Udev is the best way to add persistent values for pretty much everything in the sysfs. That being said, it can be a bit obtuse when first learning about it. Here are some tips
udevadm test /sys/path/to/device will tell you if your rule is running and what the state is at each step. You’ll want to look at this before you start so you can see when your rule should run
udevadm info /sys/path/to/device will tell you what the PROPERTIES of a device are. These are usually set by hwdb files to inform userspace programs about the details of a device.
udevadm info /sys/path/to/device --attribute-walk will tell you about the ATTRIBUTES of a device and all it’s parent devices. These correspond to the character file endpoints you are setting currently. You’ll want to use these to write your match rules and set the values.
udevadm monitor can be used to watch for udev events to let you know if you should match on add, change, and/or remove.
Udev rules work as a cascading match system and they run in numerical and directory order. E.g. /usr/lib/udev/rules/60-keyboard.rules will run before /etc/udev/rules.d/62-keyboard.rules but after /etc/udev/rules.d/60-keyboard.rules
For user defined rules you will want to put them in /etc/udev/rules.d/ and keep in mind any state that needs to be set before or after your rule.
Matching happens with == or !=, setting attributes is done with =, +=, -=, or :=. := is really cool because you can use that to block changes from downstream rules. E.g. MODE:="666" will make the matched attribute r/w from unprivileged users, even if a later rule tries to set 400.
Udev rules will run in order in a file, but each rule must be a single line. Each attribute will also be set in order of the rule if setting multiple attributes in a rule. Multiple rules can be useful if you need to set attributes on multiple levels of a device, or in sibling directories.
Udev is the best way to add persistent values for pretty much everything in the sysfs. That being said, it can be a bit obtuse when first learning about it. Here are some tips
udevadm test /sys/path/to/devicewill tell you if your rule is running and what the state is at each step. You’ll want to look at this before you start so you can see when your rule should runudevadm info /sys/path/to/devicewill tell you what the PROPERTIES of a device are. These are usually set by hwdb files to inform userspace programs about the details of a device.udevadm info /sys/path/to/device --attribute-walkwill tell you about the ATTRIBUTES of a device and all it’s parent devices. These correspond to the character file endpoints you are setting currently. You’ll want to use these to write your match rules and set the values.udevadm monitorcan be used to watch for udev events to let you know if you should match on add, change, and/or remove.Udev rules work as a cascading match system and they run in numerical and directory order. E.g.
/usr/lib/udev/rules/60-keyboard.ruleswill run before/etc/udev/rules.d/62-keyboard.rulesbut after/etc/udev/rules.d/60-keyboard.rulesFor user defined rules you will want to put them in
/etc/udev/rules.d/and keep in mind any state that needs to be set before or after your rule.Matching happens with
==or!=, setting attributes is done with=,+=,-=, or:=. := is really cool because you can use that to block changes from downstream rules. E.g.MODE:="666"will make the matched attribute r/w from unprivileged users, even if a later rule tries to set 400.Udev rules will run in order in a file, but each rule must be a single line. Each attribute will also be set in order of the rule if setting multiple attributes in a rule. Multiple rules can be useful if you need to set attributes on multiple levels of a device, or in sibling directories.
For a complete breakdown of everything, see the udev manual: https://man.archlinux.org/man/udev.7
I also have a guide on one of my (currently out of tree) drivers that has some examples. https://github.com/ShadowBlip/ayn-platform?tab=readme-ov-file#changing-startup-defaults
Let me know if you have questions.