Re: [PATCH v3 03/10] of: Add PWM support.

[Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org>]

On Friday 24 February 2012, Thierry Reding wrote:
> * Arnd Bergmann wrote:
> > On Thursday 23 February 2012, Thierry Reding wrote:
> > > * Arnd Bergmann wrote:

> [...]
> > > > * Why not include the pwm_request() call in this and return the
> > > >   pwm_device directly? You said that you want to get rid of the
> > > >   pwm_id eventually, which is a good idea, but this interface still
> > > >   forces one to use it.
> > > 
> > > Okay, that sounds sensible. I propose to rename the function to something like
> > > of_request_pwm().
> > 
> > Sounds good.

On second thought, I would actually prefer starting the name with pwm_ and
making it independent of device tree. There might be other ways how to
find the pwm_device from a struct device in the future, but it should always
be possible using a device together with a string and/or numeric identifier,
much in the same way that we can get a resource from a platform_device.

Ideally, there would be a common theme behind finding a memory region,
irq, gpio pin, clock, regulator, dma-channel and pwm or anything else
that requires a link between two device nodes.

> > > It would of course need an additional parameter (name) to
> > > forward to pwm_request().
> > 
> > Not necessarily, it could use the dev_name(device) or the name
> > of the property, or a combination of the two.
> 
> The problem with that is that usually the device would be named something
> generic like "pwm", while in case where the PWM is used for the backlight
> it makes sense to label the PWM device "backlight".
> 
> Looking at debugfs and seeing an entry "backlight" is much more straight-
> forward than "pwm.0". I mean "pwm.0" doesn't carry any useful information
> really, does it?

But the device name would be from the device using the pwm, not the
pwm controller, so it should be something more helpful, no?

> > > > > +EXPORT_SYMBOL(of_get_named_pwm);
> > > > 
> > > > EXPORT_SYMBOL_GPL?
> > > 
> > > It was brought up at some point that it might be nice to allow non-GPL
> > > drivers to use the PWM framework as well. I don't remember any discussion
> > > resulting from the comment. Perhaps we should have that discussion now and
> > > decide whether or not we want to keep it GPL-only or not.
> > 
> > I would definitely use EXPORT_SYMBOL_GPL for all new code unless it
> > replaces an earlier interface that was available as EXPORT_SYMBOL.
> 
> I just grepped the code and noticed this:
> 
> 	$ $ git grep -n 'EXPORT_SYMBOL.*(pwm_request)'
> 	arch/arm/mach-vt8500/pwm.c:139:EXPORT_SYMBOL(pwm_request);
> 	arch/arm/plat-mxc/pwm.c:183:EXPORT_SYMBOL(pwm_request);
> 	arch/arm/plat-samsung/pwm.c:83:EXPORT_SYMBOL(pwm_request);
> 	arch/unicore32/kernel/pwm.c:132:EXPORT_SYMBOL(pwm_request);
> 	drivers/mfd/twl6030-pwm.c:156:EXPORT_SYMBOL(pwm_request);
> 	drivers/misc/ab8500-pwm.c:108:EXPORT_SYMBOL(pwm_request);
> 	drivers/pwm/core.c:262:EXPORT_SYMBOL_GPL(pwm_request);
> 
> It seems like the legacy PWM API used to be non-GPL. Should I switch it back?
> Also does it make sense to have something like of_request_pwm() GPL when the
> rest of the API isn't?

I guess the choice is to make between you and Sascha. The implementation is
new, so you could pick EXPORT_SYMBOL_GPL, but you could also try to
keep to the current API.

	Arnd