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

[Octavian Purdila]

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.
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