[Intel-gfx] [PATCH v2 2/3] drm/i915: Setup MCR steering for RCS engine workarounds

[Daniele Ceraolo Spurio]

On 5/1/20 9:57 PM, Matt Roper wrote:
> Reads of multicast registers give the value associated with
> slice/subslice 0 by default unless we manually steer the reads to a
> different slice/subslice.  If slice/subslice 0 are fused off in hardware,
> performing unsteered reads of multicast registers will return a value of
> 0 rather than the value we wrote into the multicast register.
> 
> To ensure we can properly readback and verify workarounds that touch
> registers in a multicast range, we currently setup MCR steering to a
> known-valid slice/subslice as the very first item in the GT workaround
> list for gen10+.  That steering will then be in place as we verify the
> rest of the registers that show up in the GT workaround list, and at
> initialization the steering will also still be in effect when we move on
> to applying and verifying the workarounds in the RCS engine's workaround
> list (which is where most of the multicast registers actually show up).
> 
> However we seem run into problems during resets where RCS engine
> workarounds are applied without being preceded by application of the GT
> workaround list and the steering isn't in place.  Let's add the same MCR
> steering to the beginning of the RCS engine's workaround list to ensure
> that it's always in place and we don't get erroneous messages about RCS
> engine workarounds failing to apply.
> 

Did you check with the HW team about this behavior? AFAIU from reading 
the specs, the 0xfdc register should be nowhere close the render domain 
and therefore it should not be impacted by RCS reset. If it really is 
impacted, it might indicate that something funny is happening on the HW 
and there might be other register in the area that are also incorrectly 
losing their value, so IMO worth investigating.

Daniele

> References: https://gitlab.freedesktop.org/drm/intel/issues/1222
> Cc: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
> Cc: chris at chris-wilson.co.uk
> Signed-off-by: Matt Roper <matthew.d.roper at intel.com>
> ---
>   drivers/gpu/drm/i915/gt/intel_workarounds.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_workarounds.c b/drivers/gpu/drm/i915/gt/intel_workarounds.c
> index 4a255de13394..b11b83546696 100644
> --- a/drivers/gpu/drm/i915/gt/intel_workarounds.c
> +++ b/drivers/gpu/drm/i915/gt/intel_workarounds.c
> @@ -1345,6 +1345,9 @@ rcs_engine_wa_init(struct intel_engine_cs *engine, struct i915_wa_list *wal)
>   {
>   	struct drm_i915_private *i915 = engine->i915;
>   
> +	if (INTEL_GEN(i915) >= 10)
> +		wa_init_mcr(i915, wal);
> +
>   	if (IS_TGL_REVID(i915, TGL_REVID_A0, TGL_REVID_A0)) {
>   		/*
>   		 * Wa_1607138336:tgl
>