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

[Thierry Reding <thierry.reding-RM9K5IK7kjKj5M59NBduVrNAH6kLmebB@public.gmane.org>]

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* Thierry Reding wrote:
> * Stephen Warren wrote:
> > 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.

I guess something like idr/ida can be used to dynamically assign a PWM ID,
which would allow us to get rid of the bitmap. Then again, ida itself is not
much more than a bitmap either. It would complicate things a little in that
the ID assignment could no longer be assumed to be sequential for one given
PWM chip, so the lookup (or rather mapping the ID to a chip-relative number)
will be trickier to do.

I'm not sure that it's worth it. Perhaps I should keep the bitmap for ID
allocation and just set the number of bits to something sufficiently large,
say 1024? Then use a radix tree to store the actual descriptors.

pinctrl doesn't solve this because it uses statically allocated pin numbers.
Interestingly though it uses per-device numbering as well, which would be
fine for PWM as well if we had only device tree based probing. In order to
support other devices, we'll still need a global namespace.

Perhaps we can keep the global namespace using the bitmap as is for the time
being and introduce a per-chip API and move all users to that eventually? As
Mark already noted this will cause a lot of churn. Still, there aren't that
many users of the API yet (from Linus' latest tree):

	$ git grep -c pwm_request
	arch/arm/mach-s3c2440/mach-rx1950.c:1
	arch/arm/mach-vt8500/pwm.c:2
	arch/arm/plat-mxc/pwm.c:2
	arch/arm/plat-pxa/pwm.c:2
	arch/arm/plat-samsung/pwm.c:2
	arch/blackfin/kernel/pwm.c:2
	arch/mips/jz4740/pwm.c:1
	arch/unicore32/kernel/pwm.c:2
	drivers/input/misc/pwm-beeper.c:1
	drivers/leds/leds-pwm.c:1
	drivers/mfd/twl6030-pwm.c:2
	drivers/misc/ab8500-pwm.c:2
	drivers/video/backlight/pwm_bl.c:1
	include/linux/pwm.h:2

However, that would require a way to pass the providing PWM chip to the
driver (in addition to the PWM ID). I'm thinking that we should do this step
by step and use a global namespace for now, with a given maximum number of
PWM devices (with the current API there is only a very limited number of
devices anyway) and modify or extend the API subsequently.

Thierry

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