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

[Conor Dooley <conor@kernel.org>]

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+CC Linus, linux-gpio,

On Mon, Aug 31, 2026 at 06:01:43AM +0000, Changhuang Liang wrote:
> 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@starfivetech.com>
> > >>> ---
> > >>>  .../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@starfivetech.com>
> > >>> +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@lists.ozlabs.org
> > >>> +
> > >>> +What:
> > 	/sys/bus/platform/drivers/starfive-jhb100-uart-routing/\*/io\*
> > >>> +Date:		August 2026
> > >>> +Contact:	Changhuang Liang <changhuang.liang@starfivetech.com>
> > >>> +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@lists.ozlabs.org
> > >>
> > >> 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.


My first reaction when this flew by over the weekend was whether it
should be in the pinctrl subsystem. I know there's no sysfs interface
there, but I don't even see an explanation for why changing this at
runtime is a requirement.

I'd have thought that each BMC would only have one host, and so since
you've got like 12 uarts there'd be enough for a permanent routing.

Even without a permanent routing, the driver consuming the pinctrl should
be able perform the switching (uart in this case) whenever it was
needed?

Cheers,
Conor.

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