Re: [PATCH 7/9] drm/i915/execlists: Stop storing the CSB read pointer in the mmio register

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 27/06/2018 22:07, Chris Wilson wrote:
> As we now never read back our current head position from the CSB
> pointers register, and the HW itself doesn't use it to prevent
> overwriting unread CSB entries, we do not need to keep updating the
> register. As it turns out this register is not listed as being shadowed,
> and so requires forcewake -- but we haven't been taking forcewake around
> it so the writes has probably been regularly dropped. Fortuitously, we
> only read the value after a reset where it did not matter, and zero was
> the right answer (well, close enough).
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
> ---
>   drivers/gpu/drm/i915/intel_lrc.c | 2 --
>   1 file changed, 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> index 8b111a268697..a6268103663f 100644
> --- a/drivers/gpu/drm/i915/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/intel_lrc.c
> @@ -1100,8 +1100,6 @@ static void process_csb(struct intel_engine_cs *engine)
>   		}
>   	} while (head != tail);
>   
> -	writel(_MASKED_FIELD(GEN8_CSB_READ_PTR_MASK, head << 8),
> -	       execlists->csb_read);
>   	execlists->csb_head = head;
>   }
>   
> 

Proof is in the pudding. :)

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

Regards,

Tvrtko
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