Add spi-gpio support

[BjoernSch]

This would require to enable spi-gpio (and spi-bitbang as dependency) as module.
Afterwards you could add additional software SPI devices via device tree overlays on freely choosen GPIOs.

[pelwell]

Have you tried this on a Pi? How well does it work?

[BjoernSch]

I have tested it on a Raspberry Pi Zero W.
It works very good in my case, I tested it with an generic ADC + 2 thermocouple converter on a single bus.
I could provide a module working with the current kernel and the device tree overlay I used.

[pelwell]

I'm interested to see the overlay - we'd want to add one to the standard image to make the new driver usable.

[BjoernSch]

Here it is:
https://gist.github.com/BjoernSch/39d3bca950a3b025b806decc2b686bbc
(The choosen GPIOs may not be the best choice for the general public.)

[BjoernSch]

Current version of the overlay, includes debian packaging:
https://github.com/BjoernSch/dto-spi-gpio-wlanthermo/blob/master/spi-gpio-wlanthermo.dts
Building the module manually is still a real pain.
Enabling the option should "only" add two modules, as they are running with the stock kernel even if build on an external system, so no high costs and a lot of possibilities for SPI usage added.

[pelwell]

If you put the overlay in a Pull Request along with a README patch and the required config updates - see #2252 for a very recent example - it should get merged quickly.

[JamesH65]

@BjoernSch Did this PR happen?

[BjoernSch]

Not yet, but I will take care of it this week.

[JamesH65]

@pelwell I suspect since the referenced PR #2318 was apparently merged recently, this can be closed?

[pelwell]

Not exactly - we've enabled the required driver in the standard kernel builds, but users will need to modify and build their own overlays.

[dsvensson]

@pelwell Is the steps needed to be taken documented anywhere to get this working?

[pelwell]

1. Download an overlay (e.g. https://github.com/raspberrypi/linux/pull/2318/files#diff-b52f412d816937d23af7f07caa747039) - we'll call it spi-gpio-overlay.dts for the purposes of this example.
2. Edit the GPIOs (look for <&gpio ...>) as needed.
3. dtc -@ -I dts -O dtb -o spi-gpio.dtbo spi-gpio-overlay.dts. Don't worry about warnings - there are likely to be a few.
4. sudo cp spi-gpio.dtbo /boot/overlays
5. Add dtoverlay=spi-gpio to /boot/config.txt.
6. Reboot.

[dsvensson]

@pelwell is this kernel version available yet in Raspbian Stretch, or is there some nightly kernel repository?

My raspberrypi-kernel version is: 1.20180417-1

Linux raspberrypi 4.14.34-v7+ #1110 SMP Mon Apr 16 15:18:51 BST 2018 armv7l GNU/Linux