linux-kernel - Re: Some problems with sysfs patch (was Re: [PATCH v6] watchdog: New watchdog driver for MEN A21 watchdogs)

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Date:	Thu, 6 Jun 2013 04:31:10 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Johannes Thumshirn <johannes.thumshirn@....de>
Cc:	wim@...ana.be, linux-watchdog@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: Some problems with sysfs patch (was Re: [PATCH v6] watchdog: New
 watchdog driver for MEN A21 watchdogs)

On Thu, Jun 06, 2013 at 12:51:03PM +0200, Johannes Thumshirn wrote:
> On Mon, Jun 03, 2013 at 11:50:26AM +0200, Johannes Thumshirn wrote:
> > On Fri, May 31, 2013 at 09:15:23PM -0700, Guenter Roeck wrote:
> > > On Fri, May 31, 2013 at 02:55:19PM +0200, Johannes Thumshirn wrote:
> > > > Hi Guenther,
> > > > On Fri, May 31, 2013 at 04:40:37AM -0700, Guenter Roeck wrote:
> > > > > > +#define GPIO_WD_ENAB	169
> > > > > > +#define GPIO_WD_FAST	170
> > > > > > +#define GPIO_WD_TRIG	171
> > > > > > +
> > > > > > +#define GPIO_RST_CAUSE_BASE 166
> > > > > > +
> > > > >
> > > > > I think I asked that before ... as you are supporting devicetree, gpio pins
> > > > > should really be provided through devicetree properties and not be hardcoded.
> > > > >
> > > > Yes you did and I didn't come up with a solution to this problem yet. I understand
> > > > and agree to your concerns but I'm lacking example code/documentation for it, maybe
> > > > you can point me to an example on that and then I'll update my code accordingly.
> > > >
> > >
> > > Have a look at Documentation/devicetree/bindings/gpio/gpio-fan.txt and
> > > drivers/hwmon/gpio-fan.c.
> >
> > Thanks a lot, this really helped me out. Updated patch is comming today
> > including the bindings document Arnd Bergmann requested. I only need to rebase
> > the sysfs patch on top of that changes.
> >
> > Regards,
> > Johannes
> 
> Regarding the sysfs patch, I have a funny effect in my (rebased) sysfs code.
> 
> Given the following code:
> 
> +static ssize_t rebootcause_show(struct device *dev,
> +                               struct device_attribute *attr,
> +                               char *buf)
> +{
> +       struct a21_wdt_drv *drv = dev_get_drvdata(dev);
> +       unsigned int reset = 0;
> +
> +       if (!drv)
> +               return -EIO;
> +
> +       reset = a21_wdt_get_bootstatus(drv);
> +
> +       return sprintf(buf, "%s\n", reset_causes[reset]);
> +}
> +static DEVICE_ATTR(rebootcause, S_IRUGO, rebootcause_show, NULL);
> 
> I actually need the check for if (!drv) to prevent an OOPS, as
> dev_get_drvdata(dev) returns NULL., though it is set at the end of my probe
> function via:
> 
> [...]
> +       ret = watchdog_register_device(&a21_wdt);
> +       if (ret) {
> +               dev_err(&pdev->dev, "Cannot register watchdog device\n");
> +               goto err_register_wd;
> +       }
> +
> +       dev_set_drvdata(&pdev->dev, drv);
> +
> +       return 0;
> [...]
> 
> The watchdog driver itself is working without any troubles.
> 
> Some advice here would be worth its weight in gold.
> 
It has to be set before the attribute is created. Are you doing that ? Also, is
'dev' the same device pointer (ie is the 'dev' in your function the same as
&pdev->dev) ?

Guenter

> Thanks,
> Johannes
> 
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