Raspberry Pi GPIO module for LuaJIT

[Mike Pall <mikelu-1204@...>]

The Raspberry Pi foundation sent me a free RPi. Many thanks to
Alex Bradbury for arranging this! As expected, LuaJIT runs just
fine on the little device. :-)

To give something back, I've written a simple GPIO module, which
allows you to control the GPIO pins from a Lua script. It uses the
LuaJIT FFI and doesn't need any extra C modules!

Provided you've already compiled LuaJIT for the RPi, here's how to
install the module:
  mkdir -p /usr/local/share/lua/5.1/rpi
  cp gpio.lua /usr/local/share/lua/5.1/rpi/gpio.lua

The attached sample script testgpio.lua blinks an LED + resistor
attached to GPIO 0 and reads the input status of GPIO 6.

[Yes, you need to be root to run that, since it works without the
help of a driver, by writing directly to the device.]

You can read/write the pins with the gpio.pin[] table. These
correspond to the numbering given on the following drawing of the
GPIO connector on the RPi: http://elinux.org/File:GPIOs.png

There's also a raw API for accessing all of the GPIO pins on the
chip. Please read the source for details and do NOT use raw pin
numbers, unless you know what they do! Don't blame me, if you fry
your RPi -- you're on your own.

Arguably, the module is quite primitive right now. There's much
more you can do with the chip. Docs are available from:
  http://elinux.org/RPi_Low-level_peripherals

Since I'm already way too busy, I'd appreciate it if someone would
step forward and take over development, put it under version
control, package it up and so on. The current sample module is
public domain. I guess the Lua community would probably most
welcome a MIT licensed module. Thank you!

--Mike

-- Raspberry Pi GPIO module.
-- Written by Mike Pall. Public domain.

local setmt = setmetatable
local bit = require("bit")
local band, bor, shl, shr = bit.band, bit.bor, bit.lshift, bit.rshift
local ffi = require("ffi")
local C = ffi.C

ffi.cdef[[
int open(const char *path, int flags, int mode);
void *mmap(void *addr, size_t length, int prot, int flags, int fd, size_t ofs);
int munmap(void *addr, size_t length);
int poll(struct pollfd *fds, unsigned long nfds, int timeout);
]]

local function gpio_open()
  local fd = C.open("/dev/mem", 2, 0)
  if fd < 0 then
    error("You must be root to control the GPIO pins on the Raspberry Pi")
  end
  local gp = ffi.cast("volatile int32_t *",
		      C.mmap(nil, 4096, 3, 1, fd, 0x20200000))
  if ffi.cast("intptr_t", gp) == -1 then
    error("Mapping of GPIO registers failed")
  end
  return gp
end

local gp = ffi.gc(gpio_open(), function(gp)
  -- Restore default values.
  gp[0] = 0x00040000; gp[1] = 0x00064000; gp[2] = 0x00000000
  gp[10] = 0x03e6cf93
  C.munmap(ffi.cast("void *", gp), 4096)
end)

module(...)

-- WARNING: You may only use one of the following raw pin numbers!
-- 0, 1, 4, 7, 8, 9, 10, 11, 14, 15, 17, 18, 21, 22, 23, 24, 25
-- These correspond to the BCM2835 GPIO numbering scheme.

-- Set direction of raw GPIO pin. 0 = input, 1 = output.
local function _rawdir(pin, dir)
  local idx = pin % 10
  local reg = (pin - idx) / 10
  gp[reg] = bor(band(gp[reg], shl(7, idx*3)), shl(dir, idx*3))
end
rawdir = _rawdir

-- Turn raw GPIO output pin on.
function rawon(pin)
  gp[7] = shl(1, pin)
end

-- Turn raw GPIO output pin off.
function rawoff(pin)
  gp[10] = shl(1, pin)
end

-- Get state of raw GPIO input pin.
function rawin(pin)
  return band(shr(gp[13], pin), 1)
end

-- User-level (non-raw) GPIO API. The numbers correspond to the numbering
-- on the GPIO connector drawing: http://elinux.org/File:GPIOs.png
-- Input/output direction is automatically selected.

local function pinerror(t, i)
  error("bad GPIO pin number "..tostring(i), 2)
end
local rawmap = {[0]=17, 18, 21, 22, 23, 24, 25, 4}
local inmap = setmt({[0]=-1,-1,-1,-1,-1,-1,-1,-1,-1}, {__index=pinerror})
local outmap = setmt({[0]=-1,-1,-1,-1,-1,-1,-1,-1,-1}, {__index=pinerror})

pin = setmt({}, {
  __index = function(t, i)
    local pin = inmap[i]
    if pin < 0 then
      pin = rawmap[i]; inmap[i] = pin; outmap[i] = -1; _rawdir(pin, 0)
    end
    return band(shr(gp[13], pin), 1)
  end,
  __newindex = function(t, i, v)
    local pin = outmap[i]
    if pin < 0 then
      pin = rawmap[i]; outmap[i] = pin; inmap[i] = -1; _rawdir(pin, 1)
    end
    gp[10-band(v, 1)*3] = shl(1, pin)
  end,
})

function msleep(ms)
  C.poll(nil, 0, ms)
end

local gpio = require("rpi.gpio")

for i=1,10 do
  print("GPIO 6:", gpio.pin[6])
  gpio.pin[0] = 1
  gpio.msleep(100)
  gpio.pin[0] = 0
  gpio.msleep(100)
end