Date: Thu, 6 Jun 2013 10:08:28 -0700
From: Guenter Roeck <linux@roeck-us.net>
To: Johannes Thumshirn <johannes.thumshirn@men.de>
Cc: wim@iguana.be, linux-watchdog@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: Some problems with sysfs patch (was Re: [PATCH v6] watchdog: New
 watchdog driver for MEN A21 watchdogs)
Message-ID: <20130606170828.GA30898@roeck-us.net>
References: <20130531103654.GB5619@roeck-us.net>
 <20130531110846.GA15707@jtlinux>
 <20130531114037.GA13533@roeck-us.net>
 <20130531125519.GB8983@jtlinux>
 <20130601041523.GB4081@roeck-us.net>
 <20130603095026.GA21720@jtlinux>
 <20130606105103.GA25245@jtlinux>
 <20130606113110.GA5834@roeck-us.net>
 <20130606130055.GA27244@jtlinux>
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On Thu, Jun 06, 2013 at 03:00:55PM +0200, Johannes Thumshirn wrote:
> On Thu, Jun 06, 2013 at 04:31:10AM -0700, Guenter Roeck wrote:
> > 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
> 
> Hi Guenter,
> 
> Thanks for the quick reply. Indeed 'dev' in my function is a struct
> watchdog_device's dev (for device_create_file) and not &pdev->dev. Which in turn
> are not the same.
> 
> A call to dev_set_drvdata(a21_wdt.dev, drv); solved the NULL pointer access. So
> last question, is it save to set the drvdata of a dev inside the struct
> watchdog_device or do I break something vital that way?
> 
Good question. At issue is if your driver 'owns' struct watchdog_device,
or if the watchdog core owns it.

However, since you know that a21_wdt.dev is the device, you can use
container_of() to get a reference to a21_wdt, and watchdog_get_drvdata
to get access to drv from a21_wdt. So you should not really need it.

Thanks,
Guenter
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