Re: [PATCH] drm/i915: Rework GPU reset sequence to match driver load & thaw

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Jul 30, 2014 at 04:59:33PM +0000, Mcaulay, Alistair wrote:
> Hi Daniel,
> 
> could you please be clearer on the change you mean.  I think you mean something functionally equivalent to the code below, but done in a less hacky way.
> (This slight change has made no change to test results)
> Or is the idea to return at a different point to this?
> I couldn't find " dev_priv->mm.reload_in_reset or similar" in the code. The only thing I can find is error->reset_counter,
> which is used in check_wedge(). Bottom bit set means RESET_IN_PROGRESS, top bit means WEDGED

Well I've meant that you have to add a new dev_prive->mm.realod_in_reset.
And the below won't work since in all other places but when doing a gpu
reset we want the -EAGAIN to reach callers. Actually it's really important
that if we have an -EGAIN we don't eat it.

And I guess the check for mm.reload_in_reset should actually be in
gem_check_wedged.
-Daniel

> 
> 
>  --- a/drivers/gpu/drm/i915/intel_ringbuffer.c
>  +++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
>  @@ -1832,7 +1832,9 @@ int intel_ring_begin(struct intel_engine_cs 
>  *ring,
>   
>   	ret = i915_gem_check_wedge(&dev_priv->gpu_error,
>   				   dev_priv->mm.interruptible);
>  -	if (ret)
>  +
>  +	/* -EAGAIN means a reset is in progress, it is Ok to return */
>  +	if (ret == -EAGAIN)
>  + 		return 0;
>  + 	if (ret)
>  +		return ret;
>   
>   	ret = __intel_ring_prepare(ring, num_dwords * sizeof(uint32_t));
> 
> Alistair.
> 
> -----Original Message-----
> From: Intel-gfx [mailto:intel-gfx-bounces@lists.freedesktop.org] On Behalf Of Daniel Vetter
> Sent: Tuesday, July 29, 2014 11:33 AM
> To: Chris Wilson; Daniel Vetter; Ben Widawsky; intel-gfx@lists.freedesktop.org
> Subject: Re: [Intel-gfx] [PATCH] drm/i915: Rework GPU reset sequence to match driver load & thaw
> 
> On Tue, Jul 29, 2014 at 08:36:33AM +0100, Chris Wilson wrote:
> > On Mon, Jul 28, 2014 at 11:26:38AM +0200, Daniel Vetter wrote:
> > > Oh, I guess that's the tricky bit why the old approach never worked 
> > > - because reset_in_progress is set we failed the context/ppgtt 
> > > loading through the rings and screwed up.
> > > 
> > > Problem with your approach is that we want to bail out here if a 
> > > reset is in progress, so we can't just eat the EAGAIN. If we do that 
> > > we potentially deadlock or overflow the ring.
> > > 
> > > I think we need a different hack here, and a few layers down (i.e. 
> > > at the place where we actually generate that offending -EAGAIN).
> > > 
> > > - Around the re-init sequence in the reset function we set
> > >   dev_priv->mm.reload_in_reset or similar
> 
> . Since we hold dev->struct_mutex
> > >   no one will see that, as long as we never leak it out of the critical
> > >   section.
> > > 
> > > - In the ring_begin code that checks for gpu hangs we ignore
> > >   reset_in_progress if this bit is set.
> > > 
> > > - Both places need fairly big comments to explain what exactly is going
> > >   on.
> > 
> > This is going from bad to worse. I think you can do better if you 
> > looked at the problem afresh.
> 
> Well we can't really reset reset_in_progress at that point, since not all reset is done yet. Especially the modeset stuff. So I don't think that reordering the reset sequence would get us out of this ugly spot. And I don't see any other solution really. Do you?
> -Daniel
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> +41 (0) 79 365 57 48 - http://blog.ffwll.ch
> _______________________________________________
> Intel-gfx mailing list
> Intel-gfx@lists.freedesktop.org
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch