linux-kernel - Re: [PATCH 1/1] platform/x86/tuxedo: Add virtual LampArray for TUXEDO NB04 devices

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Message-ID: <cpt37kj2xqv6f5pb6bxgl53rxmmew6jdd647rsnrhowlphjq7i@dhp2655sl2sn>
Date: Thu, 3 Oct 2024 18:01:30 +0200
From: Benjamin Tissoires <bentiss@...nel.org>
To: Armin Wolf <W_Armin@....de>
Cc: Werner Sembach <wse@...edocomputers.com>, Pavel Machek <pavel@....cz>, 
	Hans de Goede <hdegoede@...hat.com>, Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>, 
	dri-devel@...ts.freedesktop.org, jelle@...aa.nl, jikos@...nel.org, lee@...nel.org, 
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org, linux-leds@...r.kernel.org, 
	miguel.ojeda.sandonis@...il.com, ojeda@...nel.org, onitake@...il.com, 
	platform-driver-x86@...r.kernel.org
Subject: Re: [PATCH 1/1] platform/x86/tuxedo: Add virtual LampArray for
 TUXEDO NB04 devices

On Oct 02 2024, Armin Wolf wrote:
> Am 02.10.24 um 10:42 schrieb Benjamin Tissoires:
> 
> > On Oct 01 2024, Werner Sembach wrote:
> > > Hi Armin,
> > > 
> > > Am 01.10.24 um 18:45 schrieb Armin Wolf:
> > [...snipped...]
> > > > Why not having a simple led driver for HID LampArray devices which exposes the
> > > > whole LampArray as a single LED?
> > > Yes that is my plan, but see my last reply to Benjamin, it might not be
> > > trivial as different leds in the same LampArray might have different max
> > > values for red, green, blue, and intensity. And the LampArray spec even
> > > allows to mix RGB and non-RGB leds.
> > > > If userspace wants to have direct control over the underlying LampArray device,
> > > > it just needs to unbind the default driver (maybe udev can be useful here?).
> > > There was something in the last discussion why this might not work, but i
> > > can't put my finger on it.
> > We recently have the exact same problem, so it's still fresh in my
> > memory. And here are what is happening:
> > - you can unbind the driver with a sysfs command for sure
> > - but then the device is not attached to a driver so HID core doesn't
> >    expose the hidraw node
> > - you'd think "we can just rebind it to hid-generic", but that doesn't
> >    work because hid-generic sees that there is already a loaded driver
> >    that can handle the device and it'll reject itself because it gives
> >    priority over the other driver
> > - what works is that you might be able to unload the other driver, but
> >    if it's already used by something else (like hid-multitouch), you
> >    don't want to do that. And also if you unload that driver, whenever
> >    the driver gets re-inserted, hid-generic will unbind itself, so back
> >    to square one
> > 
> > So unless we find a way to forward the "manual" binding to hid-generic,
> > and/or we can also quirk the device with
> > HID_QUIRK_IGNORE_SPECIAL_DRIVER[0] just unbinding the device doesn't
> > work.
> > 
> > Cheers,
> > Benjamin
> 
> I see, maybe we can add support for the driver_override mechanism to the HID bus?

hmm, we can, but only a couple of drivers would be valid: hid-multitouch
and hid-generic AFAICT. All of the others are device specific, so
allowing anybody to map a device to it might not work (if the driver
requires driver_data).

> Basically userspace could use the driver_override mechanism to forcefully bind hid-generic
> to a given HID device even if a compatible HID driver already exists.
> 

that coud be an option. But in that case, I wonder if the LampArray
implementation should be done in hid-led or in hid-input.c (the generic
part). I don't know if the new devices will export one HID device for
LampArray and one other for the rest, when the rest might need a
specific driver.

Anyway, thanks for the tip :)

Cheers,
Benjamin

> Thanks,
> Armin Wolf
> 
> > PS: brain fart:
> > if HID LampArray support (whatever the implementation, through Pavel's
> > new API or simple LED emulation) is in hid-input, we can also simply add
> > a new HID quirk to enable this or not, and use that quirk dynamically
> > (yes, with BPF :-P ) to rebind the device...
> > 
> > [0] https://lore.kernel.org/linux-input/20241001-hid-bpf-hid-generic-v3-0-2ef1019468df@kernel.org/T/#t