Hi Marcel,
On 07/25/2016 08:45 PM, Marcel Holtmann wrote:
Hi David,
short version: I have a use case for leds where I want to be able to use the
triggers in the leds subsystem without having a physical hardware led.
long version: I am working on a program to make one embedded system
(http://fatcatlab.com/product/evb/) compatible with another
(http://mindstorms.lego.com). One has physical red/green LEDs, that use the
gpio leds driver to control them. The other system does not have physical leds.
However, it does have a color screen. So, my idea is
to create virtual LEDs on the screen that emulate the physical LEDs on the
other device.
I would like to make a userspace program that works the same on both devices.
If the leds were simple on-off, then it would of course be simpler to make the
virtual leds completely in userspace. However, we are currently using other
triggers (disk activity/heatbeat/etc.) with the leds. I would like for the
virtual LEDs to be able to use these triggers as well.
this is funny since I have just written a ledsim.c using debugfs to emulate a
LED and read out its state via a simple cat command. I found that useful for
testing triggers in the Bluetooth subsystem and see if they behave correctly
without bothering to run this on real hardware.
Proposed implementation:
My thought is to create a new module that can be used to create user-defined
leds using configfs. Below is some sample code that I have been experimenting
with. Be aware, it does not quite match the description I am about to give.
In configfs you simply create a new node for each leds and assign the name. We
could have separate attributes for devicename, color and function to enforce
the naming convention or we could just allow arbitrary names.
Once the leds class devices are configured in configfs and exported, it will
create the usual leds class device in sysfs as well as a character device that
can be polled by the userspace program. The character device would work a bit
like evdev where any time the brightness is changed, it spits out an event on
the character device.
It would probably be simplest to have a single character device for all virtual
leds. In this case the leds device name or some other id would need to be part
of the event structure.
Any thoughts? Does this seem like a sane thing to do? Or maybe there is a
different way to accomplish the same thing using existing drivers?
So I used debugfs since I was lazy and only needed it for testing. However
doing something similar to what /dev/uinput, /dev/uhid, /dev/rfkill, /dev/vhci
etc. are doing seems to be reasonable here. What I like with these userspace
driven interfaces is that the process has to keep the character device open.
Once it closes it, then the kernel removes the data structures. So I would vote
for creating a /dev/uled.
However creating /dev/uled would lead to using a dedicated tool to drive it.
Which might be useful anyway since it could be easily extended with some easy
to use functionality for testing purposes. It would also be simple to create
unit test integration.
For my test of ledsim.c I opted for debugfs and cat just so that I can do it
really easily. I can share my code if that is interesting to people.
Yes, it would be nice to see your code, it is always beneficial
to compare other approaches.
--
Best regards,
Jacek Anaszewski
