Re: [PATCH v1 2/2] soc: starfive: Add JHB100 UART Routing driver

[Changhuang Liang]

Hi, Krzysztof

Thanks for the review.

> On 30/08/2026 08:51, Changhuang Liang wrote:
> > Add driver support for JHB100 UART Routing control, allowing runtime
> > configuration of RX muxes between UART controllers and I/O pins.
> >
> > A sysfs interface is provided for easy checking and updating of
> > routing paths.
> >
> > Signed-off-by: Changhuang Liang <changhuang.liang@xxxxxxxxxxxxxxxx>
> > ---
> >  .../sysfs-driver-starfive-jhb100-uart-routing |  45 +++
> >  MAINTAINERS                                   |   7 +
> >  drivers/soc/starfive/Kconfig                  |   1 +
> >  drivers/soc/starfive/Makefile                 |   1 +
> >  drivers/soc/starfive/uart-routing/Kconfig     |  14 +
> >  drivers/soc/starfive/uart-routing/Makefile    |   2 +
> >  .../uart-routing/jhb100-uart-routing.c        | 262
> ++++++++++++++++++
> >  7 files changed, 332 insertions(+)
> >  create mode 100644
> > Documentation/ABI/testing/sysfs-driver-starfive-jhb100-uart-routing
> >  create mode 100644 drivers/soc/starfive/uart-routing/Kconfig
> >  create mode 100644 drivers/soc/starfive/uart-routing/Makefile
> >  create mode 100644
> > drivers/soc/starfive/uart-routing/jhb100-uart-routing.c
> >
> > diff --git
> > a/Documentation/ABI/testing/sysfs-driver-starfive-jhb100-uart-routing
> > b/Documentation/ABI/testing/sysfs-driver-starfive-jhb100-uart-routing
> > new file mode 100644
> > index 000000000000..2844133bea1b
> > --- /dev/null
> > +++ b/Documentation/ABI/testing/sysfs-driver-starfive-jhb100-uart-rout
> > +++ ing
> > @@ -0,0 +1,45 @@
> > +What:
> 	/sys/bus/platform/drivers/starfive-jhb100-uart-routing/\*/uart\*
> > +Date:		August 2026
> > +Contact:	Changhuang Liang <changhuang.liang@xxxxxxxxxxxxxxxx>
> > +Description:	Selects the RX source of the UARTx device.
> > +
> > +		When read, each file shows the list of available options with
> currently
> > +		selected option marked by brackets "[]". The list of available options
> > +		depends on the selected file.
> > +
> > +		e.g.
> > +		cat
> /sys/bus/platform/drivers/starfive-jhb100-uart-routing/\*.uart-routing/uart1
> > +		io0 [io1] io2 io3 io4 io5 io6 io7 io8 io9 io10 io11 io12 io13 io14 uart0
> uart1
> > +		uart2 uart3 uart4 uart5 uart6 uart7 uart8 uart9 uart10 uart11
> > +uart12 uart13 uart14
> > +
> > +		In this case, UART1 gets its input from IO1 (physical serial port 1).
> > +
> > +		To switch the RX source of UART1 to UART2, write the desired
> source to the file:
> > +		echo uart2 >
> > +/sys/bus/platform/drivers/starfive-jhb100-uart-routing/*.uart-routing
> > +/uart1
> > +
> > +		This indicates that UART1 now receives its input from UART2.
> > +
> > +Users:		OpenBMC.  Proposed changes should be mailed to
> > +		openbmc@xxxxxxxxxxxxxxxx
> > +
> > +What:		/sys/bus/platform/drivers/starfive-jhb100-uart-routing/\*/io\*
> > +Date:		August 2026
> > +Contact:	Changhuang Liang <changhuang.liang@xxxxxxxxxxxxxxxx>
> > +Description:	Selects the RX source of IOx serial port. The current
> selection
> > +		will be marked by brackets "[]". The list of available options
> > +		depends on the selected file.
> > +
> > +		e.g.
> > +		cat
> /sys/bus/platform/drivers/starfive-jhb100-uart-routing/\*.uart-routing/io9
> > +		uart0 uart1 uart2 uart3 uart4 uart5 uart6 uart7 uart8 [uart9] uart10
> uart11 uart12
> > +		uart13 uart14 io0 io1 io2 io3 io4 io5 io6 io7 io8 io9 io10 io11
> > +io12 io13 io14
> > +
> > +		In this case, IO9 (physical serial port 9) gets its input from UART9.
> > +
> > +		To switch the RX source of IO9 to UART10, write the desired source
> to the file:
> > +		echo uart10 >
> > +/sys/bus/platform/drivers/starfive-jhb100-uart-routing/*.uart-routing
> > +/io9
> > +
> > +		This indicates that IO9 now receives its input from UART10.
> > +
> > +Users:		OpenBMC.  Proposed changes should be mailed to
> > +		openbmc@xxxxxxxxxxxxxxxx
> 
> drivers/soc/ should not define user-space interfaces. This is not the place for
> them. You need to route user-spaces interfaces only through one of other
> approved subsystems, after their review.
> 
> This looks like pin multiplexing interface.

I may have misunderstood something，please correct me if I'm wrong:

I have found two subsystems related to multiplexing so far:

/drivers/pinctrl and /drivers/mux. However, neither of them seems to provide a 
user-space interface for switching multiplexing values.

Do you have any suggestions on this?

Also, could you confirm whether the implementation in drivers/soc/aspeed/aspeed-uart-routing.c 
is there for historical reasons?

Best Regards,
Changhuang