Re: 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 26.03.24 um 16:39 schrieb 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?
> TBH I didn't look at the OpenRGB code yet and LampArray there is currently
> only planned. I somewhat hope that until the kernel driver is ready someone
> else already picked up implementing LampArray in OpenRGB.
> > 
> > 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...
> 
> My first thought here also: What is if something else is reading hidraw devices?
> 
> Especially for hidraw devices that are not just LampArray.
> 
> > 
> > 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.
> 
> I think a read-only mode is not part of the current led class UAPI. Also I
> don't want to associate AutonomousMode=true with led class is used.
> AutonomousMode=true could for example mean that it is controlled via
> keyboard shortcuts that are directly handled in the keyboard firmware, aka a
> case where you want neither OpenRGB nor led class make any writes to the
> keyboard.
> 
> Or AutonomousMode=true could mean that on a device that implements both a
> LampArray interface as well as a proprietary legacy interface is currently
> controlled via the proprietary legacy interface (a lot of which are
> supported by OpenRGB).

Then how is the kernel supposed to handle LampArrays?

If you need the kernel to use a ledclass, the kernel will have to set
the device into AutonomousMode=false. When the kernel is done
configuring the leds, it can not switch back to AutonomousMode=true or
its config will likely be dumped by the device.

OpenRGB can open the device, switch it to AutonomousMode=false and we
can rely on it to do the right things as long as it is alive. But I do
not see how the kernel could do the same.

FWIW, I also have a couple of crazy ideas currently boiling in my head
to "solve" that but I'd rather have a consensus on the high level side
of things before we go too deep into the technical workaround.

Cheers,
Benjamin

> 
> Regards,
> 
> Werner
> 
> > 
> > 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