linux-kernel - Re: [PATCH] pwm: sysfs: Set class on pwm devices

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Message-ID: <20200930100126.rtjfnmbc54m7vrwd@pengutronix.de>
Date:   Wed, 30 Sep 2020 12:01:26 +0200
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Lars Poeschel <poeschel@...onage.de>,
        Thierry Reding <thierry.reding@...il.com>,
        Lee Jones <lee.jones@...aro.org>,
        "open list:PWM SUBSYSTEM" <linux-pwm@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] pwm: sysfs: Set class on pwm devices

On Wed, Sep 30, 2020 at 11:52:04AM +0200, Greg Kroah-Hartman wrote:
> On Wed, Sep 30, 2020 at 11:41:46AM +0200, Uwe Kleine-König wrote:
> > Hello,
> > 
> > I added Greg Kroah-Hartman who I discussed this with via irc a bit to
> > Cc:.
> > 
> > On Wed, Sep 30, 2020 at 11:20:56AM +0200, Lars Poeschel wrote:
> > > thank you for your review!
> > > 
> > > On Wed, Sep 30, 2020 at 08:57:26AM +0200, Uwe Kleine-König wrote:
> > > > On Tue, Sep 29, 2020 at 02:19:53PM +0200, poeschel@...onage.de wrote:
> > > > > From: Lars Poeschel <poeschel@...onage.de>
> > > > > 
> > > > > This adds a class to exported pwm devices.
> > > > > Exporting a pwm through sysfs did not yield udev events. The
> > > > 
> > > > I wonder what is your use-case here. This for sure also has a place to
> > > > be mentioned in the commit log. I suspect there is a better way to
> > > > accomplish you way.
> > > 
> > > Use-case is to be able to use a pwm from a non-root userspace process.
> > > I use udev rules to adjust permissions.
> > 
> > Hmm, how do you trigger the export? Without being aware of all the
> > details in the sysfs code I would expect that the exported stuff is
> > available instantly once the write used to export the PWM is completed.
> > So changing the permissions can be done directly after triggering the
> > export in the same process.
> 
> It looks like userspace wants to see when a pwmX device shows up, right?
> 
> And it's not because those devices do not belong to any class or bus, so
> they are just "floating" out there (they might show up under
> /sys/bus/virtual, if you set things up right, which I don't think is
> happening here...)
> 
> So yes, you need to create a class, or assign this to a bus, which is
> fine, but it looks like no one is doing that.  Don't create new classes
> dynamically, but rather, just assign this to the existing pwm class.
> What's wrong with that?  I saw an older patch that did that, what did
> that break?

Are you refering to 7e5d1fd75c3dde9fc10c4472b9368089d1b81d00? Did you
read the reverting commit's log message? (i.e.
c289d6625237aa785b484b4e94c23b3b91ea7e60)

I guess the breakage is that the resulting name then is:

	"pwm%d", pwm->id

where pwm->id is a number unique to the pwmchip. So doing

	echo 0 > pwmchip1/export
	echo 0 > pwmchip2/export

breaks because both want to create pwm0 in the class directory.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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