Messages in this thread | | | Date | Wed, 30 Sep 2020 16:13:52 +0200 | From | Lars Poeschel <> | Subject | Re: [PATCH] pwm: sysfs: Set class on pwm devices |
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On Wed, Sep 30, 2020 at 01:51:06PM +0200, Greg Kroah-Hartman wrote: > On Wed, Sep 30, 2020 at 01:27:20PM +0200, Lars Poeschel wrote: > > On Wed, Sep 30, 2020 at 12:52:38PM +0200, Greg Kroah-Hartman wrote: > > > On Wed, Sep 30, 2020 at 12:01:26PM +0200, Uwe Kleine-König wrote: > > > > 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@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. > > > > > > > > > > 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. > > > > > > Ah, that makes more sense why that didn't work. > > > > > > Ok, can the "name" of the new export chip be changed? Is that > > > hard-coded somewhere in userspace tools already? Depending on that, the > > > solution for this will change... > > > > I know that back then, when sysfs for pwm was created, Thierry didn't > > want to have one global namespace like gpio sysfs has. What you ask for > > is something like: > > pwm-{chipnumber}-{pwmnumber} > > Right ? Can that be considered non-global ? > > Yes, and that's just "global" for the pwm class namespace. > > > Thierry's mail from back then is here: > > https://lore.kernel.org/lkml/20130408081745.GA21392@avionic-0098.mockup.avionic-design.de/ > > > > A short search on github I found this: > > https://github.com/vsergeev/c-periphery/blob/d34077d7ee45fa7d1947cc0174919452fac31597/src/pwm.c#L74 > > > > Seems to match your hardcoded criteria ? > > Yes, ugh :( > > Ok, now I see why the "lots of pwm classes!" patch was proposed. > > And maybe that's really the only way forward here, as the chip namespace > is the only unique thing. > > But wow, it feels wrong...
Would the following feel better: * use the new naming scheme you proposed for pwm's : pwm-{chipnumber}-{pwmnumber} * assign the normal pwm class to the exported pwm devices. That lets them appear in the global /sys/class/pwm directory as e.g. pwm-0-0 * maintain backward compatibility through symlinks e.g.: pwmchip0/pwm0 -> ../pwm-0-0
Or does this feel even more horrible ? I may miss some subtleties.
Regards, Lars
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