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Configuring the Raspberry Pi

Reference material for kernel LIRC Device Interface

https://www.kernel.org/doc/html/v6.1/userspace-api/media/rc/lirc-dev.html

https://www.mess.org/2020/01/26/Moving-from-lirc-tools-to-rc-core-tooling/

Some tips

raspberrypi/linux#2993 (comment) (the beginning, not the part regarding lircd which is not relevant here)

Initial setup

sudo apt update
sudo apt full-upgrade -y
sudo reboot
sudo rpi-update # currently updates to rpi-6.6.y
sudo reboot
# optional
sudo apt install -y etckeeper
sudo apt install -y unattended-upgrades

HDMI no signal issue

My screen went black after initial boot messages. No login prompt. I found this trouble-shooting guide.

cat /sys/class/drm/card?-HDMI-A-1/edid

did not return an EDID, suggesting that KMS had problems reading the displays EDID. Appending the following video setting to /boot/firmware/cmdline.txt solved the problem:

video=HDMI-A-1:1280x720@60D

LIRC

No LIRC user space packages are needed. There are many example projects out there that use LIRC to receive and send IR signals, but it turns out that the LIRC packages are not needed for this project. Parts of LIRC have been moved into the kernel, and the remaining user space tools that were needed are now provided by the v4l-utils package (see the blog). The Panasonic inverter remote control does not transmit simple button presses, therefore the lircd functionality for translating IR pulse/space sequences into button presses is of no use. Instead we must ourselves read (and write) the raw LIRC data provided on the /dev/lirc devices by the kernel LIRC device drivers.

Configuring the IR modules

The kernel IR modules are overlays that are enabled by editing /boot/firmware/config.txt:

# InfraRed
dtoverlay=gpio-ir,gpio_pin=23
#dtoverlay=gpio-ir-tx,gpio_pin=18
dtoverlay=pwm-ir-tx,gpio_pin=18

Note that there are two different drivers for transmission:

  • gpio-ir-tx uses what is known as "bit-banging", where the processor must control the IR output directly. I was not able to get this to work.
  • pwm-ir-tx uses PWM to control the IR output. This does not require as much of the processor, but it still needs to trigger the edges, and is therefore sensitive to delays.

Note

Regarding the pwm-ir-tx, the duration of pulses and spaces is not sufficiently precise in kernel 6.1 and 6.6, and I had problems where transmissions were not reliably received by the inverter. See below for kernel patches that increase the precision.

To be able to test both transmission drivers, I chose GPIO pin 18 for output, since it can be used with PWM. In the end, I was only able to get PWM to work.

Note that the receiver output is high at rest and drops to low when it receives IR light. The GPIO pin should therefore be configured with pull-up, which is the default for the gpio-ir module. I chose GPIO pin 23 for the input, becasue it was physically close to pin 18.

Configuring udev

A problem when both the receiver and transmitter modules are enabled is that the kernel will create to devices, /dev/lirc0 and /dev/lirc1, but there is no direct way to know which is which. Therefore it is useful to add udev rules like this:

/etc/udev/rules.d/70-lirc.rules

ACTION=="add", SUBSYSTEM=="lirc", DRIVERS=="gpio_ir_recv", SYMLINK+="lirc-rx"
ACTION=="add", SUBSYSTEM=="lirc", DRIVERS=="gpio-ir-tx", SYMLINK+="lirc-tx"
ACTION=="add", SUBSYSTEM=="lirc", DRIVERS=="pwm-ir-tx", SYMLINK+="lirc-tx"

These rules will match on the driver name, and create symbolic links that point to the associated device. This allows you to use /dev/lirc-rx for reception and /dev/lirc-tx for transmission.

Configuring limits

To allow running a process with a higher priority, the following can be added:

/etc/security/limits.d/pi.conf

pi     -       nice    -20

This will allow the pi user to set process niceness all the way to -20 (the highest priority).

Configuring unattended upgrades

/etc/apt/apt.conf.d/50unattanded-upgrades

diff --git a/apt/apt.conf.d/50unattended-upgrades b/apt/apt.conf.d/50unattended-upgrades
index 7fbd3d4..b26d125 100644
--- a/apt/apt.conf.d/50unattended-upgrades
+++ b/apt/apt.conf.d/50unattended-upgrades
@@ -26,11 +26,12 @@ Unattended-Upgrade::Origins-Pattern {
         // archives (e.g. from testing to stable and later oldstable).
         // Software will be the latest available for the named release,
         // but the Debian release itself will not be automatically upgraded.
-//      "origin=Debian,codename=${distro_codename}-updates";
+        "origin=Debian,codename=${distro_codename}-updates";
 //      "origin=Debian,codename=${distro_codename}-proposed-updates";
         "origin=Debian,codename=${distro_codename},label=Debian";
         "origin=Debian,codename=${distro_codename},label=Debian-Security";
         "origin=Debian,codename=${distro_codename}-security,label=Debian-Security";
+        "a=stable,c=main,o=Raspberry Pi Foundation,l=Raspberry Pi Foundation";

         // Archive or Suite based matching:
         // Note that this will silently match a different release after
@@ -45,7 +46,7 @@ Unattended-Upgrade::Origins-Pattern {
 // Python regular expressions, matching packages to exclude from upgrading
 Unattended-Upgrade::Package-Blacklist {
     // The following matches all packages starting with linux-
-//  "linux-";
+    "linux-";

     // Use $ to explicitely define the end of a package name. Without
     // the $, "libc6" would match all of them.
@@ -112,7 +113,7 @@ Unattended-Upgrade::Package-Blacklist {

 // Automatically reboot *WITHOUT CONFIRMATION* if
 //  the file /var/run/reboot-required is found after the upgrade
-//Unattended-Upgrade::Automatic-Reboot "false";
+Unattended-Upgrade::Automatic-Reboot "true";

 // Automatically reboot even if there are users currently logged in
 // when Unattended-Upgrade::Automatic-Reboot is set to true

Using a custom Raspberry Pi OS kernel

At the time of writing (February 2024), patches for pwm-ir-tx using hrtimers (high resolution timers) have been merged into Linux kernel 6.8. Raspberry Pi OS is still based on 6.1, on the verge of upgrading to 6.6, so the pwm-ir-tx patches are not yet available. The patches were published e.g. [PATCH v10 0/6] Improve pwm-ir-tx precision, and can be found in Raspberry Pi OS branch rpi-6.8.y. These are the patches:

  1. c748a6d77c06 - pwm: Rename pwm_apply_state() to pwm_apply_might_sleep() (2023-12-20) <Sean Young>
  2. dc518b378dce - pwm: Replace ENOTSUPP with EOPNOTSUPP (2023-12-20) <Sean Young>
  3. 752193da3f8b - pwm: renesas: Remove unused include (2023-12-20) <Sean Young>
  4. 7170d3beafc2 - pwm: Make it possible to apply PWM changes in atomic context (2023-12-20) <Sean Young>
  5. fcc760729359 - pwm: bcm2835: Allow PWM driver to be used in atomic context (2023-12-20) <Sean Young>
  6. 363d0e56285e - media: pwm-ir-tx: Trigger edges from hrtimer interrupt context (2023-12-20) <Sean Young>
  7. 346c84e281a9 - media: pwm-ir-tx: Depend on CONFIG_HIGH_RES_TIMERS (2024-02-01) <Sean Young>

Backporting these patches to branch rpi-6.6.y makes the pwm-ir-tx kernel module work much better. Before this patch I had to re-send the same message at least four times in the hope that one of the transmissions would be accepted by the inverter unit.

I have created an issue to request that these patches be backported to kernel 6.6.

Building a custom kernel: https://www.raspberrypi.com/documentation/computers/linux_kernel.html#kernel

Installing a custom 6.6 kernel

Notes on installing a custom kernel8-pwm.img kernel. During the build a custom LOCALVERSION was set, which generates a unique name for the modules directory. This simplifies installation.

DATE=$(date +'%Y-%m-%d')

sudo cp -a /boot/firmware /boot/firmware.backup.$DATE
sudo cp -r BUILD/boot/firmware /boot/
sudo cp -r BUILD/lib/modules/6.6.* /lib/modules/

Add kernel=kernel8-pwm.img to /boot/firmware/config.txt.