Re: [PATCH 1/2] pwm: sysfs: Set class on pwm devices

[Lars Poeschel <poeschel@lemonage.de>]

On Fri, Oct 02, 2020 at 03:35:12PM +0200, Greg Kroah-Hartman wrote:
> On Fri, Oct 02, 2020 at 03:08:44PM +0200, Lars Poeschel wrote:
> > On Fri, Oct 02, 2020 at 02:46:16PM +0200, Greg Kroah-Hartman wrote:
> > > On Fri, Oct 02, 2020 at 02:30:47PM +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
> > > > dev_uevent_filter function does filter-out devices without a bus or
> > > > class.
> > > > This was already addressed in commit
> > > > commit 7e5d1fd75c3d ("pwm: Set class for exported channels in sysfs")
> > > > but this did cause problems and the commit got reverted with
> > > > commit c289d6625237 ("Revert "pwm: Set class for exported channels in
> > > > sysfs"")
> > > > Problem with the previous approach was, that there is a clash if we have
> > > > multiple pwmchips:
> > > > 	echo 0 > pwmchip0/export
> > > > 	echo 0 > pwmchip1/export
> > > > would both export /sys/class/pwm/pwm0 .
> > > > 
> > > > Now this patch changes the sysfs interface. We do include the pwmchip
> > > > number into the pwm directory that gets exported.
> > > > With the example above we get:
> > > > 	/sys/class/pwm/pwm-0-0
> > > > 	/sys/class/pwm/pwm-1-0
> > > > We maintain ABI backward compatibility through symlinks.
> > > > 	/sys/class/pwm/pwmchip0/pwm0
> > > > 	/sys/class/pwm/pwmchip1/pwm0
> > > > are now symbolic links to the new names.
> > > > 
> > > > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > > > Signed-off-by: Lars Poeschel <poeschel@lemonage.de>
> > > > ---
> > > >  drivers/pwm/sysfs.c | 57 +++++++++++++++++++++++++++++++++++++--------
> > > >  1 file changed, 47 insertions(+), 10 deletions(-)
> > > > 
> > > > diff --git a/drivers/pwm/sysfs.c b/drivers/pwm/sysfs.c
> > > > index 449dbc0f49ed..c708da17a857 100644
> > > > --- a/drivers/pwm/sysfs.c
> > > > +++ b/drivers/pwm/sysfs.c
> > > > @@ -240,8 +240,10 @@ static void pwm_export_release(struct device *child)
> > > >  
> > > >  static int pwm_export_child(struct device *parent, struct pwm_device *pwm)
> > > >  {
> > > > +	struct pwm_chip *chip = dev_get_drvdata(parent);
> > > >  	struct pwm_export *export;
> > > >  	char *pwm_prop[2];
> > > > +	char *link_name;
> > > >  	int ret;
> > > >  
> > > >  	if (test_and_set_bit(PWMF_EXPORTED, &pwm->flags))
> > > > @@ -256,25 +258,39 @@ static int pwm_export_child(struct device *parent, struct pwm_device *pwm)
> > > >  	export->pwm = pwm;
> > > >  	mutex_init(&export->lock);
> > > >  
> > > > +	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, "pwm-%u-%u", chip->base, pwm->hwpwm);
> > > >  
> > > >  	ret = device_register(&export->child);
> > > > -	if (ret) {
> > > > -		clear_bit(PWMF_EXPORTED, &pwm->flags);
> > > > -		put_device(&export->child);
> > > > -		export = NULL;
> > > > -		return ret;
> > > > +	if (ret)
> > > > +		goto error;
> > > > +
> > > > +	link_name = kasprintf(GFP_KERNEL, "pwm%u", pwm->hwpwm);
> > > > +	if (link_name == NULL) {
> > > > +		ret = -ENOMEM;
> > > > +		goto dev_unregister;
> > > >  	}
> > > > -	pwm_prop[0] = kasprintf(GFP_KERNEL, "EXPORT=pwm%u", pwm->hwpwm);
> > > > +
> > > > +	pwm_prop[0] = kasprintf(GFP_KERNEL, "EXPORT=%s",
> > > > +			export->child.kobj.name);
> > > >  	pwm_prop[1] = NULL;
> > > >  	kobject_uevent_env(&parent->kobj, KOBJ_CHANGE, pwm_prop);
> > > 
> > > Do you still need to do this by hand?  Why can't this uevent field
> > > belong to the class and have it create this for you automatically when
> > > the device is added?
> > 
> > I did not add this with my patch, it was there before and I wonder, what
> > purpose it served, since the uevent was filtered because there was no
> > class there.
> > Now we have a class and now it works and this is what happens:
> > 
> > /sys/class/pwm# echo 0 > pwmchip1/export 
> > KERNEL[2111.952725] add      /devices/platform/ocp/48302000.epwmss/48302200.pwm/pwm/pwmchip1/pwm-1-0 (pwm)
> > ACTION=add
> > DEVPATH=/devices/platform/ocp/48302000.epwmss/48302200.pwm/pwm/pwmchip1/pwm-1-0
> > SEQNUM=1546
> > SUBSYSTEM=pwm
> > 
> > KERNEL[2111.955155] change   /devices/platform/ocp/48302000.epwmss/48302200.pwm/pwm/pwmchip1 (pwm)
> > ACTION=change
> > DEVPATH=/devices/platform/ocp/48302000.epwmss/48302200.pwm/pwm/pwmchip1
> > EXPORT=pwm-1-0
> > SEQNUM=1547
> > SUBSYSTEM=pwm
> > 
> > The first event is the event from device_register. It informs us that we
> > now have a new pwm-1-0. Nice.
> > The second is the event done here "by hand". It informs us, that
> > pwmchip1 changed. It has a new export now. For me personally this is not
> > needed, but also I don't think it is wrong.
> > You decide!
> 
> If the uevent was being filtered out anyway, and never sent, then let's
> just drop the thing as there is nothing to keep backwards compatible.

I had a sleepless night about this. I felt something is wrong with this.
I investigated a bit:
- for the kobject_uevent_env line git blame found this commit 552c02e3e7cfe
  ("pwm: Send a uevent on the pwmchip device upon channel sysfs
  (un)export")
- the commit message explicitly mentions my use case! (udev event to
  change permissions of exported pwm)
- git log says the commit right before is this one: commit c289d6625237
  ("Revert "pwm: Set class for exported channels in sysfs""). This is
  the commit, that reverts the previous addition of the pwmchip class
  right as is to the pwm. The one that had the sysfs name clash.

I must have done something horribly wrong. I tried again and now I can
get this udev change event. I don't know what I did wrong. The event is
not filtered, because this is sent on behalf of the parent that had the
class attached right from the start. This is my mistake. I am very sorry
for the noise.
I think best is now to keep everything as is, right ?

Regards,
Lars