Re: [PATCH v5 09/11] drm/panfrost: Introduce a reset lock

[Boris Brezillon]

On Tue, 11 Aug 2026 22:42:18 +0100
Adrián Larumbe <adrian.larumbe@xxxxxxxxxxxxx> wrote:

> So as to avoid accessing HW register while a reset is ongoing, a read/write
> semaphore that envelopes the reset sequence will help driver entry points
> avoid racing with it. For now, the only such racy entry point is the ioctl
> that returns the current GPU timestmap.
> 
> Signed-off-by: Adrián Larumbe <adrian.larumbe@xxxxxxxxxxxxx>

Reviewed-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>

> ---
>  drivers/gpu/drm/panfrost/panfrost_device.c | 2 ++
>  drivers/gpu/drm/panfrost/panfrost_device.h | 1 +
>  drivers/gpu/drm/panfrost/panfrost_drv.c    | 9 ++++++---
>  drivers/gpu/drm/panfrost/panfrost_job.c    | 1 +
>  4 files changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panfrost/panfrost_device.c b/drivers/gpu/drm/panfrost/panfrost_device.c
> index 5b66173c75b9..e0390b6c0d22 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_device.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_device.c
> @@ -602,6 +602,8 @@ bool panfrost_exception_needs_reset(const struct panfrost_device *pfdev,
>  
>  void panfrost_device_reset(struct panfrost_device *pfdev, bool enable_job_int)
>  {
> +	guard(rwsem_read)(&pfdev->reset.lock);
> +
>  	panfrost_gpu_soft_reset(pfdev);
>  	panfrost_gpu_power_on(pfdev);
>  	panfrost_mmu_reset(pfdev);
> diff --git a/drivers/gpu/drm/panfrost/panfrost_device.h b/drivers/gpu/drm/panfrost/panfrost_device.h
> index 4bbaaaf827a5..a2a68e042225 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_device.h
> +++ b/drivers/gpu/drm/panfrost/panfrost_device.h
> @@ -166,6 +166,7 @@ struct panfrost_device {
>  	struct {
>  		struct workqueue_struct *wq;
>  		struct work_struct work;
> +		struct rw_semaphore lock;
>  		atomic_t pending;
>  	} reset;
>  
> diff --git a/drivers/gpu/drm/panfrost/panfrost_drv.c b/drivers/gpu/drm/panfrost/panfrost_drv.c
> index 958f1d36ab10..ff23b1a979bb 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_drv.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_drv.c
> @@ -41,9 +41,12 @@ static int panfrost_ioctl_query_timestamp(struct panfrost_device *pfdev,
>  	if (ret)
>  		return ret;
>  
> -	panfrost_cycle_counter_get(pfdev);
> -	*arg = panfrost_timestamp_read(pfdev);
> -	panfrost_cycle_counter_put(pfdev);
> +	/* We should not read timestamp register while the GPU  is being reset */
> +	scoped_guard(rwsem_read, &pfdev->reset.lock) {
> +		panfrost_cycle_counter_get(pfdev);
> +		*arg = panfrost_timestamp_read(pfdev);
> +		panfrost_cycle_counter_put(pfdev);
> +	}
>  
>  	pm_runtime_put(pfdev->base.dev);
>  	return 0;
> diff --git a/drivers/gpu/drm/panfrost/panfrost_job.c b/drivers/gpu/drm/panfrost/panfrost_job.c
> index c761379851da..9d7dafa29f19 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_job.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_job.c
> @@ -874,6 +874,7 @@ int panfrost_jm_init(struct panfrost_device *pfdev)
>  
>  	INIT_WORK(&pfdev->reset.work, panfrost_reset_work);
>  	spin_lock_init(&js->job_lock);
> +	init_rwsem(&pfdev->reset.lock);
>  
>  	js->irq = platform_get_irq_byname(to_platform_device(pfdev->base.dev), "job");
>  	if (js->irq < 0)
>