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

[Rodrigo Vivi]

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.

[1] - https://blog.ffwll.ch/2022/07/locking-engineering.html

> 
> v2: remove unused var, flags.
> 
> Signed-off-by: Nirmoy Das <nirmoy....@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?

>  
>       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.

>               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
>