Re: [PATCH v4 18/18] drm/panthor: Add debugfs knobs to simulate reset failures

[Adrian Larumbe]

I've tried poking the knob that fakes an error in the reset path and got two different kinds of oputput:

Either
```
[ 1950.080236] panthor fb000000.gpu: [drm] *ERROR* Failed to boot MCU after reset, making device unusable.
[ 1951.167384] panthor fb000000.gpu: [drm] Timed out waiting for MCU to halt
[ 1951.167543] panthor fb000000.gpu: [drm] Failed to cleanly suspend MCU
```
or just

```
[ 2022.804466] panthor fb000000.gpu: [drm] *ERROR* Failed to boot MCU after reset, making device unusable.
```

I guess the former happens when after device unplug, there are still inflight jobs that keep a PM reference.

On 26.08.2026 16:56, Boris Brezillon wrote:
> It's almost impossible to trigger a situation where the reset
> doesn't work now that the driver is more mature, so let's add two
> knobs to exercise this error path:
> 
> - a knob to trigger a reset
> - a knob to fake an error in the reset path
> 
> Signed-off-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>
> ---
>  drivers/gpu/drm/panthor/panthor_device.c | 48 +++++++++++++++++++++++++++++++-
>  drivers/gpu/drm/panthor/panthor_device.h |  8 ++++++
>  2 files changed, 55 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_device.c b/drivers/gpu/drm/panthor/panthor_device.c
> index 328e601d80e8..8bdc511310c0 100644
> --- a/drivers/gpu/drm/panthor/panthor_device.c
> +++ b/drivers/gpu/drm/panthor/panthor_device.c
> @@ -5,6 +5,7 @@
>  /* Copyright 2025 ARM Limited. All rights reserved. */
>  
>  #include <linux/clk.h>
> +#include <linux/debugfs.h>
>  #include <linux/mm.h>
>  #include <linux/platform_device.h>
>  #include <linux/pm_domain.h>
> @@ -182,7 +183,10 @@ static void panthor_device_reset_work(struct work_struct *work)
>  	panthor_hw_soft_reset(ptdev);
>  	panthor_hw_l2_power_on(ptdev);
>  	panthor_mmu_post_reset(ptdev);
> -	ret = panthor_fw_post_reset(ptdev);
> +	if (ptdev->reset.fake_failure)
> +		ret = -EIO;
> +	else
> +		ret = panthor_fw_post_reset(ptdev);
>  	atomic_set(&ptdev->reset.pending, 0);
>  	panthor_sched_post_reset(ptdev, ret != 0);
>  	drm_dev_exit(cookie);
> @@ -690,8 +694,50 @@ int panthor_device_suspend(struct device *dev)
>  }
>  
>  #ifdef CONFIG_DEBUG_FS
> +static int panthor_device_fake_fw_reset_failure_get(void *data, u64 *val)
> +{
> +	struct panthor_device *ptdev = data;
> +
> +	*val = ptdev->reset.fake_failure ? 1 : 0;
> +	return 0;
> +}
> +
> +static int panthor_device_fake_fw_reset_failure_set(void *data, u64 val)
> +{
> +	struct panthor_device *ptdev = data;
> +
> +	ptdev->reset.fake_failure = val ? true : false;
> +	return 0;
> +}
> +
> +DEFINE_DEBUGFS_ATTRIBUTE(panthor_device_fake_fw_reset_failure_fops,
> +			 panthor_device_fake_fw_reset_failure_get,
> +			 panthor_device_fake_fw_reset_failure_set, "%llu\n");
> +
> +static ssize_t panthor_device_reset_file_write(struct file *file,
> +					       const char __user *, size_t size,
> +					       loff_t *)
> +{
> +	struct panthor_device *ptdev = file_inode(file)->i_private;
> +
> +	panthor_device_schedule_reset(ptdev);
> +	return size;
> +}
> +
> +static const struct debugfs_short_fops panthor_device_reset_fops = {
> +	.write = panthor_device_reset_file_write,
> +};
> +
>  void panthor_device_debugfs_init(struct drm_minor *minor)
>  {
> +	struct panthor_device *ptdev = container_of(minor->dev, struct panthor_device, base);
> +
> +	debugfs_create_file("fake_fw_reset_failure", 0644,
> +			    minor->debugfs_root, ptdev,
> +			    &panthor_device_fake_fw_reset_failure_fops);
> +	debugfs_create_file("reset", 0200,
> +			    minor->debugfs_root, ptdev,
> +			    &panthor_device_reset_fops);
>  	panthor_mmu_debugfs_init(minor);
>  	panthor_gem_debugfs_init(minor);
>  }
> diff --git a/drivers/gpu/drm/panthor/panthor_device.h b/drivers/gpu/drm/panthor/panthor_device.h
> index e12049961912..82ec34347eba 100644
> --- a/drivers/gpu/drm/panthor/panthor_device.h
> +++ b/drivers/gpu/drm/panthor/panthor_device.h
> @@ -294,6 +294,14 @@ struct panthor_device {
>  		 * all FW sections to make sure we start from a fresh state.
>  		 */
>  		bool fast;
> +
> +		/**
> +		 * @fake_failure: When true, pretend the FW boot in the reset path failed.
> +		 *
> +		 * This is important to check that we're doing the right thing in this very
> +		 * unlikely case.
> +		 */
> +		bool fake_failure;
>  	} reset;
>  
>  	/** @pm: Power management related data. */
> 
> -- 
> 2.55.0

Adrian Larumbe