[LinuxPPS] PPS on Raspberry Pi - with device tree

[tlhackque]

I recently updated my RPi to the latest released kernel (4.1.19+ ),
which has PPS built-in and a module for the GPIO input driver.

This release uses device trees.  I ran into some problems.  Here is the
recipe for any who follow.

Device Tree will load pps-gpio, so remove it from /etc/modules.  Remove
any reference to pps from the kernel command line.

In config.txt, add

dtoverlay=pps-gpio,gpiopin=24  (Or whatever pin number you're using).

If (as I do), you require assert_falling_edge = true, you need to
customize the dtoverlay.

The driver supports it, but it's not in the DT overlay.  (A patch has
been committed by Phil Elwell.)

Meantime, to fix:

copy
https://raw.githubusercontent.com/raspberrypi/linux/b761ab196d84a93a8ca7e9cfb77ead85347a57a2/arch/arm/boot/dts/overlays/pps-gpio-overlay.dts
to your RPi.

apt-get install device-tree-compiler

Run

sudo dtc -@ -o pps-gpio-overlay.dtb -O dtb -I dts pps-gpio-overlay.dts

rename the existing pps-gpio-overlay.dtb in /boot/overlays to .bak, and

cp -p pps-gpio-overlay.dtb to /boot/overlays.

Add assert_falling_edge=true to config.txt, e.g.:

dtoverlay=pps-gpio,gpiopin=24,assert_falling_edge=true

Reboot, and you're in business.

In debugging this, I noticed that the driver doesn't have a
"capture_clear" parameter for the device tree.
I think one should be added - there is code for this on the "pdata"
path.  Something like:
if (of_get_property(np, "capture-clear", NULL))
            data->capture_clear = true;
at line 122 should do it.  Then the DT and pdata paths should be equivalent.

For reference, the RPi issue for this is
https://github.com/raspberrypi/linux/issues/1590

Enjoy.