Re: [PATCH v2 2/3] drm/panthor: Revisit reqs_lock handling in flush/reset paths

[Boris Brezillon <boris.brezillon@collabora.com>]

Hello Nicolas,

On Thu, 30 Jul 2026 13:45:15 +0200
Nicolas Frattaroli <nicolas.frattaroli@collabora.com> wrote:

> panthor_gpu_flush_caches() and panthor_gpu_soft_reset() would read (and
> even reset) the contents of the pending_reqs register outside of holding
> the reqs_lock.

Can you elaborate a bit on the race being fixed here? If pending_reqs bits
are truly cleared before the wake_up_all() call (which would require a
WRITE_ONCE() to be enforced, admittedly), there's no risk for the
wait_event() call to do a test before the bits have been updated,
and this holds even if the test is done without the lock held.

The other race I could think of is two threads calling
panthor_gpu_flush_caches() concurrently, and the second one stealing
the FLUSH_COMPLETED event the first thread waits on and re-issuing a
second flush on top, thus delaying the completion for the first thread.
But that should be covered by the cache_flush_lock.


> Additionally, when it did hold the lock, it did so with
> the irqsave/irqrestore variants, even though the spinlock was never
> acquired in an atomic context, just the threaded handler.
> 
> Use the new wait_event_lock_timeout() macro to check pending_reqs under
> the lock, and only do so without disabling interrupts.
> 
> Fixes: 5cd894e258c4 ("drm/panthor: Add the GPU logical block")
> Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
> ---
>  drivers/gpu/drm/panthor/panthor_gpu.c | 25 +++++++++++--------------
>  1 file changed, 11 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_gpu.c b/drivers/gpu/drm/panthor/panthor_gpu.c
> index c013d6bf9a59..f015bde80abf 100644
> --- a/drivers/gpu/drm/panthor/panthor_gpu.c
> +++ b/drivers/gpu/drm/panthor/panthor_gpu.c
> @@ -330,35 +330,34 @@ int panthor_gpu_flush_caches(struct panthor_device *ptdev,
>  			     u32 l2, u32 lsc, u32 other)
>  {
>  	struct panthor_gpu *gpu = ptdev->gpu;
> -	unsigned long flags;
>  	int ret = 0;
>  
>  	/* Serialize cache flush operations. */
>  	guard(mutex)(&ptdev->gpu->cache_flush_lock);
>  
> -	spin_lock_irqsave(&ptdev->gpu->reqs_lock, flags);
> +	spin_lock(&ptdev->gpu->reqs_lock);

Can we make the _irq{save,restore}-drop its own patch?

>  	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)
> +	if (ret) {
> +		spin_unlock(&ptdev->gpu->reqs_lock);
>  		return ret;
> +	}
>  
> -	if (!wait_event_timeout(ptdev->gpu->reqs_acked,
> +	if (!wait_event_lock_timeout(ptdev->gpu->reqs_acked,
>  				!(ptdev->gpu->pending_reqs & GPU_IRQ_CLEAN_CACHES_COMPLETED),

Assuming we really need to do the test with the lock held, could we add
a patch at the beginning of the series that fixes the race without depending
on the new wait macro, so that we have a version that can easily be backported?

> -				msecs_to_jiffies(100))) {
> -		spin_lock_irqsave(&ptdev->gpu->reqs_lock, flags);
> +				ptdev->gpu->reqs_lock, msecs_to_jiffies(100))) {
>  		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);
>  	}
> +	spin_unlock(&ptdev->gpu->reqs_lock);

I think a scoped_guard() could make things a bit cleaner, and given you
already turn the regular lock/unlock sequence into a guard in
panthor_gpu_soft_reset(), I'd do that here as well.

Regards,

Boris

[1]https://elixir.bootlin.com/linux/v7.2-rc5/source/drivers/gpu/drm/panthor/panthor_gpu.c#L114