[Mesa-dev] [PATCH] i965/fs: Follow pow(16) instructions with a NOP.

[Francisco Jerez]

Matt Turner <mattst88 at gmail.com> writes:

> Beginning with commit 7b208a73, Unigine Valley began hanging the GPU on
> Gen >= 8 platforms. This patch avoids the GPU hangs, but does not
> implement a full work around for the restriction (dispatch_width == 16
> is an imperfect proxy).
>
Sounds like for a proper fix we could just check the last instruction in
the assembly at the top of the generator loop and emit a NOP in cases
where the previous instruction is one of the affected math opcodes?

> Evidently that commit allowed the scheduler to make different choices
> that somehow finally ran afoul of a hardware bug in which POW and FDIV
> instructions may not be followed by an instruction with two destination
> registers (including compressed instructions). I presume the conditions
> are more complex than that, but the internal hardware bug report (BDWGFX
> bug_de 1696294) does not contain much more information.
>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94924
> ---
>  src/mesa/drivers/dri/i965/brw_fs_generator.cpp | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_fs_generator.cpp b/src/mesa/drivers/dri/i965/brw_fs_generator.cpp
> index 8654ca4..2b3544b 100644
> --- a/src/mesa/drivers/dri/i965/brw_fs_generator.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_fs_generator.cpp
> @@ -2086,6 +2086,19 @@ fs_generator::generate_code(const cfg_t *cfg, int dispatch_width)
>           assert(inst->conditional_mod == BRW_CONDITIONAL_NONE);
>  	 if (devinfo->gen >= 7 && inst->opcode == SHADER_OPCODE_POW) {
>              gen6_math(p, dst, brw_math_function(inst->opcode), src[0], src[1]);
> +
> +            /* From the Broadwell PRM, Volume 7, "3D-Media-GPGPU", in the
> +             * "Register Region Restrictions" section: for BDW, SKL:
> +             *
> +             *    "A POW/FDIV operation must not be followed by an instruction
> +             *     that requires two destination registers."
> +             *
> +             * The documentation is often lacking annotations for Atom parts,
> +             * and empirically this affects CHV as well.
> +             */
> +            if (devinfo->gen >= 8 && dispatch_width == 16) {
> +               brw_NOP(p);
> +            }
>  	 } else if (devinfo->gen >= 6) {
>  	    generate_math_gen6(inst, dst, src[0], src[1]);
>  	 } else {
> -- 
> 2.7.3
>
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