Re: [PATCH v3 3/4] drm/atomic: Return user readable error in atomic_ioctl

[Jani Nikula <jani.nikula@linux.intel.com>]

On Fri, 22 Aug 2025, Arun R Murthy <arun.r.murthy@intel.com> wrote:
> Add user readable error codes for failure cases in drm_atomic_ioctl() so
> that user can decode the error code and take corrective measurements.
>
> Signed-off-by: Arun R Murthy <arun.r.murthy@intel.com>
> ---
>  drivers/gpu/drm/drm_atomic.c      |  6 ++++
>  drivers/gpu/drm/drm_atomic_uapi.c | 60 ++++++++++++++++++++++++++++++++-------
>  2 files changed, 56 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
> index cd15cf52f0c9144711da5879da57884674aea9e4..5f25e6d3cf6cf246f83a8c39450b410e97fe45bb 100644
> --- a/drivers/gpu/drm/drm_atomic.c
> +++ b/drivers/gpu/drm/drm_atomic.c
> @@ -1513,6 +1513,9 @@ int drm_atomic_check_only(struct drm_atomic_state *state)
>  			if (drm_atomic_crtc_needs_modeset(new_crtc_state)) {
>  				drm_dbg_atomic(dev, "[CRTC:%d:%s] requires full modeset\n",
>  					       crtc->base.id, crtc->name);
> +				state->error_code->failure_flags =
> +					DRM_MODE_ATOMIC_CRTC_NEED_FULL_MODESET;

It says flags, implying multiple, but you're just adding one there
anyway. Just like it was a regular enum.

> +
>  				return -EINVAL;
>  			}
>  		}
> @@ -1537,8 +1540,11 @@ int drm_atomic_check_only(struct drm_atomic_state *state)
>  		drm_dbg_atomic(dev,
>  			       "driver added CRTC to commit: requested 0x%x, affected 0x%0x\n",
>  			       requested_crtc, affected_crtc);
> +		state->error_code->failure_flags = DRM_MODE_ATOMIC_NEED_FULL_MODESET;
>  		WARN(!state->allow_modeset, "adding CRTC not allowed without modesets: requested 0x%x, affected 0x%0x\n",
>  		     requested_crtc, affected_crtc);
> +
> +		return -EINVAL;

This changes behaviour.

>  	}
>  
>  	return 0;
> diff --git a/drivers/gpu/drm/drm_atomic_uapi.c b/drivers/gpu/drm/drm_atomic_uapi.c
> index ecc73d52bfae41a7ef455a7e13649ec56c690b90..94eaf9c98eb4ac2455799f1416010d366e1b5bbc 100644
> --- a/drivers/gpu/drm/drm_atomic_uapi.c
> +++ b/drivers/gpu/drm/drm_atomic_uapi.c
> @@ -1058,6 +1058,9 @@ int drm_atomic_set_property(struct drm_atomic_state *state,
>  			ret = drm_atomic_crtc_get_property(crtc, crtc_state,
>  							   prop, &old_val);
>  			ret = drm_atomic_check_prop_changes(ret, old_val, prop_value, prop);
> +			if (ret)
> +				state->error_code->failure_flags =
> +					DRM_MODE_ATOMIC_ASYNC_PROP_CHANGED;
>  			break;
>  		}
>  
> @@ -1089,6 +1092,8 @@ int drm_atomic_set_property(struct drm_atomic_state *state,
>  
>  			/* ask the driver if this non-primary plane is supported */
>  			if (plane->type != DRM_PLANE_TYPE_PRIMARY) {
> +				state->error_code->failure_flags =
> +					DRM_MODE_ATOMIC_ASYNC_NOT_SUP_PLANE;
>  				ret = -EINVAL;
>  
>  				if (plane_funcs && plane_funcs->atomic_async_check)
> @@ -1380,6 +1385,13 @@ set_async_flip(struct drm_atomic_state *state)
>  	}
>  }
>  
> +#define FAILURE_REASON(flag, reason) #reason,
> +const char *drm_mode_atomic_failure_string[] = {
> +	DRM_MODE_ATOMIC_FAILURE_REASON
> +};
> +
> +#undef FAILURE_REASON
> +
>  int drm_mode_atomic_ioctl(struct drm_device *dev,
>  			  void *data, struct drm_file *file_priv)
>  {
> @@ -1389,9 +1401,11 @@ int drm_mode_atomic_ioctl(struct drm_device *dev,
>  	uint32_t __user *props_ptr = (uint32_t __user *)(unsigned long)(arg->props_ptr);
>  	uint64_t __user *prop_values_ptr = (uint64_t __user *)(unsigned long)(arg->prop_values_ptr);
>  	unsigned int copied_objs, copied_props;
> -	struct drm_atomic_state *state;
> +	struct drm_atomic_state *state = NULL;
>  	struct drm_modeset_acquire_ctx ctx;
>  	struct drm_out_fence_state *fence_state;
> +	struct drm_mode_atomic_err_code error_code;
> +	struct drm_mode_atomic_err_code __user *error_code_ptr;
>  	int ret = 0;
>  	unsigned int i, j, num_fences;
>  	bool async_flip = false;
> @@ -1400,6 +1414,11 @@ int drm_mode_atomic_ioctl(struct drm_device *dev,
>  	if (!drm_core_check_feature(dev, DRIVER_ATOMIC))
>  		return -EOPNOTSUPP;
>  
> +	if (!arg->reserved)
> +		drm_err(dev, "memory not allocated for drm_atomic error reporting\n");

This right here makes me suspect you never really tried this with your
regular desktop environment.

> +
> +	memset(&error_code, 0, sizeof(struct drm_mode_atomic_err_code));
> +
>  	/* disallow for userspace that has not enabled atomic cap (even
>  	 * though this may be a bit overkill, since legacy userspace
>  	 * wouldn't know how to call this ioctl)
> @@ -1407,24 +1426,25 @@ int drm_mode_atomic_ioctl(struct drm_device *dev,
>  	if (!file_priv->atomic) {
>  		drm_dbg_atomic(dev,
>  			       "commit failed: atomic cap not enabled\n");
> -		return -EINVAL;
> +		error_code.failure_flags = DRM_MODE_ATOMIC_CAP_NOT_ENABLED;
> +		ret = -EINVAL;
> +		goto out;
>  	}
>  
>  	if (arg->flags & ~DRM_MODE_ATOMIC_FLAGS) {
>  		drm_dbg_atomic(dev, "commit failed: invalid flag\n");
> -		return -EINVAL;
> -	}
> -
> -	if (arg->reserved) {
> -		drm_dbg_atomic(dev, "commit failed: reserved field set\n");
> -		return -EINVAL;
> +		error_code.failure_flags = DRM_MODE_ATOMIC_INVALID_FLAG;
> +		ret = -EINVAL;
> +		goto out;
>  	}
>  
>  	if (arg->flags & DRM_MODE_PAGE_FLIP_ASYNC) {
>  		if (!dev->mode_config.async_page_flip) {
>  			drm_dbg_atomic(dev,
>  				       "commit failed: DRM_MODE_PAGE_FLIP_ASYNC not supported\n");
> -			return -EINVAL;
> +			error_code.failure_flags = DRM_MODE_ATOMIC_PAGE_FLIP_ASYNC;
> +			ret = -EINVAL;
> +			goto out;
>  		}
>  
>  		async_flip = true;
> @@ -1435,7 +1455,9 @@ int drm_mode_atomic_ioctl(struct drm_device *dev,
>  			(arg->flags & DRM_MODE_PAGE_FLIP_EVENT)) {
>  		drm_dbg_atomic(dev,
>  			       "commit failed: page-flip event requested with test-only commit\n");
> -		return -EINVAL;
> +		error_code.failure_flags = DRM_MODE_ATOMIC_FLIP_EVENT_WITH_CHECKONLY;
> +		ret = -EINVAL;
> +		goto out;
>  	}
>  
>  	state = drm_atomic_state_alloc(dev);
> @@ -1446,6 +1468,8 @@ int drm_mode_atomic_ioctl(struct drm_device *dev,
>  	state->acquire_ctx = &ctx;
>  	state->allow_modeset = !!(arg->flags & DRM_MODE_ATOMIC_ALLOW_MODESET);
>  
> +	state->error_code = &error_code;
> +
>  retry:
>  	copied_objs = 0;
>  	copied_props = 0;
> @@ -1542,6 +1566,22 @@ int drm_mode_atomic_ioctl(struct drm_device *dev,
>  	}
>  
>  out:
> +	/* update the error code if any error to allow user handling it */
> +	if (ret < 0 && arg->reserved) {
> +		error_code_ptr = (struct drm_mode_atomic_err_code __user *)
> +				 (unsigned long)arg->reserved;
> +
> +		strscpy_pad(error_code.failure_string,
> +			    drm_mode_atomic_failure_string[error_code.failure_flags],
> +			    sizeof(error_code.failure_string));
> +
> +		if (copy_to_user(error_code_ptr, &error_code, sizeof(error_code)))
> +			return -EFAULT;
> +	}
> +
> +	if (!state)
> +		return ret;
> +
>  	complete_signaling(dev, state, fence_state, num_fences, !ret);
>  
>  	if (ret == -EDEADLK) {

-- 
Jani Nikula, Intel