[PATCH v2 2/2] drm/i915: Never return 0 if not all requests retired

[Janusz Krzysztofik]

Hi Andrzej,

Thanks for your comment.

On Monday, 21 November 2022 11:17:42 CET Andrzej Hajda wrote:
> 
> On 21.11.2022 09:30, Janusz Krzysztofik wrote:
> > Hi Nimroy,
> >
> > Thanks for looking at this.
> >
> > On Friday, 18 November 2022 20:56:50 CET Das, Nirmoy wrote:
> >> On 11/18/2022 11:42 AM, Janusz Krzysztofik wrote:
> >>> Users of intel_gt_retire_requests_timeout() expect 0 return value on
> >>> success.  However, we have no protection from passing back 0 potentially
> >>> returned by a call to dma_fence_wait_timeout() when it succedes right
> >>> after its timeout has expired.
> >>>
> >>> Replace 0 with -ETIME before potentially using the timeout value as return
> >>> code, so -ETIME is returned if there are still some requests not retired
> >>> after timeout, 0 otherwise.
> >>>
> >>> v2: Move the added lines down so flush_submission() is not affected.
> >>>
> >>> Fixes: f33a8a51602c ("drm/i915: Merge wait_for_timelines with
> > retire_request")
> >>> Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik at linux.intel.com>
> >>> Cc: stable at vger.kernel.org # v5.5+
> >>> ---
> >>>    drivers/gpu/drm/i915/gt/intel_gt_requests.c | 3 +++
> >>>    1 file changed, 3 insertions(+)
> >>>
> >>> diff --git a/drivers/gpu/drm/i915/gt/intel_gt_requests.c b/drivers/gpu/
> > drm/i915/gt/intel_gt_requests.c
> >>> index edb881d756309..3ac4603eeb4ee 100644
> >>> --- a/drivers/gpu/drm/i915/gt/intel_gt_requests.c
> >>> +++ b/drivers/gpu/drm/i915/gt/intel_gt_requests.c
> >>> @@ -199,6 +199,9 @@ out_active:	spin_lock(&timelines->lock);
> >>>    	if (remaining_timeout)
> >>>    		*remaining_timeout = timeout;
> >>>    
> >>> +	if (!timeout)
> >>> +		timeout = -ETIME;
> >> This will return error, -ETIME when 0 timeout is passed,
> >> intel_gt_retire_requests().
> > Yes, but only when active_count is not 0 after we loop through
> > timelines->active_list calling retire_requests() on each and counting up
> > failures in active_count.
> 
> Moving this line just after the call to dma_fence_wait_timeout should 
> solve the controversy.

But that would break our need to pass 0, not -ETIME, to flush_submission() in 
case the initial value of timeout was 0, as pointed out by Chris during our 
discussion on v2.

Maybe an inline comment above the added lines that explains why we are doing 
this could help?

Thanks,
Janusz

> 
> Regards
> Andrzej
> 
> >
> >> We don't want that.
> > When 0 timeout is passed to intel_gt_retire_requests(), do we really want it
> > to return 0 unconditionally, or are we rather interested if those calls to
> > retire_requests() succeeded?
> >
> >> I think you can use a separate variable to store
> >> return val from the dma_fence_wait_timeout()
> >>
> >>
> >> Regards,
> >>
> >> Nirmoy
> >>
> >>> +
> >>>    	return active_count ? timeout : 0;
> > If active count is 0, we return 0 regardless of timeout value, and that's OK.
> > However, if active_count is not 0, we shouldn't return 0, I believe, we should
> > return either remaining time if some left, or error (-ETIME) if not.  If you
> > think I'm wrong, please explain why.
> >
> > Thanks,
> > Janusz
> >
> >>>    }
> >>>    
> >
> >
> >
> 
>