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