Re: [libcamera-devel] [PATCH 2/4] media: v4l2-dev: Add v4l2_device_register_ro_subdev_node()

[Sakari Ailus <sakari.ailus@linux.intel.com>]

Hi Jacopo,

On Wed, Mar 25, 2020 at 12:23:42PM +0100, Jacopo Mondi wrote:
> Hi Andrey,
>    thanks for the review
> 
> On Wed, Mar 25, 2020 at 11:42:27AM +0300, Andrey Konovalov wrote:
> > Hi Jacopo,
> >
> > Thank you for your patch set!
> >
> > On 24.03.2020 23:28, Jacopo Mondi wrote:
> > > Add to the V4L2 code a function to register device nodes for video
> > > subdevices in read-only mode.
> > >
> > > Registering a device node in read-only mode is useful to expose to
> > > userspace the current sub-device configuration, without allowing
> > > application to change it by using the V4L2 subdevice ioctls.
> > >
> > > Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
> > > ---
> > >   drivers/media/v4l2-core/v4l2-device.c | 16 +++++++++++++++-
> > >   drivers/media/v4l2-core/v4l2-subdev.c | 19 +++++++++++++++++++
> > >   include/media/v4l2-dev.h              |  7 +++++++
> > >   include/media/v4l2-device.h           | 10 ++++++++++
> > >   4 files changed, 51 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/media/v4l2-core/v4l2-device.c b/drivers/media/v4l2-core/v4l2-device.c
> > > index 63d6b147b21e..6f9dba36eda1 100644
> > > --- a/drivers/media/v4l2-core/v4l2-device.c
> > > +++ b/drivers/media/v4l2-core/v4l2-device.c
> > > @@ -188,7 +188,8 @@ static void v4l2_device_release_subdev_node(struct video_device *vdev)
> > >   	kfree(vdev);
> > >   }
> > > -int v4l2_device_register_subdev_nodes(struct v4l2_device *v4l2_dev)
> > > +int __v4l2_device_register_subdev_nodes(struct v4l2_device *v4l2_dev,
> > > +					bool read_only)
> > >   {
> > >   	struct video_device *vdev;
> > >   	struct v4l2_subdev *sd;
> > > @@ -217,6 +218,8 @@ int v4l2_device_register_subdev_nodes(struct v4l2_device *v4l2_dev)
> > >   		vdev->fops = &v4l2_subdev_fops;
> > >   		vdev->release = v4l2_device_release_subdev_node;
> > >   		vdev->ctrl_handler = sd->ctrl_handler;
> > > +		if (read_only)
> > > +			vdev->flags |= V4L2_FL_RO_DEVNODE;
> >
> > <snip>
> >
> > > @@ -331,6 +331,7 @@ static long subdev_do_ioctl(struct file *file, unsigned int cmd, void *arg)
> > >   	struct v4l2_fh *vfh = file->private_data;
> > >   #if defined(CONFIG_VIDEO_V4L2_SUBDEV_API)
> > >   	struct v4l2_subdev_fh *subdev_fh = to_v4l2_subdev_fh(vfh);
> > > +	bool ro_devnode = !!(vdev->flags & V4L2_FL_RO_DEVNODE);

No need to shout, i.e.

	bool ro_devnode = ... & ...;

> >
> > So V4L2_FL_RO_DEVNODE is a bit mask, ...
> >
> > <snip>
> >
> > > diff --git a/include/media/v4l2-dev.h b/include/media/v4l2-dev.h
> > > index 48531e57cc5a..029873a338f2 100644
> > > --- a/include/media/v4l2-dev.h
> > > +++ b/include/media/v4l2-dev.h
> > > @@ -82,11 +82,18 @@ struct v4l2_ctrl_handler;
> > >    *	but the old crop API will still work as expected in order to preserve
> > >    *	backwards compatibility.
> > >    *	Never set this flag for new drivers.
> > > + * @V4L2_FL_RO_DEVNODE:
> > > + *	indicates that the video device node is registered in read-only mode.
> > > + *	The flag only applies to device nodes registered for sub-devices, it is
> > > + *	set by the core when the sub-devices device nodes are registered with
> > > + *	v4l2_device_register_ro_subdev_nodes() and used by the sub-device ioctl
> > > + *	handler to restrict access to some ioctl calls.
> > >    */
> > >   enum v4l2_video_device_flags {
> > >   	V4L2_FL_REGISTERED		= 0,
> > >   	V4L2_FL_USES_V4L2_FH		= 1,
> > >   	V4L2_FL_QUIRK_INVERTED_CROP	= 2,
> > > +	V4L2_FL_RO_DEVNODE		= 3,
> >
> > ... then V4L2_FL_RO_DEVNODE should rather be equal to 4, than to (V4L2_FL_USES_V4L2_FH | V4L2_FL_QUIRK_INVERTED_CROP)
> 
> Ups!
> 
> I should have noticed looking at V4L2_FL_REGISTERED=0 that these
> flags are actually meant to be used as bit shifts. I can't say I like
> the idea, but I should have known better than this.
> 
> I'll resend using set_bit() and test_bit().

You can also use BIT(flag name).

Either works, but IMO test_bit() is a bit of overkill.

-- 
Sakari Ailus