Re: [PATCH 3/6] drm/i915: Update ring position from request on retiring

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

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

> When wedged, we do not update the ring->tail as we submit the requests
> causing us to leak the ring->space upon cleaning up the wedged driver.
> We can just use the value stored in rq->tail, and keep the submission
> backend details away from set-wedge.
>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
> ---
>  drivers/gpu/drm/i915/i915_request.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
> index efa9ee557f31..69b378a323fc 100644
> --- a/drivers/gpu/drm/i915/i915_request.c
> +++ b/drivers/gpu/drm/i915/i915_request.c
> @@ -358,7 +358,7 @@ static void advance_ring(struct i915_request *request)
>  		 * is just about to be. Either works, if we miss the last two
>  		 * noops - they are safe to be replayed on a reset.
>  		 */
> -		tail = READ_ONCE(request->ring->tail);
> +		tail = READ_ONCE(request->tail);

I tried to think if we need the READ_ONCE here anymore.

But as this is the safest version,
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>

Noticed that request->tail is not cleared on i915_request_alloc.

If we would set rq->head = rq->tail = rq->ring->emit
we could use rq->head == rq->tail to assert that
we screw up something major during the request lifetime.

-Mika


>  	} else {
>  		tail = request->postfix;
>  	}
> -- 
> 2.16.2
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