On Tue, Nov 17, 2009 at 1:27 AM, David Brownell <david-b@xxxxxxxxxxx> 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. ;) I think you've missed my point and taken it to an illogical extreme to counter it. I agree that PWMs are not GPIOs and visa versa. However, *some* devices are both GPIOs and PWMs. Also what is needed to manage GPIO and PWM pins is pretty much identical. >> The difference being >> that PWMs cannot do input, and has additional capabilities (can be >> programmed with a signal; not just on/off/tristate) > > 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.) > > >> What is the reason for bringing in an entirely new framework instead >> of extending the GPIO API or gpiolib? I'm not too excited about >> having two entirely different frameworks for what basically boils down >> to "numbered signal pins". > > You seem to mis-understand what PWM is all about, then. > The whole point of a PWM is to set up a periodic activity > that will run without CPU intervention. I understand that. > 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). But that *isn't* the primary purpose of the GPIO subsystem. All that stuff is layered on top of the GPIO pin management code and doesn't really play into this debate. g. -- Grant Likely, B.Sc., P.Eng. Secret Lab Technologies Ltd. -- 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
