David Brownell wrote: > On Friday 13 November 2009, Grant Likely wrote: > >> 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. >> > > PWM is not GPIO, and doesn't fit into a GPIO framework. > > Since *everything* boils down to one or more signal lines, > your argument leads directly to Linux having no native > hardware interface except GPIOs. Not ... practical. ;) > Wait. Isn't that what Ubicom's chips do? :) > If you want to combine PWM with something else ... timers would > be a better target. They're both fundamentally about periodic > phenomena. And quite a lot of timers support PWM output modes... > > (A generic interface to hardware timers is lacking, too.) > True, and I have code (not yet published) to support a couple of timer/counter peripherals under the PWM interface. So for that functionality at least, I don't think a more generic or standalone API is necessary. I don't know how to define a generic interface for the counter a.k.a. "input capture" behavior of such devices, though. Still an unsolved problem, but I don't think it will be a part of the PWM API. It's a different metaphor. > GPIOs, on the other hand, are packaged for manual bit > twiddling. While it's possible to create low-speed > implementations of serial protocols using GPIOs (like > 2-wire/I2C, one-wire, and various SPI variants), those > are explicitly the high-overhead (and low performance) > substitutes, to be used only when native hardware isn't > available (or is broken etc). > And when using GPIO to generate I2C or SPI signals, you don't do it through the GPIO interface--- you do it through the I2C or SPI interface, and a driver behind that API talks to the GPIO API.... or to a different driver if you change your mind. Same idea for the PWM API. -- Bill Gatliff bgat@xxxxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-embedded" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
