linux-kernel - Re: [PATCH RFC 1/8] Introduce dynamic clock devices

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Message-Id: <201012061514.50055.arnd@arndb.de>
Date:	Mon, 6 Dec 2010 15:14:49 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Richard Cochran <richardcochran@...il.com>
Cc:	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Christoph Lameter <cl@...ux.com>,
	John Stultz <johnstul@...ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH RFC 1/8] Introduce dynamic clock devices

On Saturday 04 December 2010, Richard Cochran wrote:
> On Mon, Nov 08, 2010 at 07:38:41AM +0100, Arnd Bergmann wrote:
> > On Thursday 04 November 2010, Richard Cochran wrote:
> > > +struct clock_device {
> > > +   struct file_operations fops;
> > > +   struct file_operations *driver_fops;
> > > +   struct clock_device_operations *ops;
> > > +   struct cdev cdev;
> > > +   struct kref kref;
> > > +   struct mutex mux;
> > > +   void *priv;
> > > +   int index;
> > > +   bool zombie;
> > > +};
> > 
> > You should really not need the file_operations here, neither the
> > struct nor the pointer. Just define a static file_operations
> > structure containing clock_device_open and clock_device_release,
> > and whatever else you might need, then add the driver's operations
> > to clock_device_operations, and pass the clock_device pointer
> > directly to them, instead of passing the file/inode pointers.
> 
> Arnd, I'm working a revision of this series, and I am not sure I
> understand your comment.
> 
> The intent here was to allow clock drivers to register a character
> device through the clock_device API, since some clocks (hpet, rtc)
> already offer a chardev interface.
> 
> The same FD from the open character device will also be usable as a
> clockid for the generic posix clock_get/settime calls. Thus, the
> clock_device layer needs to hook into the file open/release functions.

Ok, it wasn't clear that you use this to hook the existing file
operations, I now understand why you did it this way, but I think
my point is still valid.

> Are you suggesting that I simply offer all of the functions from a
> 'struct file_operations' (sans file/inode) in the 'struct
> clock_device_operations' too?

Yes, exactly.

> I wanted to avoid duplicating the file_operations functions, so that
> future changes in those functions would automatically trickle down to
> the clock drivers.

No need, these rarely change. More importantly, if you want to offer
a consistent interface across all these, I would make the interface
as restrictive as possible rather than offering all of the file
operations. Have a look which operations are actually used by the
character devices you want to support, and then pass through the
superset of those, but not more.

	Arnd
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