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debian_kernel_rebuild.md

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Recompile debian kernel to support APU2 GPIO

http://www.pcengines.ch/howto.htm#gpio

Get appropriate debian kernel source, e.g. from linux-source-* package.

Copy source to host PC and compile with gpio module included:

CONFIG_GPIO_NCT5104D=m
CONFIG_GPIO_SYSFS=y

make menuconfig
sudo apt-get install debhelper
sudo apt-get install modutils
sudo apt-get install kernel-package
make-kpkg clean
fakeroot make-kpkg  -j8 --initrd --revision=1.2.custom kernel_image

Copy .deb into sd card, boot and install:

dpkg-i linux-image-3.16.36_1.3.custom_amd64.deb

vi /etc/modules

add:

gpio-nct5104d

cd /sys/class/gpio
echo 0 > export
cd gpio0
echo out > direction
echo 1 > value

First test

  1. Export gpio17:
cd /sys/class/gpio
echo 17 > export
  1. Check it's value and direction:
root@debian:/sys/class/gpio/gpio17# cat direction 
in
root@debian:/sys/class/gpio/gpio17# cat value 
0
  1. Change to out and 1:
root@debian:/sys/class/gpio/gpio17# echo out > direction 
root@debian:/sys/class/gpio/gpio17# cat direction 
out
root@debian:/sys/class/gpio/gpio17# echo 1 > value 
root@debian:/sys/class/gpio/gpio17# cat value 
1

At this point voltage on pin rises from 0V to 0.8V.

  1. Reboot:
reboot

During coreboot-bios phase voltage rises to 1.2V.

When OS boots into login propmt voltage drops back to 0.8V.

  1. Check gpio17 direction and value:
cd /sys/class/gpio
echo 17 > export
cd gpio17

root@debian:/sys/class/gpio/gpio17# cat direction 
in
root@debian:/sys/class/gpio/gpio17# cat value 
1

It claims to be an input in high state. But the voltage is still 0.8V so it is actually an output (?)

That's because gpios are configured by default as OD

  1. Reboot again with those settings and check:
root@debian:/sys/class/gpio/gpio17# cat direction 
in
root@debian:/sys/class/gpio/gpio17# cat value 
1
  1. Set output to low (0V and reboot)
root@debian:/sys/class/gpio/gpio17# cat direction 
in
root@debian:/sys/class/gpio/gpio17# cat value 
0

After reboot voltage on this pin is 0V, even if boot into other OS. Voltage on the rest of the pins is ~0.9V.

After coldboot voltage is back at ~0.9V.

APU1 test

Voyage on USB stick, without gpio kernel module Debian on SDcard, with gpio kernel module

Pins as GPIO

  1. Log into Debian, set pin17 as output with value = 0. Voltage = 0V.
  2. Reboot into Debian. Voltage = 0V for the entire time of reboot as well as in the OS.
  3. Reboot into Voyage. Voltage = 0V for the entire time of reboot as well as in the OS.

Pins disabled

  1. Log into Debian, set pin17 as output with value = 0. Voltage = 0V.
  2. Reboot into Debian. Voltage rises to 1.2V for the entire time of reboot but drops to 0 when login prompt appears (when gpio module is loaded ? it initialises pins as GPIOs).
  3. Reboot into Voyage. Voltage = 1.2V for the entire time of reboot as well as in the OS.