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@xxxxxxxxx> > > > --- > > > 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(>->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(>->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(>->reset.mutex); > > > for_each_engine_masked(engine, gt, awake, tmp) { > > > if (wait_for_invalidate(engine)) > > > -- > > > 2.41.0 > > >
