Re: [PATCH 00/13] HID: ft260: add UART and GPIO support, plus I2C fixes

[Benjamin Tissoires <bentiss@kernel.org>]

On Sep 17 2026, Lee Jones wrote:
> On Thu, 17 Sep 2026, Benjamin Tissoires wrote:
> 
> > On Sep 16 2026, Lee Jones wrote:
> > > On Mon, 14 Sep 2026, Linus Walleij wrote:
> > > 
> > > > Hi Benjamin, Lee,
> > > > 
> > > > On Tue, Sep 1, 2026 at 4:03 PM Benjamin Tissoires <bentiss@kernel.org> wrote:
> > > > 
> > > > > TBH, I'm not a big fan of having multiple subsystems children into HID.
> > > > > Mostly because I can't review the best practive in each of them. However,
> > > > > for quite a long time, HID was mostly for input devices, and input is a
> > > > > different subsystem.
> > > > >
> > > > > That being said, there are 2 types of HID devices:
> > > > > - ones with defined standard usages (keyboards, mice, touchscreen,
> > > > >         battery, etc) and using MFD for those would certainly be overthinking
> > > > > - others use raw HID device with a custom protocol (cp2112, mcp2221,
> > > > >         ft260), these could be MFD candidates
> > > > 
> > > > Surely, as HID start to attract chips which clearly fall into the MFD
> > > > category of things, with a plethora of subsystems hooking into the
> > > > same HID device, we must find a way for HID devices to spawn
> > > > MFD cells?
> > > > 
> > > > MFD solved and evolved a system for handling exactly this type
> > > > of situation.
> > > > 
> > > > Whether there should be an MFD device in the middle spawning
> > > > each a HID, GPIO, I2C, UART cell or whether HID device itself should
> > > > sit in the nexus and gain the ability to simply spawn out MFD cells
> > > > from itself is what we need to figure out.
> > > 
> > > There's no figuring that part out.
> > > 
> > > If you want to use the MFD API, the part that uses it must reside in
> > > drivers/mfd.  Else it becomes a nightmare to maintain and things get
> > > wild, quickly.
> > 
> > Historically speaking, HID devices always have been under drivers/hid.
> > They are usually leaf drivers, only hooking to input/battery/LEDs.
> > 
> > As mentioned, hid-sensor-hub is an exception but this got sorted out by
> > splitting the HID part from the IIO.
> > 
> > Looking at the various MFD-like HID drivers, they all share the common
> > point of not really being HID devices: they rely on a custom protocol
> > handled in .raw_event().
> > 
> > So I think I'd be OK to have those drivers moved to the mfd tree as long
> > as they only rely on low level HID API, and do not have to deal with
> > "generic" HID report descriptors. If that arise, I think we should split
> > the HID/MFD parts like hid-sensor-hub does.
> > 
> > So you can have my unformal acked-by for transfering those drivers.
> 
> A couple of caveats to this:
> 
> Only the most basic set-up parts need to go into MFD (shared resource
> allocation, core device initialisation: clocks, resets, regulators,
> IRQs, etc.  Anything that does a 'thing' needs to reside in the
> applicable sub-system that supports the 'thing'.

Sure, as long as I'm not responsible for those :)

> 
> Device need to straddle at least 2 additional subsystems to qualify as
> an MFD which undoubtibly would be HID and then something else: gpio,
> regulator, uart, iio, clk, what have you. 

I think HID is just the transport layer (like USB, I2C, SPI, etc...) so
it shouldn't count as one of the additional subsystems to MFD.

> 
> > Last, I'm currently using a cp2112 as a i2c-hid bridge in my upstream
> > CI. It's relying on a DSDT override in the VM to actually work, and I'd
> > like to keep that around. So my main request here is to keep the
> > equivalent of the DSDT override in the MFD cells, at least for this one.
> 
> I have no idea what that means, but we can discuss it.  Maybe this lives
> in some form of platform data?  I'd be able to advise better when I know
> what it is.

I apply the following DSDT to the VM that is running the test device:
https://gitlab.freedesktop.org/bentiss/gitlab-kernel-ci/-/blob/master/VM/0004-Add-CP2112-SSDT-override.patch?ref_type=heads#L51

Basically, this converts the CP2112 from being a simple USB device to a
known I2C + GPIO device by the ACPI table, and where I can then attach
another I2C child device (Device (TPD0) in the DSDT).
It's the same than adding a device tree to a platform where the I2C bus
is connected to a USB bus but is soldered on the chip.

My only constraint here is that the children of the MFD node properly
have the fwnode support. See commit c0be05f68a21 ("HID: cp2112: Add
fwnode support") for when this was added upstream.

I would assume MFD correctly sets fwnodes for the children devices it
handles, and hopefully it's working fine with ACPI as well.

Cheers,
Benjamin