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

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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(&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 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.


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 ?

I should've added that the commit message.

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




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