Re: [PATCH] RFT drm/i915/execlists: Flush memory before signaling ELSQ

[Mika Kuoppala <mika.kuoppala@linux.intel.com>]

Chris Wilson <chris@chris-wilson.co.uk> writes:

> We observe that the ordering of writes for a CS event is not as strong
> from the GPU as we would like, and that on occasions we see the
> ringbuffer tail updated before the event is written into the ringbuffer,
> leading us to reuse the stale data.
>
> Through around a big hammer to try and batter ELSQ into submission with
> the presumption that perhaps the UC mmio write is not flushing our
> writes into the context images.
>
> References: https://bugs.freedesktop.org/show_bug.cgi?id=108315
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>  drivers/gpu/drm/i915/intel_lrc.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> index 22b57b8926fc..ba61849fbb9b 100644
> --- a/drivers/gpu/drm/i915/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/intel_lrc.c
> @@ -454,8 +454,10 @@ static void execlists_submit_ports(struct intel_engine_cs *engine)
>  	}
>  
>  	/* we need to manually load the submit queue */
> -	if (execlists->ctrl_reg)
> +	if (execlists->ctrl_reg) {
> +		wmb(); /* XXX Big hammer or paper? XXX */
>  		writel(EL_CTRL_LOAD, execlists->ctrl_reg);

Will be difficult to know if it is the additional delay or
not.

The question is then why would it only be with ctrl_reg and
not legacy submission mode. So lets try with legacy mode?

-Mika

> +	}
>  
>  	execlists_clear_active(execlists, EXECLISTS_ACTIVE_HWACK);
>  }
> -- 
> 2.19.1
>
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