[RFC 2/7] pwm: Allow chips to support multiple PWMs.

[Thierry Reding]

* Stephen Warren wrote:
> Thierry Reding wrote at Tuesday, December 20, 2011 3:32 AM:
> > many PWMs it controls using the npwm member. Each of the functions in
> > the pwm_ops structure gets an additional argument that specified the PWM
> > number (it can be converted to a per-chip index by subtracting the
> > chip's base).
> 
> I think it'd be much more convenient for drivers if the PWM core passed
> the device-relative PWM ID to the driver functions instead of the global
> PWM ID; the common case is going to be for the driver to want to index
> into some local array using the value, which means all drivers will have
> to subtract the chip base. I doubt many drivers will care about the global
> ID at all, and if they do, they can add the base back on.

That was my initial plan as well, but then I looked to gpiolib for
inspiration and decided to go the same way. But from what I hear now that
wasn't so good. If there is a concensus that gpiolib has some flaws in this
respect, then I think we should try to avoid them in the PWM framework.

> > The total maximum number of PWM devices is currently fixed to 64, but
> > can easily be made configurable via Kconfig.
> 
> GPIO (and IRQ?) have static sized arrays for this kind of purpose too,
> and I'm pretty sure I've seen discussions that this was a mistake, or
> at least something that will hopefully be reworked. pinctrl was similar,
> but it was requested this be reworked, and now I think the pin ID ->
> pin descriptor table is a radix tree.
> 
> In other words, can you do away with NR_PWM, and make it completely
> dynamic?

IRQ can be configured to use a radix tree if CONFIG_SPARSE_IRQ=y. I guess it
doesn't hurt to always use a radix tree for PWM, so I'll read up on it and
will try to address that in the next version.

Thierry
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