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

[Changhuang Liang]

Hi, Krzysztof

Thanks for the review.

> On 31/08/2026 03:35, Changhuang Liang wrote:
> > 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-routin
> >>> g
> >>> b/Documentation/ABI/testing/sysfs-driver-starfive-jhb100-uart-routin
> >>> g
> >>> new file mode 100644
> >>> index 000000000000..2844133bea1b
> >>> --- /dev/null
> >>> +++ b/Documentation/ABI/testing/sysfs-driver-starfive-jhb100-uart-ro
> >>> +++ ut
> >>> +++ 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-routin
> >> g/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-routi
> >>> +ng
> >>> +/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-routin
> >> g/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-routi
> >>> +ng
> >>> +/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?
> 
> pinctrl has some interface, not sure if writable, though. If interface is missing,
> it should be added via such subsystem.

Okay, this needs a bit more time for deeper research.

> 
> >
> > Also, could you confirm whether the implementation in
> > drivers/soc/aspeed/aspeed-uart-routing.c
> > is there for historical reasons?
> 
> I supposed sneaked in without SoC maintainers noticing.
> 
> >
> > Best Regards,
> > Changhuang
> 
> 
> Best regards,
> Krzysztof

Best Regards,
Changhuang