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

[Janusz Krzysztofik]

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.

> 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

> >   }
> >   
>