[Intel-xe] [PATCH] drm/xe: fix tlb_invalidation_seqno_past()

[Matthew Brost]

On Fri, May 05, 2023 at 03:49:10PM +0100, Matthew Auld wrote:
> Checking seqno_recv >= seqno looks like it will incorrectly report true
> when the seqno has wrapped (not unlikely given
> TLB_INVALIDATION_SEQNO_MAX). Calling xe_gt_tlb_invalidation_wait() might
> then return before the flush has been completed by the GuC.
> 
> Fix this by treating a large negative delta as an indication that the
> seqno has wrapped around. Similar to how we treat a large positive delta
> as an indication that the seqno_recv must have wrapped around, but in
> that case the seqno has likely also signalled.
> 
> It looks like we could also potentially make the seqno use the full
> 32bits as supported by the GuC.

Yea we def could use more of the space but in the end we have the seqno
wrap issue. I think I set this to a low value to prove the wrapping
protection worked (it didn't) by triigering wraps more often than the
wrapping 32 bits.

With, this patch LGTM.

Reviewed-by: Matthew Brost <matthew.brost at intel.com>

> 
> Signed-off-by: Matthew Auld <matthew.auld at intel.com>
> Cc: Thomas Hellström <thomas.hellstrom at linux.intel.com>
> Cc: Matthew Brost <matthew.brost at intel.com>
> ---
>  drivers/gpu/drm/xe/xe_gt_tlb_invalidation.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_gt_tlb_invalidation.c b/drivers/gpu/drm/xe/xe_gt_tlb_invalidation.c
> index 604f189dbd70..67822b3dd353 100644
> --- a/drivers/gpu/drm/xe/xe_gt_tlb_invalidation.c
> +++ b/drivers/gpu/drm/xe/xe_gt_tlb_invalidation.c
> @@ -251,14 +251,15 @@ int xe_gt_tlb_invalidation_vma(struct xe_gt *gt,
>  
>  static bool tlb_invalidation_seqno_past(struct xe_gt *gt, int seqno)
>  {
> -	if (gt->tlb_invalidation.seqno_recv >= seqno)
> -		return true;
> +	if (seqno - gt->tlb_invalidation.seqno_recv <
> +	    -(TLB_INVALIDATION_SEQNO_MAX / 2))
> +		return false;
>  
>  	if (seqno - gt->tlb_invalidation.seqno_recv >
>  	    (TLB_INVALIDATION_SEQNO_MAX / 2))
>  		return true;
>  
> -	return false;
> +	return gt->tlb_invalidation.seqno_recv >= seqno;
>  }
>  
>  /**
> -- 
> 2.40.0
>