using libgpio to bitbang LCDs!

[Luiz Otavio O Souza]

On 11 April 2015 at 22:00, Rui Paulo wrote:
> On Saturday 11 April 2015 17:11:47 Adrian Chadd wrote:
>> Hi!
>>
>> I just ported an adafruit LCD driver to FreeBSD. This was a pretty trivial
>> task:
>>
>> * convert C++ to C, which was done primarily to not rely on the
>> arduino-isms; * use libgpio to bitbang the SPI bus needed to write to the
>> LCD.
>>
>> It worked third time. First was "oh it runs on 5v power and 3.3v
>> signaling", and I had it hooked up to 3.3. Second was "Oh, I haven't
>> set the pins to be output pins yet." Third time worked - just slowly.
>>
>> However - using libgpio was just pleasant. Get a handle, set the pin
>> config, set the pins high/low. It was pretty damned wonderful.
>>
>> It turns out that to fill the screen with individual pixel writes
>> takes quite a few seconds pinning my AR9331 CPU doing ~ 190,000
>> syscalls a second. Ian's suggested something sensible - a bulk data
>> ioctl() that can bit bang a series of GPIO pins in the kernel. Ie, one
>> syscall, a big chunk of data with instructions and timings.
>
> Since this is an LCD, you probably want a way to export the GPIO bank via mmap
> with write-combining enabled.  It's not really a safe operation as it could
> hang the GPIO driver, but it could be useful.
>
>> The other interesting thing would be a kind of bulk pin set/get - ie,
>> instead of doing rmw for one pin at a time, communicate down masks of
>> pins to do together.
>
> I think that's sensible and could be done via a separate (new) ioctl.

I've patches for this (bank read and writing), it is pretty simple and
should not block other approaches (mmap, bulk transfers, ...).

Luiz