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

[Krzysztof Kozlowski <krzk@kernel.org>]

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-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@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-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@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-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@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.

> 
> 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