netdev - Re: [PATCH V7 3/8] posix clocks: introduce dynamic clocks

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Message-Id: <201012161716.42520.arnd@arndb.de>
Date:	Thu, 16 Dec 2010 17:16:42 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Richard Cochran <richardcochran@...il.com>
Cc:	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	netdev@...r.kernel.org, Alan Cox <alan@...rguk.ukuu.org.uk>,
	Christoph Lameter <cl@...ux.com>,
	David Miller <davem@...emloft.net>,
	John Stultz <johnstul@...ibm.com>,
	Krzysztof Halasa <khc@...waw.pl>,
	Peter Zijlstra <peterz@...radead.org>,
	Rodolfo Giometti <giometti@...ux.it>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH V7 3/8] posix clocks: introduce dynamic clocks

On Thursday 16 December 2010, Richard Cochran wrote:
> This patch adds support for adding and removing posix clocks. The
> clock lifetime cycle is patterned after usb devices. Each clock is
> represented by a standard character device. In addition, the driver
> may optionally implemented custom character device operations.
> 
> The dynamic posix clocks do not yet do anything useful. This patch
> merely provides some needed infrastructure.
> 
> Signed-off-by: Richard Cochran <richard.cochran@...cron.at>

> +struct posix_clock_fops {
> +	int (*fasync)  (void *priv, int fd, struct file *file, int on);
> +	int (*mmap)    (void *priv, struct vm_area_struct *vma);
> +	int (*open)    (void *priv, fmode_t f_mode);
> +	int (*release) (void *priv);
> +	long (*ioctl)  (void *priv, unsigned int cmd, unsigned long arg);
> +	long (*compat_ioctl) (void *priv, unsigned int cmd, unsigned long arg);
> +	ssize_t (*read) (void *priv, uint flags, char __user *buf, size_t cnt);
> +	unsigned int (*poll) (void *priv, struct file *file, poll_table *wait);
> +};

Thanks for the change to a private ops structure. Three more
suggestions for this:

* I would recommend starting without a compat_ioctl operation if you can.
Just mandate that all ioctls for posix clocks are compatible and call
fops->ioctl from the posix_clock_compat_ioctl() function.
If you ever need compat_ioctl handling, it can still be added later.

* Instead of passing a void pointer, why not pass a struct posix_clock
pointer to the lower device and use container_of? That follows what
we do in other subsystems and allows you save one allocation, by
including the posix_clock into the specific clock struct like
ptp_clock.

> +struct posix_clock_operations {
> +	struct module *owner;
> +	struct posix_clock_fops fops;
> +	int  (*clock_adjtime)(void *priv, struct timex *tx);

You can easily combine the two structures and get rid of the extra
struct posix_clock_fops by moving its operations into
posix_clock_operations.

Looks really good otherwise.

	Arnd
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