Re: [PATCH] pwm: sysfs: Set class on pwm devices

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Thu, Oct 01, 2020 at 11:05:31AM +0200, Lars Poeschel 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@lemonage.de wrote:
> > > > > From: Lars Poeschel <poeschel@lemonage.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.
> 
> The export is triggered through the userspace process itself. Why can it
> do this ? Because there is another udev rule, that changes permissions
> when a pwmchip appears.
> Then I'd like to have the second udev rule, that changes permissions on
> the freshly exported pwm. The userspace process can't do this.
> You are right I could propably do everything from within udev: If a
> pwmchip appears, export certain pwms and right away change their
> permissions. It does not also not feel right. It'd require knowledge
> from the userspace application to be mapped to udev.

The way the kernel code is now, yes, you will not have any way to
trigger it by userspace as the kernel is creating a "raw" struct device
that isn't assigned to anything.  That is what needs to be fixed here.

> > Out of interest: What do you use the pwm for? Isn't there a suitable
> > kernel driver that can do the required stuff? Compared to the kernel-API
> > the sysfs interface isn't atomic. Is this an annoyance?
> 
> Use-case is generating a voltage from the pwm. This voltage is used to
> signal different states and does not change very often. This is
> absolutely not annoying that this is not atomic. We just change the duty
> cycle on the fly. Everything else is configured one time at startup.
> I'd call what I need pwm-dac. I could not find a ready to use driver.
> Maybe I could misuse some kernel driver for this. Maybe I could use
> pwm-led or pwm-brightness or pwm-fan. Propably pwm-regulator could work,
> there is even a userspace facing part for this, but this is not
> devicetree ready.
> ...and the worst, please don't blame me: The application is java, so
> ioctl is a problem.

I thought java could do ioctls, otherwise how would it ever be able to
talk to serial ports?

Anyway, this needs to be fixed in the kernel...

thanks,

greg k-h