Messages in this thread | | | From | David Brownell <> | Subject | Re: [RFC 2.6.27 1/1] gpiolib: add support for batch set of pins | Date | Mon, 29 Dec 2008 11:32:14 -0800 |
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On Saturday 27 December 2008, Jaya Kumar wrote: > Oh, gosh darn it, how time has flown. My email above was to make sure > I have understood the feedback. I assume I should just get started on > implementing. Just to double check, the plan is: > - add bitmask support. > - add get_batch support > - improve naming. I think gpio_get_batch/gpio_set_batch sounds good.
I was expecting to get some agreement on interfaces first.
> I plan to have something like: > > void gpio_set_batch(unsigned gpio, u32 values, u32 bitmask, int bitwidth); > u32 gpio_get_batch(unsigned gpio, u32 bitmask, int bitwidth);
I still don't like the word "batch" here. Did you look at the <linux/bitmask.h> interfaces as I suggested? They would suggest something rather different:
- passing bitmasks as {unsigned long *bits, int count} tuples - separate calls to: * zero a set of bits (loop: gpio_set_value to 0) * fill a set of bits (loop: gpio_set_value to 1) * copy a set of bits (loop: gpio_set_value to src[i]) * read a set of bits (loop: dst[i] = gpio_get_value) * ... maybe more
Any restriction to 32 bit value seems shortsighted. It would make sense to wrap the GPIO bitmask descriptions in a struct, letting drivers work with smaller sets -- probably most would fit into a single "unsigned long".
> I think I should explain the bitmask and bitwidth choice. The intended > use case is: > for (i=0; i < 800*600; i++) { > gpio_set_batch(...) > } > > bitwidth (needed to iterate and map to chip ngpios) could be > calculated from bitmask, but that involves iteratively counting bits > from the mask, so we would have to do 800*600 bit counts. Unless, we > do ugly things like cache the previous bitwidth/mask and compare > against the current caller arguments. Given that the bitwidth would > typically be a fixed value, I believe it is more intuitive for the > caller to provide it, eg: > > gpio_set_batch(DB0, value, 0xFFFF, 16) > > which has the nice performance benefit of skipping all the bit > counting in the most common use case scenario.
That doesn't explain the bit mask and bitwidth parameters at all.
> While we are here, I was thinking about it, and its better if I give > gpio_request/free/direction_batch a miss for now.
Let's not and say we didn't. Providing incomplete interfaces isn't really much help.
> Nothing prevents > those features being added at a later point. That way I can focus on > getting gpio_set/get_batch implemented correctly and merged as the > first step.
First step needs to be defining the interface extensions needed.
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