Re: [PATCH] drm/i915: Click the ACTHD three times to go home

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

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

> One POSTING_READ of ACTHD may not be enough to ensure that the seqno
> write has been posted from the GPU and is now visible. So do three!
>

Is this about posting read triggering or just adding enough delay
that the write gets through?

-Mika

> References: https://bugs.freedesktop.org/show_bug.cgi?id=97557
> References: https://bugs.freedesktop.org/show_bug.cgi?id=100007
> References: https://bugs.freedesktop.org/show_bug.cgi?id=100052
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>  drivers/gpu/drm/i915/intel_ringbuffer.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
> index ab5e2e0623b7..2b5cd6039a6e 100644
> --- a/drivers/gpu/drm/i915/intel_ringbuffer.c
> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
> @@ -766,6 +766,7 @@ static void
>  gen6_seqno_barrier(struct intel_engine_cs *engine)
>  {
>  	struct drm_i915_private *dev_priv = engine->i915;
> +	int loop;
>  
>  	/* Workaround to force correct ordering between irq and seqno writes on
>  	 * ivb (and maybe also on snb) by reading from a CS register (like
> @@ -783,7 +784,8 @@ gen6_seqno_barrier(struct intel_engine_cs *engine)
>  	 * take the spinlock to guard against concurrent cacheline access.
>  	 */
>  	spin_lock_irq(&dev_priv->uncore.lock);
> -	POSTING_READ_FW(RING_ACTHD(engine->mmio_base));
> +	for (loop = 0; loop < 3; loop++)
> +		POSTING_READ_FW(RING_ACTHD(engine->mmio_base));
>  	spin_unlock_irq(&dev_priv->uncore.lock);
>  }
>  
> -- 
> 2.11.0
>
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