Re: [Intel-gfx] [PATCH v2] drm/i915: Reduce MCR lock surface

[Rodrigo Vivi <rodrigo.vivi@intel.com>]

On Wed, Oct 04, 2023 at 03:54:59PM +0200, Nirmoy Das wrote:
> Hi Rodrigo,
> 
> On 10/4/2023 2:44 PM, Rodrigo Vivi wrote:
> > On Wed, Oct 04, 2023 at 02:04:07PM +0200, Nirmoy Das wrote:
> > > Take the mcr lock only when driver needs to write into a mcr based
> > > tlb based registers.
> > > 
> > > To prevent GT reset interference, employ gt->reset.mutex instead, since
> > > intel_gt_mcr_multicast_write relies on gt->uncore->lock not being held.
> > This looks a lot like protecting code and not protecting data [1]
> > 
> > But to be really honest I'm afraid we were already doing this before
> > this patch but with 2 other locks instead.
> 
> I haven't thought about that but yes, the issue was there already.
> 
> 
> > 
> > [1] - https://blog.ffwll.ch/2022/07/locking-engineering.html
> > 
> > > v2: remove unused var, flags.
> > > 
> > > Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
> > > ---
> > >   drivers/gpu/drm/i915/gt/intel_tlb.c | 13 +++++--------
> > >   1 file changed, 5 insertions(+), 8 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/gt/intel_tlb.c b/drivers/gpu/drm/i915/gt/intel_tlb.c
> > > index 139608c30d97..0ad905df4a98 100644
> > > --- a/drivers/gpu/drm/i915/gt/intel_tlb.c
> > > +++ b/drivers/gpu/drm/i915/gt/intel_tlb.c
> > > @@ -52,15 +52,13 @@ static void mmio_invalidate_full(struct intel_gt *gt)
> > >   	struct intel_engine_cs *engine;
> > >   	intel_engine_mask_t awake, tmp;
> > >   	enum intel_engine_id id;
> > > -	unsigned long flags;
> > >   	if (GRAPHICS_VER(i915) < 8)
> > >   		return;
> > >   	intel_uncore_forcewake_get(uncore, FORCEWAKE_ALL);
> > > -	intel_gt_mcr_lock(gt, &flags);
> > > -	spin_lock(&uncore->lock); /* serialise invalidate with GT reset */
> > > +	mutex_lock(&gt->reset.mutex);/* serialise invalidate with GT reset */
> > I'm still looking at this and the commit message above and trying to understand
> > why we are doing this and changing the previous 2 by this other one. why?
> 
> 
> We need the MCR lock only for intel_gt_mcr_multicast_*() so I am not
> replacing the two locks here but moving the mcr lock down
> 
> where we were doing intel_gt_mcr_multicast_write_fw()
> 
> 
> why s/spin_lock(&uncore->lock)/mutex_lock(&gt->reset.mutex):
> 
> intel_gt_mcr_multicast_*() expects gt->uncore->lock to be not held

is there any lockdep assert or primitive that we could/should do
that to avoid this same issue in the future?
anyway, this is also another thing that it is important for the
commit message.

and why is that? what I have in mind goes along with the comment
above intel_de_read_fw():
"""
Access to registers should
 * therefore generally be serialised, by either the dev_priv->uncore.lock or
"""

> and to
> achieve this, I could do something like:
> 
> if (engine->tlb_inv.mcr) {
> 
>      spin_unlock(&uncore->lock);
> 
>      intel_gt_mcr_lock(gt, &flags);
> 
>      intel_gt_mcr_multicast_write_fw
> 
>      intel_gt_mcr_unlock(gt, flags);
> 
>     spin_lock(&uncore->lock);
> 
> }
> 
> Or take gt->reset.mutex instead which should block any concurrent gt reset.
> 
> If this is not acceptable then I can pick the above 1st option but I am not
> sure how safe is it do release uncore->lock and then take it back again.

hmm... probably the gt_reset one is better than releasing and grabbing it
again.

> 
> > 
> > >   	awake = 0;
> > >   	for_each_engine(engine, gt, id) {
> > > @@ -68,9 +66,9 @@ static void mmio_invalidate_full(struct intel_gt *gt)
> > >   			continue;
> > >   		if (engine->tlb_inv.mcr)
> > > -			intel_gt_mcr_multicast_write_fw(gt,
> > > -							engine->tlb_inv.reg.mcr_reg,
> > > -							engine->tlb_inv.request);
> > > +			intel_gt_mcr_multicast_write(gt,
> > > +						     engine->tlb_inv.reg.mcr_reg,
> > > +						     engine->tlb_inv.request);
> > you are already taking the forcewake_all domain above, so you wouldn't
> > need to convert this to the variant that grabs the forcewake underneath.
> > 
> > Also this is not mentioned in the commit message above.
> 
> intel_gt_mcr_multicast_write() takes the mcr lock for us, helps replacing multiple lines into one.
> Will there be any side-effects for that ?

hmm... I can't forsee side-effects here... but I'm asking myself why on the non
MCR ones we are using the global forcewake_all and the _fw to start with.
Maybe there was a reason for that? Because in general we should prefer the non _fw
variants to start with. Maybe we should dig into the history there to understand
why the line below started with the intel_uncore_write_fw below?

> 
> I should've added that the commit message.

I'm even wondering if this should be 2 separated patches?!

> 
> Regards,
> Nirmoy
> 
> 
> > 
> > >   		else
> > >   			intel_uncore_write_fw(uncore,
> > >   					      engine->tlb_inv.reg.reg,
> > > @@ -90,8 +88,7 @@ static void mmio_invalidate_full(struct intel_gt *gt)
> > >   	     IS_ALDERLAKE_P(i915)))
> > >   		intel_uncore_write_fw(uncore, GEN12_OA_TLB_INV_CR, 1);
> > > -	spin_unlock(&uncore->lock);
> > > -	intel_gt_mcr_unlock(gt, flags);
> > > +	mutex_unlock(&gt->reset.mutex);
> > >   	for_each_engine_masked(engine, gt, awake, tmp) {
> > >   		if (wait_for_invalidate(engine))
> > > -- 
> > > 2.41.0
> > >