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

[Lee Jones <lee@kernel.org>]

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'.

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. 

> 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.

-- 
Lee Jones