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

On 10/15/2014 06:32 AM, Octavian Purdila wrote:

On Wed, Oct 15, 2014 at 4:13 PM, Guenter Roeck [off-list ref] wrote:

quoted

On 10/15/2014 04:49 AM, Octavian Purdila wrote:

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On Tue, Oct 14, 2014 at 6:41 PM, Guenter Roeck [off-list ref] wrote:

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On Tue, Oct 14, 2014 at 05:48:14PM +0300, Octavian Purdila wrote:

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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>

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+
+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.

quoted

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