Re: [PATCH] drm/i915: Handle sync_seqno correctly when seqno has wrapped.

[Ben Widawsky <ben@bwidawsk.net>]

On Tue, 13 Nov 2012 15:51:36 +0200
Mika Kuoppala <mika.kuoppala@linux.intel.com> wrote:

> If seqno has wrapped, normal compare operation will give wrong results.
> i915_seqno_passed can handle the wrap so use it instead.
> 
> Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
> ---
>  drivers/gpu/drm/i915/i915_gem.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index cdcf19d..35d5db8 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -2138,7 +2138,7 @@ i915_gem_retire_requests_ring(struct intel_ring_buffer *ring)
>  	seqno = ring->get_seqno(ring, true);
>  
>  	for (i = 0; i < ARRAY_SIZE(ring->sync_seqno); i++)
> -		if (seqno >= ring->sync_seqno[i])
> +		if (i915_seqno_passed(seqno, ring->sync_seqno[i]))
>  			ring->sync_seqno[i] = 0;
>  
>  	while (!list_empty(&ring->request_list)) {
> @@ -2376,7 +2376,7 @@ i915_gem_object_sync(struct drm_i915_gem_object *obj,
>  	idx = intel_ring_sync_index(from, to);
>  
>  	seqno = obj->last_read_seqno;
> -	if (seqno <= from->sync_seqno[idx])
> +	if (i915_seqno_passed(from->sync_seqno[idx], seqno))
>  		return 0;
>  
>  	ret = i915_gem_check_olr(obj->ring, seqno);

Can you add another patch on top of this to have the starting seqno be a
near pre-wrap value so we can catch bugs even without igt.

Also as an overall comment, I want the patches to guarantee to catch
the bug you found, which I think with the randomness of
gem_stress - isn't. Specifically, we want the waiting ring to be
waiting on a pre-wrapped value. Maybe I missed that guarantee, but if
there is a quick/dirty way to make that happen, that would better than
running an arbitrary number of gem_stress tests.

-- 
Ben Widawsky, Intel Open Source Technology Center