p, BRW_OPCODE_SEND);
>> >> -      brw_set_dest(p, insn_send, retype(dst, BRW_REGISTER_TYPE_UW));
>> >
>> > I'm just reading this through again and noticed that the destination type
>> > changes here to BRW_REGISTER_TYPE_UD (set in brw_send_indirect_message()).
>>
>> Ah, yes, that change is intentional.  The type being set to UW was a
>> remnant from Gen4-5 times -- Those used to require the destination type
>> of SEND to be W/UW when doing 16-wide (even if the message was actually
>> writing dwords back...).  None of the codepaths modified in this patch
>> (or in the rest of the series) should be executed on Gen4 or 5.
>>
>> Anyway good catch. :)
>
> I'm pretty sure this does still apply on Gen7 - notably, BLORP uses UW
> destinations for SIMD16 sends, and I believe Paul Berry tracked actual
> issues down to that.  BLORP only exists on Gen6+.
>
> I also seem to recall Chris Forbes hitting an issue relating to that
> which he and Paul fixed during XDC 2013.
>
> CC'ing Chris and Matt in case they remember any details.

I believe he was helping Abdiel track down a hang when using the
resource streamer, and it turned out that using UW typed-destinations
on sends in blorp fixed it:

commit 4c4934636cb286e7d7836afc26e9d392e2f0f155
Author: Paul Berry <stereotype441 at gmail.com>
Date:   Tue Sep 24 15:18:52 2013 -0700

    i965/blorp: retype destination register for texture SEND instruction to UW.

The resource streamer only exists on HSW+, so the UW dest is certainly
needed for things after Gen5.