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

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Date:   Mon, 24 Sep 2018 16:23:18 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     Fabrice Gasnier <fabrice.gasnier@...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 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. If so, perhaps we can
workaround that by creating a new class that is not parent->class?

Thierry

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