Re: [PATCH v2 2/2] mfd: Add initial synology microp driver

[Lee Jones <lee@kernel.org>]

On Mon, 09 Mar 2026, Markus Probst wrote:

> On Mon, 2026-03-09 at 14:32 +0100, Danilo Krummrich wrote:
> > On Mon Mar 9, 2026 at 1:52 PM CET, Markus Probst wrote:
> > > Yes. I will split it into multiple drivers using the aux bus in the
> > > next revision.
> > 
> > Independent of the other discussion whether this belongs into the kernel in the
> > first place, reading over the cover letter and commit message I understood the
> > following.
> > 
> >   "Synology uses a microcontroller in their NAS devices connected to a serial
> >   port [...]" controlling LEDs, fan speeds, a beeper, etc.
> > 
> >   I.e. it muliplexes several physical functions that belong to different
> >   subsystems, such as hwmon, input, etc. over a single serial port.
> > 
> > This sounds like a textbook candidate for MFD to me.

Then you do not know what a textbook candidate for MFD is. :)

What part of the MFD API does this device utilise?

> > I.e. there is a very loose coupling of the different functions that make up for
> > entirely independent drivers, except that they share the same serial port
> > connection.
> > 
> > Whereas the auxiliary bus is more for very complicated devices to be broken down
> > into more managable (sometimes optional) sub-domains, where the corresponding
> > drivers usually have driver specific APIs to interact with each other.
> > 
> > - Danilo
> 
> QNAP and Synology do things very similarly.
> There is already a driver for QNAP devices:
> 
> drivers/mfd/qnap-mcu.c
> drivers/leds/leds-qnap-mcu.c
> drivers/input/misc/qnap-mcu-input.c
> drivers/hwmon/qnap-mcu-hwmon.c
> drivers/nvmem/qnap-mcu-eeprom.c
> 
> drivers/power/reset/qnap-poweroff.c (this one is not part of the mfd)
> 
> and I try to implement the equivalent for Synology devices.
> Given its a MFD I would assume the same applies to this driver?

Of course not.

The QNAP driver above calls devm_mfd_add_devices().

This one uses none of the MFD functionality provided.

Linux supports 10's if not 100's of devices which do more than one
thing.  Only a fraction of them are Linux MFDs.  MFD in Linux is an API,
not a type of device.

Don't get me wrong, you could probably code up this device as a Linux
MFD, but you have chosen not to, so therefore it cannot reside here.

-- 
Lee Jones [李琼斯]