In kernel virtual HID devices (was Future handling of complex RGB devices on Linux v3)

[Benjamin Tissoires]

On Mar 26 2024, Werner Sembach wrote:
> Hi all,
> 
> Am 25.03.24 um 19:30 schrieb Hans de Goede:
> 
> [snip]
> > > > If the kernel already handles the custom protocol into generic HID, the
> > > > work for userspace is not too hard because they can deal with a known
> > > > protocol and can be cross-platform in their implementation.
> > > > 
> > > > I'm mentioning that cross-platform because SDL used to rely on the
> > > > input, LEDs, and other Linux peculiarities and eventually fell back on
> > > > using hidraw only because it's way more easier that way.
> > > > 
> > > > The other advantage of LampArray is that according to Microsoft's
> > > > document, new devices are going to support it out of the box, so they'll
> > > > be supported out of the box directly.
> > > > 
> > > > Most of the time my stance is "do not add new kernel API, you'll regret
> > > > it later". So in that case, given that we have a formally approved
> > > > standard, I would suggest to use it, and consider it your API.
> > > The only new UAPI would be the use_leds_uapi switch to turn on/off the backwards compatibility.

I have my reserves with such a kill switch (see below).

> > Actually we don't even need that. Typically there is a single HID
> > driver handling both keys and the backlight, so userspace cannot
> > just unbind the HID driver since then the keys stop working.

I don't think Werner meant unbinding the HID driver, just a toggle to
enable/disable the basic HID core processing of LampArray.

> > 
> > But with a virtual LampArray HID device the only functionality
> > for an in kernel HID driver would be to export a basic keyboard
> > backlight control interface for simple non per key backlight control
> > to integrate nicely with e.g. GNOME's backlight control.
> 
> Don't forget that in the future there will be devices that natively support
> LampArray in their firmware, so for them it is the same device.

Yeah, the generic LampArray support will not be able to differentiate
"emulated" devices from native ones.

> 
> Regards,
> 
> Werner
> 
> > And then when OpenRGB wants to take over it can just unbind the HID
> > driver from the HID device using existing mechanisms for that.

Again no, it'll be too unpredicted.

> > 
> > Hmm, I wonder if that will not also kill hidraw support though ...
> > I guess getting hidraw support back might require then also manually
> > binding the default HID input driver.  Bentiss any input on this?

To be able to talk over hidraw you need a driver to be bound, yes. But I
had the impression that LampArray would be supported by default in
hid-input.c, thus making this hard to remove. Having a separate driver
will work, but as soon as the LampArray device will also export a
multitouch touchpad, we are screwed and will have to make a choice
between LampArray and touch...

> > 
> > Background info: as discussed earlier in the thread Werner would like
> > to have a basic driver registering a /sys/class/leds/foo::kbd_backlight/
> > device, since those are automatically supported by GNOME (and others)
> > and will give basic kbd backlight brightness control in the desktop
> > environment. This could be a simple HID driver for
> > the hid_allocate_device()-ed virtual HID device, but userspace needs
> > to be able to move that out of the way when it wants to take over
> > full control of the per key lighting.

Do we really need to entirely unregister the led class device? Can't we
snoop on the commands and get some "mean value"?

> > 
> > Regards,
> > 
> > Hans
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > > The control flow for the whole system would look something like this:
> > > 
> > > - System boots
> > > 
> > >      - Kernel driver initializes keyboard (maybe stops rainbowpuke boot effects, sets brightness to a default value, or initializes a solid color)
> > > 
> > >      - systemd-backlight restores last keyboard backlight brightness
> > > 
> > >      - UPower sees sysfs leds entry and exposes it to DBus for DEs to do keyboard brightness handling
> > > 
> > > - If the user wants more control they (auto-)start OpenRGB
> > > 
> > >      - OpenRGB disables sysfs leds entry via use_leds_uapi to prevent double control of the same device by UPower
> > > 
> > >      - OpenRGB directly interacts with hidraw device via LampArray API to give fine granular control of the backlight
> > > 
> > >      - When OpenRGB closes it should reenable the sysfs leds entry

That's where your plan falls short: if OpenRGB crashes, or is killed it
will not reset that bit.

Next question: is OpenRGB supposed to keep the hidraw node opened all
the time or not?

If it has to keep it open, we should be able to come up with a somewhat
similar hack that we have with hid-steam: when the hidraw node is
opened, we disable the kernel processing of LampArray. When the node is
closed, we re-enable it.

But that also means we have to distinguish steam/SDL from OpenRGB...

I just carefully read the LampArray spec. And it's simpler than what
I expected. But the thing that caught my attention was that it's
mentioned that there is no way for the host to query the current
color/illumination of the LEDs when the mode is set to
AutonomousMode=false. Which means that the kernel should be able to
snoop into any commands sent from OpenRGB to the device, compute a mean
value and update its internal state.

Basically all we need is the "toggle" to put the led class in read-only
mode while OpenRGB kicks in. Maybe given that we are about to snoop on
the commands sent, we can detect that there is a LampArray command
emitted, attach this information to the struct file * in hidraw, and
then re-enable rw when that user closes the hidraw node.

Cheers,
Benjamin

> > > 
> > > - System shutdown
> > > 
> > >      - Since OpenRGB reenables the sysfs leds entry, systemd-backlight can correctly store a brightness value for next boot
> > > 
> > > Regards,
> > > 
> > > Werner
> > > 
> > > > Side note to self: I really need to resurrect the hidraw revoke series
> > > > so we could export those hidraw node to userspace with uaccess through
> > > > logind...
> > > > 
> > > > Cheers,
> > > > Benjamin