linux-kernel - Re: [PATCH v3 3/5] media: mc: add debugfs node to keep track of requests

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Message-ID: <aEa3azYxM5Sc6cZC@kekkonen.localdomain>
Date: Mon, 9 Jun 2025 10:28:59 +0000
From: Sakari Ailus <sakari.ailus@...ux.intel.com>
To: Nicolas Dufresne <nicolas.dufresne@...labora.com>
Cc: Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Mauro Carvalho Chehab <mchehab@...nel.org>,
	Hans Verkuil <hverkuil@...all.nl>,
	Tiffany Lin <tiffany.lin@...iatek.com>,
	Andrew-CT Chen <andrew-ct.chen@...iatek.com>,
	Yunfei Dong <yunfei.dong@...iatek.com>,
	Matthias Brugger <matthias.bgg@...il.com>,
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-mediatek@...ts.infradead.org, kernel@...labora.com,
	linux-media@...r.kernel.org,
	Sebastian Fricke <sebastian.fricke@...labora.com>
Subject: Re: [PATCH v3 3/5] media: mc: add debugfs node to keep track of
 requests

Hi Nicolas,

On Wed, Jun 04, 2025 at 07:08:53PM -0400, Nicolas Dufresne wrote:
> Le mercredi 04 juin 2025 à 21:33 +0000, Sakari Ailus a écrit :
> > Hi Nicolas, Hans,
> > 
> > Thanks for the update.
> 
> thanks for the review, these things are precious.
> 
> > 
> > On Wed, Jun 04, 2025 at 04:09:37PM -0400, Nicolas Dufresne wrote:
> > > From: Hans Verkuil <hverkuil@...all.nl>
> > > 
> > > Keep track of the number of requests and request objects of a media
> > > device. Helps to verify that all request-related memory is freed.
> > > 
> > > Signed-off-by: Hans Verkuil <hverkuil-cisco@...all.nl>
> > > Signed-off-by: Nicolas Dufresne <nicolas.dufresne@...labora.com>
> > > ---
> > >  drivers/media/mc/mc-device.c  | 30 ++++++++++++++++++++++++++++++
> > >  drivers/media/mc/mc-devnode.c |  5 +++++
> > >  drivers/media/mc/mc-request.c |  6 ++++++
> > >  include/media/media-device.h  |  9 +++++++++
> > >  include/media/media-devnode.h |  4 ++++
> > >  include/media/media-request.h |  2 ++
> > >  6 files changed, 56 insertions(+)
> > > 
> > > diff --git a/drivers/media/mc/mc-device.c b/drivers/media/mc/mc-device.c
> > > index c0dd4ae5722725f1744bc6fd6282d5c765438059..5a458160200afb540d8014fed42d8bf2dab9c8c3 100644
> > > --- a/drivers/media/mc/mc-device.c
> > > +++ b/drivers/media/mc/mc-device.c
> > > @@ -679,6 +679,23 @@ void media_device_unregister_entity(struct media_entity *entity)
> > >  }
> > >  EXPORT_SYMBOL_GPL(media_device_unregister_entity);
> > >  
> > > +#ifdef CONFIG_DEBUG_FS
> > > +/*
> > > + * Log the state of media requests.
> > > + * Very useful for debugging.
> > > + */
> > 
> > Fits on a single line.
> 
> Ack.
> 
> > 
> > > +static int media_device_requests(struct seq_file *file, void *priv)
> > > +{
> > > +	struct media_device *dev = dev_get_drvdata(file->private);
> > > +
> > > +	seq_printf(file, "number of requests: %d\n",
> > > +		   atomic_read(&dev->num_requests));
> > > +	seq_printf(file, "number of request objects: %d\n",
> > > +		   atomic_read(&dev->num_request_objects));
> > 
> > Newline here?
> 
> I prefer that too.
> 
> > 
> > > +	return 0;
> > > +}
> > > +#endif
> > > +
> > >  void media_device_init(struct media_device *mdev)
> > >  {
> > >  	INIT_LIST_HEAD(&mdev->entities);
> > > @@ -697,6 +714,9 @@ void media_device_init(struct media_device *mdev)
> > >  		media_set_bus_info(mdev->bus_info, sizeof(mdev->bus_info),
> > >  				   mdev->dev);
> > >  
> > > +	atomic_set(&mdev->num_requests, 0);
> > > +	atomic_set(&mdev->num_request_objects, 0);
> > > +
> > >  	dev_dbg(mdev->dev, "Media device initialized\n");
> > >  }
> > >  EXPORT_SYMBOL_GPL(media_device_init);
> > > @@ -748,6 +768,15 @@ int __must_check __media_device_register(struct media_device *mdev,
> > >  
> > >  	dev_dbg(mdev->dev, "Media device registered\n");
> > >  
> > > +#ifdef CONFIG_DEBUG_FS
> > > +	if (!media_debugfs_root)
> > > +		media_debugfs_root = debugfs_create_dir("media", NULL);
> > > +	mdev->media_dir = debugfs_create_dir(dev_name(&devnode->dev),
> > > +					     media_debugfs_root);
> > > +	debugfs_create_devm_seqfile(&devnode->dev, "requests",
> > > +				    mdev->media_dir, media_device_requests);
> > > +#endif
> > 
> > I have no objection to this but it would have been great to have the Media
> > device lifetime set in first and MC device and devnode merged. But maybe
> > it's too late for that. Well, at least this won't change error handling...
> 
> Since this specific patch is not required to fix the MTK VCODEC issue, I can
> delay this a little. Is that comment related to an existing patch ?

Yes.

I've pushed the current set here:
<URL:https://git.linuxtv.org/sailus/media_tree.git/log/?h=media-ref>. I've
rebased it recently but it's still WiP.

...

> > > diff --git a/include/media/media-devnode.h b/include/media/media-devnode.h
> > > index d27c1c646c2805171be3997d72210dd4d1a38e32..dbcabeffcb572ae707f5fe1f51ff719d451c6784 100644
> > > --- a/include/media/media-devnode.h
> > > +++ b/include/media/media-devnode.h
> > > @@ -20,9 +20,13 @@
> > >  #include <linux/fs.h>
> > >  #include <linux/device.h>
> > >  #include <linux/cdev.h>
> > > +#include <linux/debugfs.h>
> > >  
> > >  struct media_device;
> > >  
> > > +/* debugfs top-level media directory */
> > > +extern struct dentry *media_debugfs_root;
> > > +
> > >  /*
> > >   * Flag to mark the media_devnode struct as registered. Drivers must not touch
> > >   * this flag directly, it will be set and cleared by media_devnode_register and
> > > diff --git a/include/media/media-request.h b/include/media/media-request.h
> > > index 7f9af68ef19ac6de0184bbb0c0827dc59777c6dc..610ccfe8d7b20ec38e166383433f9ee208248640 100644
> > > --- a/include/media/media-request.h
> > > +++ b/include/media/media-request.h
> > > @@ -292,6 +292,7 @@ struct media_request_object_ops {
> > >   * struct media_request_object - An opaque object that belongs to a media
> > >   *				 request
> > >   *
> > > + * @mdev: Media device this object belongs to
> > 
> > This deserves at least a comment what this may be used for: generally once
> > object is unbound, it's not related to a request anymore (nor a Media
> > device). This field also adds a new Media device lifetime issue: nothing
> 
> We could make it explicit by clearing the mdev pointer ?

That would probably be out of scope of this patch(set). Also see the
patchset I referred to earlier.

> 
> > guarantees the mdev is not disappearing at a wrong time albeit this is
> > very, very likely not user-triggerable without physically removing
> > hardware.
> 
> I'm not too familiar with the subject, if the MC knows it has open request
> FD(s), why would it allow userspace from unloading its module ?

Drivers nor MC currently have a list of request file handles.

Apart from the file handles, that was the original thinking, yes, but
devices can be also unbound without touching the driver (or other) modules.

> 
> > 
> > >   * @ops: object's operations
> > >   * @priv: object's priv pointer
> > >   * @req: the request this object belongs to (can be NULL)
> > > @@ -303,6 +304,7 @@ struct media_request_object_ops {
> > >   * another struct that contains the actual data for this request object.
> > >   */
> > >  struct media_request_object {
> > > +	struct media_device *mdev;
> > >  	const struct media_request_object_ops *ops;
> > >  	void *priv;
> > >  	struct media_request *req;
> > > 

-- 
Kind regards,

Sakari Ailus