Wednesday, March 20, 2024

Converting GPIO from the legacy sysfs ABI to the ioctl ABI in Diminuto and Hazer

It could be that no one but me is using my "Hazer" GNSS library and toolkit, or the "Diminuto" C-based systems programming library on which it depends. But just in case: I'm close to finishing testing of the develop branch of both repos, both of which have some major changes to how General Purpose Input/Output (GPIO) - the generic term for controlling digital input and output pins in software - and merging develop back into the master branch.

This was all motivated by my being one of the lucky few to get a backordered Raspberry Pi 5, and putting the latest version of Raspberry Pi OS, based on Debian "bookworm" and Linux 6.1, on it, only to find when unit and functional testing my code that the deprecated sysfs-based GPIO ABI no long worked. This wasn't a big surprise - I had read at least two years ago that the old ABI was being phased out in favor of a new ioctl-based ABI. My code makes heavy use of GPIO for a lot of my stuff, e.g. interrupts from real-time sensors, the One Pulse Per Second (1PPS) signal from GNSS receivers, status LEDS, etc. So it was finally time to bite the bullet and replace all the places where I used the sysfs-based Diminuto Pin feature (diminuto_pin) with a new ABI using the ioctl. Hence, the Diminuto Line (borrowing a term from the new ABI) feature (diminuto_line).

Line is now used in place of Pin in all of the Diminuto functional tests that run on  hardware test fixtures I wired up many years ago just for this purpose, and all the functional tests work. The Hazer gpstool utility has similarly been converted to using Line instead of Pin and has been tested with an Ardusimple board using a u-blox UBX-NEO-F10T GNSS receiver.


(That's a Pi4 on the left connected to my test fixture, and a Pi5 on the right connected to the GNSS receiver.)

Two complaints.

[1] The new ABI is woefully under documented. However, I found some code examples in the Linux kernel source repo under tools/gpio that were essential to my understanding. (I chose not to use the new-ish libgpiod for my work, for reasons, but that is a story for another time. I have no reason to believe that it's not perfectly fine to use.)

[2] The way the ioctl GPIO driver is implemented on older versus newer Raspberry Pi OS versions makes it difficult - I am tempted to say impossible, but maybe I'm just not that smart - to write code that easily runs on either platform using the new ABI. Specifically, the GPIO device drivers in the OSes use a different symbolic naming scheme, making it impossible for application code to select the correct GPIO device and line offset portably on the two platforms. But maybe I'm just missing something. (I hope so.)

I like the new ioctl ABI, and expect to use it exclusively moving forward, even though this will orphan Pis I have that might run older versions of the OS. (I think I have at least one example of every Pi version ever sitting around the Palatial Overclock Estate. Ten of them run 24x7 in roles such as a web server, an Open Street Map server, a Differential GNSS base station, and an NTP server with a cesium atomic clock for holdover.) I have tagged the last version of both repos that still use the sysfs ABI.

That's it.

Update (2024-03-23)

I merged the develop branch in to the master branch this morning. Both the Diminuto build and the Hazer build passed sanity and functional testing (and I'm currently running the long-running "geologic" unit test suite against Diminuto). I had tagged the old master branch in both repos with the name sysfsgpio in case I needed to build them, but I don't anticipate any further development of the old code.

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