[PATCH 4/5] drm/panthor: Be robust against resume failures

[Steven Price]

On 13/11/2024 15:42, Boris Brezillon wrote:
> When the runtime PM resume callback returns an error, it puts the device
> in a state where it can't be resumed anymore. Make sure we can recover
> from such transient failures by calling pm_runtime_set_suspended()
> explicitly after a pm_runtime_resume_and_get() failure.
> 
> Signed-off-by: Boris Brezillon <boris.brezillon at collabora.com>
> ---
>  drivers/gpu/drm/panthor/panthor_device.c |  1 +
>  drivers/gpu/drm/panthor/panthor_device.h | 17 +++++++++++++++++
>  drivers/gpu/drm/panthor/panthor_drv.c    |  2 +-
>  drivers/gpu/drm/panthor/panthor_sched.c  |  4 ++--
>  4 files changed, 21 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_device.c b/drivers/gpu/drm/panthor/panthor_device.c
> index 353f3aabef42..d3276b936141 100644
> --- a/drivers/gpu/drm/panthor/panthor_device.c
> +++ b/drivers/gpu/drm/panthor/panthor_device.c
> @@ -486,6 +486,7 @@ int panthor_device_resume(struct device *dev)
>  
>  err_set_suspended:
>  	atomic_set(&ptdev->pm.state, PANTHOR_DEVICE_PM_STATE_SUSPENDED);
> +	atomic_set(&ptdev->pm.recovery_needed, 1);
>  	return ret;
>  }
>  
> diff --git a/drivers/gpu/drm/panthor/panthor_device.h b/drivers/gpu/drm/panthor/panthor_device.h
> index 0e68f5a70d20..cc74e99e53f9 100644
> --- a/drivers/gpu/drm/panthor/panthor_device.h
> +++ b/drivers/gpu/drm/panthor/panthor_device.h
> @@ -9,6 +9,7 @@
>  #include <linux/atomic.h>
>  #include <linux/io-pgtable.h>
>  #include <linux/regulator/consumer.h>
> +#include <linux/pm_runtime.h>
>  #include <linux/sched.h>
>  #include <linux/spinlock.h>
>  
> @@ -180,6 +181,9 @@ struct panthor_device {
>  		 * is suspended.
>  		 */
>  		struct page *dummy_latest_flush;
> +
> +		/** @recovery_needed: True when a resume attempt failed. */
> +		atomic_t recovery_needed;
>  	} pm;
>  
>  	/** @profile_mask: User-set profiling flags for job accounting. */
> @@ -243,6 +247,19 @@ int panthor_device_mmap_io(struct panthor_device *ptdev,
>  int panthor_device_resume(struct device *dev);
>  int panthor_device_suspend(struct device *dev);
>  
> +static inline int panthor_device_resume_and_get(struct panthor_device *ptdev)
> +{
> +	int ret = pm_runtime_resume_and_get(ptdev->base.dev);
> +
> +	/* If the resume failed, we need to clear the runtime_error, which we
> +	 * can done by forcing the RPM state to suspended.
> +	 */
> +	if (ret && atomic_cmpxchg(&ptdev->pm.recovery_needed, 1, 0) == 1)

I'm unclear what this atomic achieves. At first glance it appears
pointless: with this change if panthor_device_resume() fails then
recovery_needed is set to 1. So logically if ret != 0 then also
recovery_needed == 1.

My second thought was is this to avoid races? If multiple threads are
calling this then only one will win the cmpxchg and call
pm_runtime_set_suspended. But it's not safe - it's quite possible for
the first thread to get past the cmpxchg and be suspended before the
second thread comes along and reaches the same point - leading to
multiple calls to pm_runtime_set_suspended().

So I'm afraid I don't understand this atomic - what am I missing?

Thanks,
Steve

> +		pm_runtime_set_suspended(ptdev->base.dev);
> +
> +	return ret;
> +}
> +
>  enum drm_panthor_exception_type {
>  	DRM_PANTHOR_EXCEPTION_OK = 0x00,
>  	DRM_PANTHOR_EXCEPTION_TERMINATED = 0x04,
> diff --git a/drivers/gpu/drm/panthor/panthor_drv.c b/drivers/gpu/drm/panthor/panthor_drv.c
> index 1498c97b4b85..b7a9adc918e3 100644
> --- a/drivers/gpu/drm/panthor/panthor_drv.c
> +++ b/drivers/gpu/drm/panthor/panthor_drv.c
> @@ -763,7 +763,7 @@ static int panthor_query_timestamp_info(struct panthor_device *ptdev,
>  {
>  	int ret;
>  
> -	ret = pm_runtime_resume_and_get(ptdev->base.dev);
> +	ret = panthor_device_resume_and_get(ptdev);
>  	if (ret)
>  		return ret;
>  
> diff --git a/drivers/gpu/drm/panthor/panthor_sched.c b/drivers/gpu/drm/panthor/panthor_sched.c
> index 97ed5fe5a191..77b184c3fb0c 100644
> --- a/drivers/gpu/drm/panthor/panthor_sched.c
> +++ b/drivers/gpu/drm/panthor/panthor_sched.c
> @@ -2364,7 +2364,7 @@ static void tick_work(struct work_struct *work)
>  	if (!drm_dev_enter(&ptdev->base, &cookie))
>  		return;
>  
> -	ret = pm_runtime_resume_and_get(ptdev->base.dev);
> +	ret = panthor_device_resume_and_get(ptdev);
>  	if (drm_WARN_ON(&ptdev->base, ret))
>  		goto out_dev_exit;
>  
> @@ -3131,7 +3131,7 @@ queue_run_job(struct drm_sched_job *sched_job)
>  		return dma_fence_get(job->done_fence);
>  	}
>  
> -	ret = pm_runtime_resume_and_get(ptdev->base.dev);
> +	ret = panthor_device_resume_and_get(ptdev);
>  	if (drm_WARN_ON(&ptdev->base, ret))
>  		return ERR_PTR(ret);
>