[Intel-gfx] [PATCH v9 63/70] drm/i915: Move gt_revoke() slightly

[Ville Syrjälä]

On Wed, Mar 24, 2021 at 06:00:12PM +0100, Daniel Vetter wrote:
> On Tue, Mar 23, 2021 at 04:50:52PM +0100, Maarten Lankhorst wrote:
> > We get a lockdep splat when the reset mutex is held, because it can be
> > taken from fence_wait. This conflicts with the mmu notifier we have,
> > because we recurse between reset mutex and mmap lock -> mmu notifier.
> > 
> > Remove this recursion by calling revoke_mmaps before taking the lock.
> > 
> > The reset code still needs fixing, as taking mmap locks during reset
> > is not allowed.
> > 
> > Signed-off-by: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> > Reviewed-by: Thomas Hellström <thomas.hellstrom at linux.intel.com>
> > ---
> >  drivers/gpu/drm/i915/gt/intel_reset.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/gt/intel_reset.c b/drivers/gpu/drm/i915/gt/intel_reset.c
> > index 990cb4adbb9a..447f589750c2 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_reset.c
> > +++ b/drivers/gpu/drm/i915/gt/intel_reset.c
> > @@ -970,8 +970,6 @@ static int do_reset(struct intel_gt *gt, intel_engine_mask_t stalled_mask)
> >  {
> >  	int err, i;
> >  
> > -	gt_revoke(gt);
> > -
> >  	err = __intel_gt_reset(gt, ALL_ENGINES);
> >  	for (i = 0; err && i < RESET_MAX_RETRIES; i++) {
> >  		msleep(10 * (i + 1));
> > @@ -1026,6 +1024,9 @@ void intel_gt_reset(struct intel_gt *gt,
> >  
> >  	might_sleep();
> >  	GEM_BUG_ON(!test_bit(I915_RESET_BACKOFF, &gt->reset.flags));
> > +
> 
> I've added a FIXME comment here just so we don't totally forget. This will
> also blow up again when we wrap the entire reset path into a dma_fence
> critical section annotation (at least going forward, we can't do that on
> hw that needs display reset with the current code unfortunately).
> 
> But I did look at the code which originally added this in
> 
> commit 2caffbf1176256cc4f8d4e5c3c524fc689cb9876
> Author: Chris Wilson <chris at chris-wilson.co.uk>
> Date:   Fri Feb 8 15:37:03 2019 +0000
> 
>     drm/i915: Revoke mmaps and prevent access to fence registers across reset
> 
> and noped right out.
> 
> I think this complexity needs to go entirely, and instead we just protect
> the fence register state to make sure that after reset they are all good
> again:
> - add a new mutex for low level fence register state
> - hold that mutex around fence register writes (really just the low level
>   fence writes)
> - hold it in the reset path when we restore fence registers
> 
> This means that a global reset also thrashes mmaps, but it's a global
> reset we're talking about here, everything is thrash anyway. Plus/minus
> fenced gtt mmaps really doesn't change the tally.

My recollection is that GPU reset doesn't actually clobber the fence 
registers. Though not 100% sure I can trust my brain on this. Also
dunno if it actually matter here or not, but figured I'd point it out.

-- 
Ville Syrjälä
Intel