Re: [PATCH v4 2/3] drm/panthor: Revisit reqs_lock handling in flush/reset paths

[Liviu Dudau <liviu.dudau@arm.com>]

On Wed, Aug 19, 2026 at 04:37:27PM +0100, Steven Price wrote:
> On 12/08/2026 15:07, Nicolas Frattaroli wrote:
> > panthor_gpu_flush_caches() and panthor_gpu_soft_reset() acquire their
> > reqs_lock spinlock with the IRQ-disabling variants of the spinlocking
> > functions. This isn't necessary, as the lock is never taken from an
> > atomic context, as Panthor uses threaded interrupt handlers. The result
> > of this overly strict locking is that IRQs may be disabled more
> > frequently and for longer than they should be, resulting in increased
> > system latency.
> > 
> > Switch the locking to use non-IRQ-disabling scoped_guard statements for
> > locking. The wait_event_timeout read of pending_reqs outside of the
> > spinlock is fine as wait_event_timeout is a memory barrier according to
> > the Linux Memory Model.
> > 
> > Fixes: 5cd894e258c4 ("drm/panthor: Add the GPU logical block")
> > Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
> > Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
> 
> Reviewed-by: Steven Price <steven.price@arm.com>
> 
> Although one minor formatting nit below.
> 
> > ---
> >  drivers/gpu/drm/panthor/panthor_gpu.c | 72 ++++++++++++++++-------------------
> >  1 file changed, 33 insertions(+), 39 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/panthor/panthor_gpu.c b/drivers/gpu/drm/panthor/panthor_gpu.c
> > index 7088371c6d64..55e33f145b40 100644
> > --- a/drivers/gpu/drm/panthor/panthor_gpu.c
> > +++ b/drivers/gpu/drm/panthor/panthor_gpu.c
> > @@ -345,41 +345,36 @@ int panthor_gpu_flush_caches(struct panthor_device *ptdev,
> >  			     u32 l2, u32 lsc, u32 other)
> >  {
> >  	struct panthor_gpu *gpu = ptdev->gpu;
> > -	unsigned long flags;
> >  	u64 start = 0;
> >  	int ret = 0;
> >  
> >  	/* Serialize cache flush operations. */
> >  	guard(mutex)(&ptdev->gpu->cache_flush_lock);
> >  
> > -	spin_lock_irqsave(&ptdev->gpu->reqs_lock, flags);
> > -
> > -	if (tracepoint_enabled(gpu_cache_flush))
> > -		start = ktime_get_ns();
> > -
> > -	if (!(ptdev->gpu->pending_reqs & GPU_IRQ_CLEAN_CACHES_COMPLETED)) {
> > -		ptdev->gpu->pending_reqs |= GPU_IRQ_CLEAN_CACHES_COMPLETED;
> > -		gpu_write(gpu->iomem, GPU_CMD, GPU_FLUSH_CACHES(l2, lsc, other));
> > -	} else {
> > -		ret = -EIO;
> > -	}
> > -	spin_unlock_irqrestore(&ptdev->gpu->reqs_lock, flags);
> > -
> > -	if (ret) {
> > -		panthor_gpu_emit_flush_caches_tp(ptdev, start, l2, lsc, other, ret);
> > -		return ret;
> > +	scoped_guard(spinlock, &ptdev->gpu->reqs_lock) {
> > +		if (tracepoint_enabled(gpu_cache_flush))
> > +			start = ktime_get_ns();
> > +
> > +		if (!(ptdev->gpu->pending_reqs & GPU_IRQ_CLEAN_CACHES_COMPLETED)) {
> > +			ptdev->gpu->pending_reqs |= GPU_IRQ_CLEAN_CACHES_COMPLETED;
> > +			gpu_write(gpu->iomem, GPU_CMD, GPU_FLUSH_CACHES(l2, lsc, other));
> > +		} else {
> > +			panthor_gpu_emit_flush_caches_tp(ptdev, start, l2, lsc,
> > +							 other, -EIO);
> > +			return -EIO;
> > +		}
> >  	}
> >  
> >  	if (!wait_event_timeout(ptdev->gpu->reqs_acked,
> >  				!(ptdev->gpu->pending_reqs & GPU_IRQ_CLEAN_CACHES_COMPLETED),
> >  				msecs_to_jiffies(100))) {
> > -		spin_lock_irqsave(&ptdev->gpu->reqs_lock, flags);
> > -		if ((ptdev->gpu->pending_reqs & GPU_IRQ_CLEAN_CACHES_COMPLETED) != 0 &&
> > -		    !(gpu_read(gpu->irq.iomem, INT_RAWSTAT) & GPU_IRQ_CLEAN_CACHES_COMPLETED))
> > -			ret = -ETIMEDOUT;
> > -		else
> > -			ptdev->gpu->pending_reqs &= ~GPU_IRQ_CLEAN_CACHES_COMPLETED;
> > -		spin_unlock_irqrestore(&ptdev->gpu->reqs_lock, flags);
> > +		scoped_guard(spinlock, &ptdev->gpu->reqs_lock) {
> > +			if ((ptdev->gpu->pending_reqs & GPU_IRQ_CLEAN_CACHES_COMPLETED) != 0 &&
> > +			!(gpu_read(gpu->irq.iomem, INT_RAWSTAT) & GPU_IRQ_CLEAN_CACHES_COMPLETED))
> 
> NIT: This isn't aligned correctly with the if() above any more.

I've fixed this locally before pushing it to drm-misc-next.

Best regards,
Liviu

> 
> To be honest what we really need here is a helper for this sequence as
> there's basically the same code again in panthor_gpu_soft_reset() below.
> 
> > +				ret = -ETIMEDOUT;
> > +			else
> > +				ptdev->gpu->pending_reqs &= ~GPU_IRQ_CLEAN_CACHES_COMPLETED;
> > +		}
> >  	}
> >  
> >  	panthor_gpu_emit_flush_caches_tp(ptdev, start, l2, lsc, other, ret);
> > @@ -402,27 +397,26 @@ int panthor_gpu_soft_reset(struct panthor_device *ptdev)
> >  {
> >  	struct panthor_gpu *gpu = ptdev->gpu;
> >  	bool timedout = false;
> > -	unsigned long flags;
> >  
> > -	spin_lock_irqsave(&ptdev->gpu->reqs_lock, flags);
> > -	if (!drm_WARN_ON(&ptdev->base,
> > -			 ptdev->gpu->pending_reqs & GPU_IRQ_RESET_COMPLETED)) {
> > -		ptdev->gpu->pending_reqs |= GPU_IRQ_RESET_COMPLETED;
> > -		gpu_write(gpu->irq.iomem, INT_CLEAR, GPU_IRQ_RESET_COMPLETED);
> > -		gpu_write(gpu->iomem, GPU_CMD, GPU_SOFT_RESET);
> > +	scoped_guard(spinlock, &ptdev->gpu->reqs_lock) {
> > +		if (!drm_WARN_ON(&ptdev->base,
> > +				ptdev->gpu->pending_reqs & GPU_IRQ_RESET_COMPLETED)) {
> > +			ptdev->gpu->pending_reqs |= GPU_IRQ_RESET_COMPLETED;
> > +			gpu_write(gpu->irq.iomem, INT_CLEAR, GPU_IRQ_RESET_COMPLETED);
> > +			gpu_write(gpu->iomem, GPU_CMD, GPU_SOFT_RESET);
> > +		}
> >  	}
> > -	spin_unlock_irqrestore(&ptdev->gpu->reqs_lock, flags);
> >  
> >  	if (!wait_event_timeout(ptdev->gpu->reqs_acked,
> >  				!(ptdev->gpu->pending_reqs & GPU_IRQ_RESET_COMPLETED),
> >  				msecs_to_jiffies(100))) {
> > -		spin_lock_irqsave(&ptdev->gpu->reqs_lock, flags);
> > -		if ((ptdev->gpu->pending_reqs & GPU_IRQ_RESET_COMPLETED) != 0 &&
> > -		    !(gpu_read(gpu->irq.iomem, INT_RAWSTAT) & GPU_IRQ_RESET_COMPLETED))
> > -			timedout = true;
> > -		else
> > -			ptdev->gpu->pending_reqs &= ~GPU_IRQ_RESET_COMPLETED;
> > -		spin_unlock_irqrestore(&ptdev->gpu->reqs_lock, flags);
> > +		scoped_guard(spinlock, &ptdev->gpu->reqs_lock) {
> > +			if ((ptdev->gpu->pending_reqs & GPU_IRQ_RESET_COMPLETED) != 0 &&
> > +			!(gpu_read(gpu->irq.iomem, INT_RAWSTAT) & GPU_IRQ_RESET_COMPLETED))
> 
> NIT: Same issue here.
> 
> Thanks,
> Steve
> 
> > +				timedout = true;
> > +			else
> > +				ptdev->gpu->pending_reqs &= ~GPU_IRQ_RESET_COMPLETED;
> > +		}
> >  	}
> >  
> >  	if (timedout) {
> > 
> 

-- 
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