[Intel-gfx] [PATCH] drm/i915: Fix possible overflow when recording semaphore states.

[Damien Lespiau]

On Fri, Jul 18, 2014 at 02:05:16AM -0700, Rodrigo Vivi wrote:
> semaphore _sync_seqno, _seqno and _mbox are smaller than number of rings.
> This optimization is to remove the ring itself from the list and the logic to do that
> is at intel_ring_sync_index as below:
> 
> /*
>  * rcs -> 0 = vcs, 1 = bcs, 2 = vecs, 3 = vcs2;
>  * vcs -> 0 = bcs, 1 = vecs, 2 = vcs2, 3 = rcs;
>  * bcs -> 0 = vecs, 1 = vcs2. 2 = rcs, 3 = vcs;
>  * vecs -> 0 = vcs2, 1 = rcs, 2 = vcs, 3 = bcs;
>  * vcs2 -> 0 = rcs, 1 = vcs, 2 = bcs, 3 = vecs;
> */
> 
> v2: Skip when from == to (Damien).
> v3: avoid computing idx when from == to (Damien).
>     use ring == to instead of ring->id == to->id (Damien).
>     use continue instead of return (Rodrigo).
> 
> Cc: Damien Lespiau <damien.lespiau at intel.com>
> Cc: Ben Widawsky <benjamin.widawsky at intel.com>
> Signed-off-by: Rodrigo Vivi <rodrigo.vivi at intel.com>

I guess there are still some details that look weird:

  - idx's scope could be reduced to the loop
  - there's still some code that is executed on the skipped iteration
    that doesn't need to be.

But I believe we shouldn't overflow now at least, so, whether you can be
bothered with those last 2 comments or not:

Reviewed-by: Damien Lespiau <damien.lespiau at intel.com>

-- 
Damien

> ---
>  drivers/gpu/drm/i915/i915_gpu_error.c | 15 ++++++++++-----
>  1 file changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gpu_error.c b/drivers/gpu/drm/i915/i915_gpu_error.c
> index 9faebbc..1efcf1f 100644
> --- a/drivers/gpu/drm/i915/i915_gpu_error.c
> +++ b/drivers/gpu/drm/i915/i915_gpu_error.c
> @@ -764,8 +764,8 @@ static void gen8_record_semaphore_state(struct drm_i915_private *dev_priv,
>  					struct intel_engine_cs *ring,
>  					struct drm_i915_error_ring *ering)
>  {
> -	struct intel_engine_cs *useless;
> -	int i;
> +	struct intel_engine_cs *to;
> +	int i, idx;
>  
>  	if (!i915_semaphore_is_enabled(dev_priv->dev))
>  		return;
> @@ -776,13 +776,18 @@ static void gen8_record_semaphore_state(struct drm_i915_private *dev_priv,
>  						 dev_priv->semaphore_obj,
>  						 &dev_priv->gtt.base);
>  
> -	for_each_ring(useless, dev_priv, i) {
> +	for_each_ring(to, dev_priv, i) {
>  		u16 signal_offset =
>  			(GEN8_SIGNAL_OFFSET(ring, i) & PAGE_MASK) / 4;
>  		u32 *tmp = error->semaphore_obj->pages[0];
>  
> -		ering->semaphore_mboxes[i] = tmp[signal_offset];
> -		ering->semaphore_seqno[i] = ring->semaphore.sync_seqno[i];
> +		if (ring == to)
> +			continue;
> +
> +		idx = intel_ring_sync_index(ring, to);
> +
> +		ering->semaphore_mboxes[idx] = tmp[signal_offset];
> +		ering->semaphore_seqno[idx] = ring->semaphore.sync_seqno[idx];
>  	}
>  }
>  
> -- 
> 1.9.3
>