linux-kernel - Re: [PATCH 1/10 v2] Generic Watchdog Timer Driver

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Message-Id: <201106241559.16416.arnd@arndb.de>
Date:	Fri, 24 Jun 2011 15:59:15 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Wim Van Sebroeck <wim@...ana.be>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Linux Watchdog Mailing List <linux-watchdog@...r.kernel.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH 1/10 v2] Generic Watchdog Timer Driver

On Wednesday 22 June 2011 21:50:24 Wim Van Sebroeck wrote:

> > I'm pretty sure you don't need bitops.h or uaccess.h here, because all the
> > code using those has moved into the core.
> 
> bitops will be used later on, but this can indeed be cleaned up.
> 
> > > +#include <linux/io.h>
> > 
> > This is also not needed here, although it will probably be needed in most
> > real drivers.
> 
> Same.

Nevermind then.

> > > +#include <linux/platform_device.h>
> > > +
> > > +/* Hardware heartbeat in seconds */
> > > +#define WDT_HW_HEARTBEAT 2
> > > +
> > > +/* Timer heartbeat (500ms) */
> > > +#define WDT_HEARTBEAT	(HZ/2)	/* should be <= ((WDT_HW_HEARTBEAT*HZ)/2) */
> > > +
> > > +/* User land timeout */
> > > +#define WDT_TIMEOUT 15
> > > +static int timeout = WDT_TIMEOUT;
> > > +module_param(timeout, int, 0);
> > > +MODULE_PARM_DESC(timeout, "Watchdog timeout in seconds. "
> > > +	"(default = " __MODULE_STRING(WDT_TIMEOUT) ")");
> > 
> > Should the module parameter really be part of each individual driver?
> > It would be nice if that can be moved into the core as well.
> 
> Yes, the module parameter is needed for each individual driver.
> If we go for supporting multiple watchdog devices, then we will have to support
> different timeout values. The timeout ranges also differ for different devices.

Ok, but you'd still have to worry about a single driver supporting multiple
distinct devices that each want a separate timeout, right?

OTOH, we can still find a solution when it ever gets to the point of
supporting multiple devices.

> > > +static void wdt_timer_tick(unsigned long data);
> > 
> > I usually recommend reordering all functions so that you don't need any forward
> > declarations, that makes the driver easier to read IMHO.
> > 
> > > +static DEFINE_TIMER(timer, wdt_timer_tick, 0, 0);
> > > +					/* The timer that pings the watchdog */
> 
> Yes, I also tend to do that but it's used in the DEFINE_TIMER(timer, wdt_timer_tick, 0, 0);
> just under it. No clean way to do that better imho...

Ah, right. I missed that. You could get rid of the forward declaration
by dynamically initializing the timer struct, but that would be no
better than what you have now.

> > Is it common for watchdog these days to have both a kernel timer and
> > a user space timer?
> 
> No, it is only common for watchdog devices that
> 1) don't stop once started
> 2) device that have very small (mostly < 1second) heartbeat values.
> All other watchdog device timers don't need and use the kernel timer.

Ok, I hadn't thought of these.

> > If yes, that might be good to support in the core as well, instead of
> > having to implement it in each driver.
> > 
> > If no, it might not be ideal to have in an example driver.
> 
> As said, it's an example for a common exception. You should not look at this
> as a common example driver. I added it, because it's a common exception that
> people understand less.

ok.

> > > +struct watchdog_device {
> > > +	char *name;
> > > +	const struct watchdog_ops *ops;
> > > +	long status;
> > > +};
> > > +
> > > +It contains following fields:
> > > +* name: a pointer to the (preferably unique) name of the watchdog timer device.
> > > +* ops: a pointer to the list of watchdog operations that the watchdog supports.
> > > +* status: this field contains a number of status bits that give extra
> > > +  information about the status of the device (Like: is the device opened via
> > > +  the /dev/watchdog interface or not, ...)
> > 
> > I think this should really have a pointer to the struct device implementing the
> > watchdog, so that a driver that gets called with a struct watchdog_device can
> > find its own state from there. Alternatively, you could have struct device
> > embedded in struct watchdog_device and register it as a child of the hardware
> > device.
> 
> Would go for a pointer to private data then.

Ok.

	Arnd
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