Re: [PATCH 0/6] Generic PWM API implementation

[Grant Likely <grant.likely@secretlab.ca>]

On Tue, Nov 17, 2009 at 8:39 AM, Bill Gatliff <bgat@billgatliff.com> wrote:
> Grant Likely wrote:
>> Hi Bill
>>
>> On Wed, Oct 15, 2008 at 11:14 AM, Bill Gatliff <bgat@billgatliff.com> wrote:
>>
>>> This series implements a Generic PWM Device API, including reference
>>> implementations for the Atmel PWMC device, an LED device, and an LED
>>> trigger.  It is based on linux-2.6.27.
>>>
>> [...]
>>
>>> The implementation of the Generic PWM Device API is structurally
>>> similar to the generic GPIO API, except that the PWM code uses
>>> platform bus_id strings instead of integers to identify target
>>> deviices.  A configuration structure is also provided, both to
>>> facilitate atomic hardware state changes and so that the API can be
>>> extended in a source-code-compatible way to accomodate devices with
>>> features not anticipated by the current code.
>>>
>>
>> Hey Bill,
>>
>> I'm concerned about the approach taken here.  As I understand it, the
>> PWM signals are very similar to GPIOs in that each PWM device controls
>> an external signal line, just like GPIO lines.  The difference being
>> that PWMs cannot do input, and has additional capabilities (can be
>> programmed with a signal; not just on/off/tristate).  Actually, many
>> GPIOs have these properties too.  I've got a part with output-only
>> gpios, and gpio devices that also have a PWM.
>
> It's true that PWM signals can be emitted via GPIO pins, and in fact the
> API code I submitted uses the GPIO API combined with an hrtimer to
> generate a low-speed PWM output.
>
> It's also true that things like LEDs might want to be driven by PWM
> signals, and indeed LEDs can also be driven by GPIO pins.
>
> Finally, it's accurate to say that a lot of microcontrollers multiplex
> GPIO functions onto pins that also provide PWM functionality.
>
> But in my opinion, none of the above means that the PWM API and GPIO API
> should be coupled at the interface abstraction.  All it means is that in
> some cases, one implementation might be able to "stand in" as an
> embodiment of the other's interface.  And we all agree on that for PWM
> and GPIO.

I have no problem with the PWM API.  I think it describes most of the
PWM *usage* requirements well, and I agree that GPIO devices can never
implement the PWM API.

> Given a PWM-capable chip that you control over an I2C or SPI interface,
> I don't think anyone would argue that such chips should be viewed as an
> extension of the SPI or I2C APIs just because the chips use those
> communications protocols.

Nor would I.

> I think it's a similar argument for PWM and GPIO.

I don't think that's true, but I'm approaching it from the viewpoint
of managing pins; not the usage API.  ie. the difference between
gpiolib (management) and the gpio api (usage).

g.

-- 
Grant Likely, B.Sc., P.Eng.
Secret Lab Technologies Ltd.