Re: [PATCH] drm/i915/gt: Skip rmw for masked registers

[Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>]

On 30/01/2020 14:21, Chris Wilson wrote:
> A masked register does not need rmw to update, and it is best not to use
> such a sequence.
>
> Reported-by: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> ---
>   drivers/gpu/drm/i915/gt/intel_workarounds.c | 29 ++++++++++++++-------
>   1 file changed, 19 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/gt/intel_workarounds.c b/drivers/gpu/drm/i915/gt/intel_workarounds.c
> index 5a7db279f702..89474a4fa9b9 100644
> --- a/drivers/gpu/drm/i915/gt/intel_workarounds.c
> +++ b/drivers/gpu/drm/i915/gt/intel_workarounds.c
> @@ -167,12 +167,6 @@ wa_write_masked_or(struct i915_wa_list *wal, i915_reg_t reg, u32 mask,
>   	wa_add(wal, reg, mask, val, mask);
>   }
>   
> -static void
> -wa_masked_en(struct i915_wa_list *wal, i915_reg_t reg, u32 val)
> -{
> -	wa_write_masked_or(wal, reg, val, _MASKED_BIT_ENABLE(val));
> -}
> -
>   static void
>   wa_write(struct i915_wa_list *wal, i915_reg_t reg, u32 val)
>   {
> @@ -185,14 +179,26 @@ wa_write_or(struct i915_wa_list *wal, i915_reg_t reg, u32 val)
>   	wa_write_masked_or(wal, reg, val, val);
>   }
>   
> +static void
> +wa_masked_en(struct i915_wa_list *wal, i915_reg_t reg, u32 val)
> +{
> +	wa_add(wal, reg, 0, _MASKED_BIT_ENABLE(val), val);
> +}
> +
> +static void
> +wa_masked_dis(struct i915_wa_list *wal, i915_reg_t reg, u32 val)
> +{
> +	wa_add(wal, reg, 0, _MASKED_BIT_DISABLE(val), val);
> +}
> +
>   #define WA_SET_BIT_MASKED(addr, mask) \
> -	wa_write_masked_or(wal, (addr), (mask), _MASKED_BIT_ENABLE(mask))
> +	wa_masked_en(wal, (addr), mask)
>   
>   #define WA_CLR_BIT_MASKED(addr, mask) \
> -	wa_write_masked_or(wal, (addr), (mask), _MASKED_BIT_DISABLE(mask))
> +	wa_masked_dis(wal, (addr), mask)
>   
>   #define WA_SET_FIELD_MASKED(addr, mask, value) \
> -	wa_write_masked_or(wal, (addr), (mask), _MASKED_FIELD((mask), (value)))
> +	wa_write_masked_or(wal, (addr), 0, _MASKED_FIELD((mask), (value)))
>   
>   static void gen8_ctx_workarounds_init(struct intel_engine_cs *engine,
>   				      struct i915_wa_list *wal)
> @@ -1020,7 +1026,10 @@ wa_list_apply(struct intel_uncore *uncore, const struct i915_wa_list *wal)
>   	intel_uncore_forcewake_get__locked(uncore, fw);

In the realm of pointless we could also consider the rmw vs wr when 
calculating the required forcewake domains.

>   
>   	for (i = 0, wa = wal->list; i < wal->count; i++, wa++) {
> -		intel_uncore_rmw_fw(uncore, wa->reg, wa->mask, wa->val);
> +		if (wa->mask)
> +			intel_uncore_rmw_fw(uncore, wa->reg, wa->mask, wa->val);
> +		else
> +			intel_uncore_write(uncore, wa->reg, wa->val);
>   		if (IS_ENABLED(CONFIG_DRM_I915_DEBUG_GEM))
>   			wa_verify(wa,
>   				  intel_uncore_read_fw(uncore, wa->reg),

Looks correct.

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>

What were the real world consequences of getting it wrong?

Regards,

Tvrtko
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