linux-kernel - Re: [RESEND PATCH] Revert "pwm: Set class for exported channels in sysfs"

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Date:   Mon, 24 Sep 2018 17:50:48 +0200
From:   Fabrice Gasnier <fabrice.gasnier@...com>
To:     Thierry Reding <thierry.reding@...il.com>
CC:     <stefan.wahren@...e.com>, <gohai@...zessiv.net>,
        <hsweeten@...ionengravers.com>, <gottfried.haider@...il.com>,
        <loic.pallardy@...com>, <broonie@...nel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-rpi-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <linux-pwm@...r.kernel.org>
Subject: Re: [RESEND PATCH] Revert "pwm: Set class for exported channels in
 sysfs"

On 09/24/2018 04:23 PM, Thierry Reding wrote:
> On Mon, Sep 24, 2018 at 03:59:03PM +0200, Fabrice Gasnier wrote:
>> On 09/24/2018 01:53 PM, Thierry Reding wrote:
>>> On Fri, Sep 21, 2018 at 04:02:47PM +0200, Fabrice Gasnier wrote:
>>>> This reverts commit 7e5d1fd75c3dde9fc10c4472b9368089d1b81d00 as it causes
>>>> regression with multiple pwm chip. It creates a new entry in
>>>> '/sys/class/pwm' every time a 'pwmX' is exported with 'echo X > export':
>>>> - 1st time export will create an entry in /sys/class/pwm/pwmX
>>>> - when another export happens on another pwmchip, it can't be created
>>>> (e.g. -EEXIST)
>>>>
>>>> This also changes existing ABI (Documentation/ABI/testing/sysfs-class-pwm):
>>>> - pmwX should be there: /sys/class/pwm/pwmchipN/pwmX
>>>>
>>>> Example on stm32 (stm32429i-eval) platform:
>>>> $ ls /sys/class/pwm
>>>> pwmchip0 pwmchip4
>>>>
>>>> $ cd /sys/class/pwm/pwmchip0/
>>>> $ echo 0 > export
>>>> $ ls /sys/class/pwm
>>>> pwm0 pwmchip0 pwmchip4
>>>>
>>>> $ cd /sys/class/pwm/pwmchip4/
>>>> $ echo 0 > export
>>>> sysfs: cannot create duplicate filename '/class/pwm/pwm0'
>>>> ...Exception stack follows...
>>>>
>>>> Signed-off-by: Fabrice Gasnier <fabrice.gasnier@...com>
>>>> ---
>>>>  drivers/pwm/sysfs.c | 1 -
>>>>  1 file changed, 1 deletion(-)
>>>
>>> Can we come up with an alternative that allows us to have both? We want
>>> uevent and proper sysfs creation, or is that not possible?
>>
>> Hi Thierry, all,
>>
>> With current approach:
>> - "export->child.class = parent->class"
>> - ABI (e.g. "pwm%d") device name isn't unique with multiple pwm chip.
>> I think this is not possible.
>>
>> Trying to think of an alternative... I just did a quick test, by
>> changing device name, to take pwmchip into account:
>> +       export->child.class = parent->class;
>>         export->child.release = pwm_export_release;
>>         export->child.parent = parent;
>>         export->child.devt = MKDEV(0, 0);
>>         export->child.groups = pwm_groups;
>> -       dev_set_name(&export->child, "pwm%u", pwm->hwpwm);
>> +       dev_set_name(&export->child, "pwmchip%d-pwm%u", chip->base,
>> pwm->hwpwm);
>>
>> But this also impacts existing ABI :-(
>> Would you have suggestions to send an uevent, without modifying ABI ?
> 
> I don't quite understand why, in the example you show in the commit
> message, the pwmX nodes appear in the top-level /sys/class/pwm
> directory. According to Documentation/ABI/testing/sysfs-class-pwm they
> should appear as /sys/class/pwm/pwmchipN/pwmX. I can only imagine that
> setting the class may have changed that.

Yes, adding the class makes the link to be created under /sys/class/pwm:
device_register() -> device_add() -> device_add_class_symlinks()
In device_add_class_symlinks():
...
	if (!dev->class)
		return 0;
...
	/* link in the class directory pointing to the device */
	error = sysfs_create_link(&dev->class->p->subsys.kobj,
				  &dev->kobj, dev_name(dev));
...

> If so, perhaps we can
> workaround that by creating a new class that is not parent->class?

And this link is added using dev_name(). So I doubt adding a new class
will change the current behavior.

> 
> Thierry
>