linux-kernel - Re: [PATCH] misc: always assign miscdevice to file->private

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Date:	Wed, 8 Oct 2014 06:43:07 -0700
From:	Greg KH <gregkh@...uxfoundation.org>
To:	Martin Kepplinger <martink@...teo.de>
Cc:	arnd@...db.de, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] misc: always assign miscdevice to file->private_data in
 open()

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@...teo.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 :)

> 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?

thanks,

greg k-h
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