Re: [PATCH] misc: always assign miscdevice to file->private_data in open()

[Greg KH <gregkh@linuxfoundation.org>]

On Thu, Oct 09, 2014 at 03:10:21PM +0200, Martin Kepplinger wrote:
> Am 2014-10-08 15:43, schrieb Greg KH:
> > On Wed, Oct 08, 2014 at 10:47:54AM +0200, Martin Kepplinger wrote:
> >> As of now, a miscdevice driver has to provide an implementation of
> >> the open() file operation if it wants to have misc_open() assign a
> >> pointer to struct miscdevice to file->private_data for other file
> >> operations to use (given the user calls open()).
> >>
> >> This leads to situations where a miscdevice driver that doesn't need
> >> internal operations during open() has to implement open() that only
> >> returns immediately, in order to use the data in private_data in other
> >> fops.
> > 
> > Yeah, that's messy, do we have any in-kernel misc drivers that do this?
> > 
> >> This change provides consistent behaviour for miscdevice developers by
> >> always providing the pointer in private_data. A driver's open() fop would,
> >> of course, just overwrite it, when using private_data itself.
> >>
> >> Signed-off-by: Martin Kepplinger <martink@posteo.de>
> >> ---
> >> This is really only a question: Do I understand this correctly, and,
> >> could this change then hurt any existing driver?
> > 
> > I don't know, take a look at the existing ones and see please.
> > 
> >> As a driver developer it took me a while to figure out what happens here,
> >> and in my situation it would have been nice to just have this feature as
> >> part of the miscdevice API. Possibly documented somewhere?
> > 
> > Patches always accepted for documentation :)
> 
> What would be a good place for this?
> Documentation/driver-model/device.txt or
> Documentation/filesystem/vfs.txt like so? I'm not sure.

There's no documentation for misc devices?  If not, just put it in
kerneldoc format in the misc .c file.

> >From facd10cfa7539755e960dec8cc009934200e68ec Mon Sep 17 00:00:00 2001
> From: Martin Kepplinger <martink@posteo.de>
> Date: Thu, 9 Oct 2014 14:54:28 +0200
> Subject: [PATCH] documentation: misc_open sets private_data for driver's
>  open()
> 
> Signed-off-by: Martin Kepplinger <martink@posteo.de>
> ---
>  Documentation/filesystems/vfs.txt |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/filesystems/vfs.txt
> b/Documentation/filesystems/vfs.txt
> index 61d65cc..06df9d9 100644
> --- a/Documentation/filesystems/vfs.txt
> +++ b/Documentation/filesystems/vfs.txt
> @@ -869,7 +869,8 @@ otherwise noted.
>  	done the way it is because it makes filesystems simpler to
>  	implement. The open() method is a good place to initialize the
>  	"private_data" member in the file structure if you want to point
> -	to a device structure
> +	to a device structure. In the case of "struct miscdevice", when
> +	you implement open() this is done automatically.

No, no one will notice this in the vfs.txt file, and the vfs doesn't
care about misc devices.

> >> misc_open() is called in any case, on open(). As long as miscdevice drivers
> >> don't explicitly rely on private_data being NULL exactly IF they don't
> >> implement an open() fop (which I wouldn't imagine), this would make things
> >> even more convenient.
> > 
> > I agree, but it would be great if you can audit the existing misc
> > drivers to ensure we don't break anything with this change.  Can you do
> > that please?
> > 
> 
> I would grep -r "struct miscdevice" ./drivers/; and look at struct
> file_operations of these results, see how their open() looks like, and
> where they assign something to private_data.
> 
> If you have an idea for a script that lists all relevant files for me,
> please tell me.

You just came up with one there, that should be a good start.

good luck,

greg k-h