Re: [PATCH] drm/i915: read HEAD register back in init_ring_common() to enforce ordering

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Aug 07, 2014 at 04:29:53PM +0200, Jiri Kosina wrote:
> Withtout this, ring initialization fails reliabily during resume with
> 
> 	[drm:init_ring_common] *ERROR* render ring initialization failed ctl 0001f001 head ffffff8804 tail 00000000 start 000e4000
> 
> This is not a complete fix, but it is verified to make the ring 
> initialization failures during resume much less likely.
> 
> We were not able to root-cause this bug (likely HW-specific to Gen4 chips) 
> yet. This is therefore used as a ducttape before problem is fully 
> understood and proper fix created, so that people don't suffer from 
> completely unusable systems in the meantime.
> 
> The discussion and debugging is happening at
> 
> 	https://bugs.freedesktop.org/show_bug.cgi?id=76554
> 
> Signed-off-by: Jiri Kosina <jkosina@suse.cz>

Ok, picked this one here, has the nicer commit message ;-)
-Daniel

> ---
>  drivers/gpu/drm/i915/intel_ringbuffer.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
> index 279488a..7add7ee 100644
> --- a/drivers/gpu/drm/i915/intel_ringbuffer.c
> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
> @@ -517,6 +517,9 @@ static int init_ring_common(struct intel_engine_cs *ring)
>  	else
>  		ring_setup_phys_status_page(ring);
>  
> +	/* Enforce ordering by reading HEAD register back */
> +	I915_READ_HEAD(ring);
> +
>  	/* Initialize the ring. This must happen _after_ we've cleared the ring
>  	 * registers with the above sequence (the readback of the HEAD registers
>  	 * also enforces ordering), otherwise the hw might lose the new ring
> -- 
> Jiri Kosina
> SUSE Labs

-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch