Re: [RFC PATCH v2 3/3] i2c: show and change bus frequency via sysfs

[Guenter Roeck]

On 10/15/2014 06:32 AM, Octavian Purdila wrote:
> On Wed, Oct 15, 2014 at 4:13 PM, Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
> > On 10/15/2014 04:49 AM, Octavian Purdila wrote:
> > > On Tue, Oct 14, 2014 at 6:41 PM, Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
> > > > On Tue, Oct 14, 2014 at 05:48:14PM +0300, Octavian Purdila wrote:
> > > > > This patch adds three new sysfs files: bus_frequency,
> > > > > bus_min_frequency and bus_max_frequency which allows the user to view
> > > > > or change the bus frequency on a per bus level.
> > >
> > > <snip>
> > >
> > > > > +
> > > > > +static DEVICE_ATTR(bus_frequency, S_IRUGO, i2c_sysfs_freq_show,
> > > > > +                i2c_sysfs_freq_store);
> > > >
> > > >
> > > > Consider using DEVICE_ATTR_RO here. Also, extra empty line.
> > >
> > > Unfortunately that won't work because we must transform bus_frequency
> > > to a RW entry (via is_visible) if the bus can change the frequency. We
> >
> >
> > Ah yes, you are right.
> >
> > > can't use DEVIE_ATTR_RW either, because transforming a RW entry to a
> > > RO entry with is visible is not possible:
> >
> > Why not ?
> >
> > is_visible returns the desired mode. Just like you can return mode |
> > S_IWUSR,
> > you can return mode & ~S_IWUSR.
> >
> > Am I missing something ?
>
> Here is how sysfs uses is_visible:
>
> static int create_files(struct kernfs_node *parent, struct kobject *kobj,
> ...
>                          if (grp->is_visible) {
>                                  mode = grp->is_visible(kobj, *attr, i);
>                                  if (!mode)
>                                          continue;
>                          }
>                          error = sysfs_add_file_mode_ns(parent, *attr, false,
>                                                         (*attr)->mode | mode,
>                                                         NULL);
>
> so basically is the mode set is the original mode from the attributed
> "or-ed" with the mode return by is_visible.
Ah, you are right. That's a new one for me. Thanks, I didn't notice earlier.
Good to know ;-).

Guenter

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