Re: [PATCH 1/4] drm/debugfs: Add support for dynamic debugfs initialization

[Daniel Vetter]

On Mon, Aug 27, 2018 at 08:36:24PM -0400, Lyude Paul wrote:
> Currently all debugfs related initialization for the DRM core happens in
> drm_debugfs_init(), which is called when registering the minor device.
> While this works fine for features such as atomic modesetting and GEM,
> this doesn't work at all for resources like DP MST topology managers
> which can potentially be created both before and after the minor
> device has been registered.
> 
> So, in order to add driver-wide debugfs files for MST we'll need to be
> able to handle debugfs initialization for such resources. We do so by
> introducing drm_debugfs_callback_register() and
> drm_debugfs_callback_unregister(). These functions allow driver-agnostic
> parts of DRM to add additional debugfs initialization callbacks at any
> point during a DRM driver's lifetime.
> 
> Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
> Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
> Cc: Daniel Stone <daniel@xxxxxxxxxxxxx>

So the debugfs lifetime rules are indeed silly and broken. I'm not sure
this is what we want to do though:

- Thanks to Noralf's cleanup you don't need a debugfs cleanup callback
  anymore really, it will auto-clean-up on device unregistration.

- For stuff tied to connectors we have the connector->register/unregister
  callbacks. Imo connector-related debugfs stuff, like for mst, should be
  put there.

- debugfs_init is a dead idea, as you point out it's fundamentally racy.

- Dropping the silly notion that we have to duplicate all debugfs entries
  per-minor would be really sweet (last time I checked there's not a
  single debugfs file that's actually different per minor).

Cheers, Daniel

> ---
>  drivers/gpu/drm/drm_debugfs.c  | 173 +++++++++++++++++++++++++++++++--
>  drivers/gpu/drm/drm_drv.c      |   3 +
>  drivers/gpu/drm/drm_internal.h |   5 +
>  include/drm/drm_debugfs.h      |  27 +++++
>  include/drm/drm_file.h         |   4 +
>  5 files changed, 203 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_debugfs.c b/drivers/gpu/drm/drm_debugfs.c
> index 6f28fe58f169..a53a454b167f 100644
> --- a/drivers/gpu/drm/drm_debugfs.c
> +++ b/drivers/gpu/drm/drm_debugfs.c
> @@ -39,6 +39,13 @@
>  
>  #if defined(CONFIG_DEBUG_FS)
>  
> +struct drm_debugfs_callback {
> +	void (*init)(void *data);
> +	void (*cleanup_cb)(void *data);
> +	void *data;
> +	struct list_head list;
> +};
> +
>  /***************************************************
>   * Initialization, etc.
>   **************************************************/
> @@ -67,6 +74,113 @@ static const struct file_operations drm_debugfs_fops = {
>  	.release = single_release,
>  };
>  
> +/**
> + * drm_debugfs_register_callback - Register a callback for initializing
> + *                                 dynamic driver-agnostic debugfs files
> + * @minor: device minor number
> + * @init: The callback to invoke to perform the debugfs initialization
> + * @cleanup_cb: The callback to invoke to cleanup any resources for the
> + * callback
> + * @data: Data to pass to @init and @cleanup_cb
> + * @out: Where to store the pointer to the callback struct
> + *
> + * Register an initialization callback with debugfs. This callback can be used
> + * to creating debugfs nodes for DRM resources that might get created before
> + * the debugfs node for @minor has been created.
> + *
> + * When a callback is registered with this function before the debugfs root
> + * has been created, the callback's execution will be delayed until all other
> + * debugfs nodes (including those owned by the DRM device's driver) have been
> + * instantiated. If a callback is registered with this function after the
> + * debugfs root has been created, @init and @cleanup_cb will be executed
> + * immediately without creating a &struct drm_debugfs_callback.
> + *
> + * In the event that debugfs creation for the device fails; all registered
> + * debugfs callbacks will have their @cleanup_cb callbacks invoked without
> + * having their @init callbacks invoked. This is to ensure that no resources
> + * are leaked during initialization of debugfs, even if the initialization
> + * process fails. Likewise; any callbacks that are registered after DRM has
> + * failed to initialize it's debugfs files will have their @cleanup_cb
> + * callbacks invoked immediately and all of their respective resources
> + * destroyed.
> + *
> + * Implementations of @cleanup_cb should clean up all resources for the
> + * callback, with the exception of freeing the memory for @out. Freeing @out
> + * will be handled by the DRM core automatically.
> + *
> + * Users of this function should take care to add a symmetric call to
> + * @drm_debugfs_unregister_callback to handle destroying a registered callback
> + * in case the resources for the user of this function are destroyed before
> + * debugfs root is initialized.
> + *
> + */
> +int
> +drm_debugfs_register_callback(struct drm_minor *minor,
> +			      void (*init)(void *),
> +			      void (*cleanup_cb)(void *),
> +			      void *data, struct drm_debugfs_callback **out)
> +{
> +	int ret = 0;
> +
> +	mutex_lock(&minor->debugfs_callback_lock);
> +	if (minor->debugfs_callbacks_done) {
> +		/* debugfs is already setup, so just handle the callback
> +		 * immediately
> +		 */
> +		if (minor->debugfs_root)
> +			(*init)(data);
> +		(*cleanup_cb)(data);
> +		goto out_unlock;
> +	}
> +
> +	*out = kzalloc(sizeof(**out), GFP_KERNEL);
> +	if (!*out) {
> +		ret = -ENOMEM;
> +		goto out_unlock;
> +	}
> +
> +	(*out)->init = init;
> +	(*out)->cleanup_cb = cleanup_cb;
> +	(*out)->data = data;
> +	list_add(&(*out)->list, &minor->debugfs_callback_list);
> +
> +out_unlock:
> +	mutex_unlock(&minor->debugfs_callback_lock);
> +	return ret;
> +}
> +EXPORT_SYMBOL(drm_debugfs_register_callback);
> +
> +/**
> + * drm_debugfs_unregister_callback - Unregister and release the resources
> + *                                   associated with a debugfs init callback
> + * @minor: device minor number
> + * @cb: A pointer to the &struct drm_debugfs_callback struct returned by
> + * drm_debugfs_register_callback(). May be NULL
> + *
> + * Unregisters a &struct drm_debugfs_callback struct with debugfs and destroys
> + * all of it's associated resources. This includes a call to the callback's
> + * @cleanup_cb implementation.
> + *
> + * Once the debugfs root is initialized or has failed initialization, all
> + * registered callbacks are automatically destroyed. If this function is
> + * called after that point; it will automatically be a no-op.
> + */
> +void
> +drm_debugfs_unregister_callback(struct drm_minor *minor,
> +				struct drm_debugfs_callback *cb)
> +{
> +	mutex_lock(&minor->debugfs_callback_lock);
> +	/* We don't have to do anything if we've already completed any
> +	 * registered callbacks, as they will have already been destroyed
> +	 */
> +	if (!minor->debugfs_callbacks_done) {
> +		cb->cleanup_cb(cb->data);
> +		list_del(&cb->list);
> +		kfree(cb);
> +	}
> +	mutex_unlock(&minor->debugfs_callback_lock);
> +}
> +EXPORT_SYMBOL(drm_debugfs_unregister_callback);
>  
>  /**
>   * drm_debugfs_create_files - Initialize a given set of debugfs files for DRM
> @@ -126,12 +240,24 @@ int drm_debugfs_create_files(const struct drm_info_list *files, int count,
>  }
>  EXPORT_SYMBOL(drm_debugfs_create_files);
>  
> +int drm_debugfs_alloc(struct drm_minor *minor)
> +{
> +	INIT_LIST_HEAD(&minor->debugfs_callback_list);
> +	mutex_init(&minor->debugfs_callback_lock);
> +	return 0;
> +}
> +
>  int drm_debugfs_init(struct drm_minor *minor, int minor_id,
>  		     struct dentry *root)
>  {
>  	struct drm_device *dev = minor->dev;
> +	struct drm_debugfs_callback *pos, *tmp;
>  	char name[64];
> -	int ret;
> +	int ret = 0;
> +
> +	/* Don't allow any more callbacks to be registered while we setup */
> +	mutex_lock(&minor->debugfs_callback_lock);
> +	minor->debugfs_callbacks_done = true;
>  
>  	INIT_LIST_HEAD(&minor->debugfs_list);
>  	mutex_init(&minor->debugfs_lock);
> @@ -139,7 +265,8 @@ int drm_debugfs_init(struct drm_minor *minor, int minor_id,
>  	minor->debugfs_root = debugfs_create_dir(name, root);
>  	if (!minor->debugfs_root) {
>  		DRM_ERROR("Cannot create /sys/kernel/debug/dri/%s\n", name);
> -		return -1;
> +		ret = -1;
> +		goto out_unlock;
>  	}
>  
>  	ret = drm_debugfs_create_files(drm_debugfs_list, DRM_DEBUGFS_ENTRIES,
> @@ -148,14 +275,14 @@ int drm_debugfs_init(struct drm_minor *minor, int minor_id,
>  		debugfs_remove(minor->debugfs_root);
>  		minor->debugfs_root = NULL;
>  		DRM_ERROR("Failed to create core drm debugfs files\n");
> -		return ret;
> +		goto out_unlock;
>  	}
>  
>  	if (drm_core_check_feature(dev, DRIVER_ATOMIC)) {
>  		ret = drm_atomic_debugfs_init(minor);
>  		if (ret) {
>  			DRM_ERROR("Failed to create atomic debugfs files\n");
> -			return ret;
> +			goto out_unlock;
>  		}
>  	}
>  
> @@ -163,13 +290,13 @@ int drm_debugfs_init(struct drm_minor *minor, int minor_id,
>  		ret = drm_framebuffer_debugfs_init(minor);
>  		if (ret) {
>  			DRM_ERROR("Failed to create framebuffer debugfs file\n");
> -			return ret;
> +			goto out_unlock;
>  		}
>  
>  		ret = drm_client_debugfs_init(minor);
>  		if (ret) {
>  			DRM_ERROR("Failed to create client debugfs file\n");
> -			return ret;
> +			goto out_unlock;
>  		}
>  	}
>  
> @@ -178,10 +305,23 @@ int drm_debugfs_init(struct drm_minor *minor, int minor_id,
>  		if (ret) {
>  			DRM_ERROR("DRM: Driver failed to initialize "
>  				  "/sys/kernel/debug/dri.\n");
> -			return ret;
> +			goto out_unlock;
>  		}
>  	}
> -	return 0;
> +
> +out_unlock:
> +	/* Execute any delayed callbacks if we succeeded, or just clean them
> +	 * up without running them if we failed
> +	 */
> +	list_for_each_entry_safe(pos, tmp, &minor->debugfs_callback_list,
> +				 list) {
> +		if (!ret)
> +			pos->init(pos->data);
> +		pos->cleanup_cb(pos->data);
> +		kfree(pos);
> +	}
> +	mutex_unlock(&minor->debugfs_callback_lock);
> +	return ret;
>  }
>  
>  
> @@ -223,14 +363,29 @@ static void drm_debugfs_remove_all_files(struct drm_minor *minor)
>  
>  int drm_debugfs_cleanup(struct drm_minor *minor)
>  {
> +	struct drm_debugfs_callback *pos, *tmp;
> +
> +	mutex_lock(&minor->debugfs_callback_lock);
> +	if (!minor->debugfs_callbacks_done) {
> +		list_for_each_entry_safe(pos, tmp,
> +					 &minor->debugfs_callback_list,
> +					 list) {
> +			pos->cleanup_cb(pos->data);
> +			kfree(pos);
> +		}
> +	}
> +	minor->debugfs_callbacks_done = true;
> +
>  	if (!minor->debugfs_root)
> -		return 0;
> +		goto out;
>  
>  	drm_debugfs_remove_all_files(minor);
>  
>  	debugfs_remove_recursive(minor->debugfs_root);
>  	minor->debugfs_root = NULL;
>  
> +out:
> +	mutex_unlock(&minor->debugfs_callback_lock);
>  	return 0;
>  }
>  
> diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
> index ea4941da9b27..7041b3137229 100644
> --- a/drivers/gpu/drm/drm_drv.c
> +++ b/drivers/gpu/drm/drm_drv.c
> @@ -118,6 +118,9 @@ static int drm_minor_alloc(struct drm_device *dev, unsigned int type)
>  
>  	minor->type = type;
>  	minor->dev = dev;
> +	r = drm_debugfs_alloc(minor);
> +	if (r)
> +		goto err_free;
>  
>  	idr_preload(GFP_KERNEL);
>  	spin_lock_irqsave(&drm_minor_lock, flags);
> diff --git a/drivers/gpu/drm/drm_internal.h b/drivers/gpu/drm/drm_internal.h
> index 40179c5fc6b8..d6394246967d 100644
> --- a/drivers/gpu/drm/drm_internal.h
> +++ b/drivers/gpu/drm/drm_internal.h
> @@ -118,6 +118,7 @@ void drm_gem_print_info(struct drm_printer *p, unsigned int indent,
>  
>  /* drm_debugfs.c drm_debugfs_crc.c */
>  #if defined(CONFIG_DEBUG_FS)
> +int drm_debugfs_alloc(struct drm_minor *minor);
>  int drm_debugfs_init(struct drm_minor *minor, int minor_id,
>  		     struct dentry *root);
>  int drm_debugfs_cleanup(struct drm_minor *minor);
> @@ -127,6 +128,10 @@ int drm_debugfs_crtc_add(struct drm_crtc *crtc);
>  void drm_debugfs_crtc_remove(struct drm_crtc *crtc);
>  int drm_debugfs_crtc_crc_add(struct drm_crtc *crtc);
>  #else
> +static inline int drm_debugfs_alloc(struct drm_minor *minor)
> +{
> +	return 0;
> +}
>  static inline int drm_debugfs_init(struct drm_minor *minor, int minor_id,
>  				   struct dentry *root)
>  {
> diff --git a/include/drm/drm_debugfs.h b/include/drm/drm_debugfs.h
> index ac0f75df1ac9..6ac45d96fcd1 100644
> --- a/include/drm/drm_debugfs.h
> +++ b/include/drm/drm_debugfs.h
> @@ -77,12 +77,23 @@ struct drm_info_node {
>  	struct dentry *dent;
>  };
>  
> +struct drm_debugfs_callback;
> +
>  #if defined(CONFIG_DEBUG_FS)
>  int drm_debugfs_create_files(const struct drm_info_list *files,
>  			     int count, struct dentry *root,
>  			     struct drm_minor *minor);
>  int drm_debugfs_remove_files(const struct drm_info_list *files,
>  			     int count, struct drm_minor *minor);
> +
> +int drm_debugfs_register_callback(struct drm_minor *minor,
> +				  void (*init)(void *),
> +				  void (*cleanup_cb)(void *),
> +				  void *data,
> +				  struct drm_debugfs_callback **out);
> +void drm_debugfs_unregister_callback(struct drm_minor *minor,
> +				     struct drm_debugfs_callback *cb);
> +
>  #else
>  static inline int drm_debugfs_create_files(const struct drm_info_list *files,
>  					   int count, struct dentry *root,
> @@ -96,6 +107,22 @@ static inline int drm_debugfs_remove_files(const struct drm_info_list *files,
>  {
>  	return 0;
>  }
> +
> +static inline int
> +drm_debugfs_register_callback(struct drm_minor *minor,
> +			      void (*init)(void *),
> +			      void (*cleanup_cb)(void *),
> +			      void *data,
> +			      struct drm_debugfs_callback **out)
> +{
> +	return 0;
> +}
> +
> +static inline void
> +drm_debugfs_unregister_callback(struct drm_minor *minor,
> +				struct drm_debugfs_callback *cb)
> +{
> +}
>  #endif
>  
>  #endif /* _DRM_DEBUGFS_H_ */
> diff --git a/include/drm/drm_file.h b/include/drm/drm_file.h
> index 26485acc51d7..180052b51712 100644
> --- a/include/drm/drm_file.h
> +++ b/include/drm/drm_file.h
> @@ -74,6 +74,10 @@ struct drm_minor {
>  
>  	struct dentry *debugfs_root;
>  
> +	bool debugfs_callbacks_done;
> +	struct list_head debugfs_callback_list;
> +	struct mutex debugfs_callback_lock;
> +
>  	struct list_head debugfs_list;
>  	struct mutex debugfs_lock; /* Protects debugfs_list. */
>  };
> -- 
> 2.17.1
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch