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

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Date:   Mon, 24 Sep 2018 15:59:03 +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 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 ?

Please advise,
Best regards,
Fabrice

> 
> Thierry
>