Re: [PATCH v13 3/4] gpio: rpmsg: add generic rpmsg GPIO driver

Beleswar Prasad Padhi
2026-05-05T11:16:13Z

Hi Tanmay,

On 05/05/26 00:49, Shah, Tanmay wrote:
> Hello all,
>
> I have started reviewing this work as well.
> Thanks Shenwei for this work.
>
> I have gone through only the current revision, and would like to provide
> idea on how to achieve GPIO number multiplexing with the RPMsg protocol.
> Also, have some bindings related question.
>
> Please see below:
>
> On 4/30/2026 11:40 AM, Arnaud POULIQUEN wrote:
>>
>> On 4/30/26 14:56, Beleswar Prasad Padhi wrote:
>>> Hello Arnaud,
>>>
>>> On 30/04/26 13:05, Arnaud POULIQUEN wrote:
>>>> Hello,
>>>>
>>>> On 4/29/26 21:20, Mathieu Poirier wrote:
>>>>> On Wed, 29 Apr 2026 at 12:07, Padhi, Beleswar <b-padhi@ti.com> wrote:
>>>>>> Hi Mathieu,
>>>>>>
>>>>>> On 4/29/2026 11:03 PM, Mathieu Poirier wrote:
>>>>>>> On Wed, 29 Apr 2026 at 10:53, Shenwei Wang <shenwei.wang@nxp.com>
>>>>>>> wrote:
>>>>>>>>
>>>>>>>>> -----Original Message-----
>>>>>>>>> From: Mathieu Poirier <mathieu.poirier@linaro.org>
>>>>>>>>> Sent: Wednesday, April 29, 2026 10:42 AM
>>>>>>>>> To: Shenwei Wang <shenwei.wang@nxp.com>
>>>>>>>>> Cc: Andrew Lunn <andrew@lunn.ch>; Padhi, Beleswar <b-
>>>>>>>>> padhi@ti.com>; Linus
>>>>>>>>> Walleij <linusw@kernel.org>; Bartosz Golaszewski
>>>>>>>>> <brgl@kernel.org>; Jonathan
>>>>>>>>> Corbet <corbet@lwn.net>; Rob Herring <robh@kernel.org>;
>>>>>>>>> Krzysztof Kozlowski
>>>>>>>>> <krzk+dt@kernel.org>; Conor Dooley <conor+dt@kernel.org>; Bjorn
>>>>>>>>> Andersson
>>>>>>>>> <andersson@kernel.org>; Frank Li <frank.li@nxp.com>; Sascha Hauer
>>>>>>>>> <s.hauer@pengutronix.de>; Shuah Khan
>>>>>>>>> <skhan@linuxfoundation.org>; linux-
>>>>>>>>> gpio@vger.kernel.org; linux-doc@vger.kernel.org; linux-
>>>>>>>>> kernel@vger.kernel.org;
>>>>>>>>> Pengutronix Kernel Team <kernel@pengutronix.de>; Fabio Estevam
>>>>>>>>> <festevam@gmail.com>; Peng Fan <peng.fan@nxp.com>;
>>>>>>>>> devicetree@vger.kernel.org; linux-remoteproc@vger.kernel.org;
>>>>>>>>> imx@lists.linux.dev; linux-arm-kernel@lists.infradead.org; dl-
>>>>>>>>> linux-imx <linux-
>>>>>>>>> imx@nxp.com>; Bartosz Golaszewski <brgl@bgdev.pl>
>>>>>>>>> Subject: [EXT] Re: [PATCH v13 3/4] gpio: rpmsg: add generic
>>>>>>>>> rpmsg GPIO driver
>>>>>>>>> On Tue, Apr 28, 2026 at 03:24:59PM +0000, Shenwei Wang wrote:
>>>>>>>>>>> -----Original Message-----
>>>>>>>>>>> From: Andrew Lunn <andrew@lunn.ch>
>>>>>>>>>>> Sent: Monday, April 27, 2026 3:49 PM
>>>>>>>>>>> To: Shenwei Wang <shenwei.wang@nxp.com>
>>>>>>>>>>> Cc: Padhi, Beleswar <b-padhi@ti.com>; Linus Walleij
>>>>>>>>>>> <linusw@kernel.org>; Bartosz Golaszewski <brgl@kernel.org>;
>>>>>>>>>>> Jonathan
>>>>>>>>>>> Corbet <corbet@lwn.net>; Rob Herring <robh@kernel.org>; Krzysztof
>>>>>>>>>>> Kozlowski <krzk+dt@kernel.org>; Conor Dooley
>>>>>>>>>>> <conor+dt@kernel.org>;
>>>>>>>>>>> Bjorn Andersson <andersson@kernel.org>; Mathieu Poirier
>>>>>>>>>>> <mathieu.poirier@linaro.org>; Frank Li <frank.li@nxp.com>; Sascha
>>>>>>>>>>> Hauer <s.hauer@pengutronix.de>; Shuah Khan
>>>>>>>>>>> <skhan@linuxfoundation.org>; linux-gpio@vger.kernel.org; linux-
>>>>>>>>>>> doc@vger.kernel.org; linux-kernel@vger.kernel.org; Pengutronix
>>>>>>>>>>> Kernel Team <kernel@pengutronix.de>; Fabio Estevam
>>>>>>>>>>> <festevam@gmail.com>; Peng Fan <peng.fan@nxp.com>;
>>>>>>>>>>> devicetree@vger.kernel.org; linux- remoteproc@vger.kernel.org;
>>>>>>>>>>> imx@lists.linux.dev; linux-arm- kernel@lists.infradead.org;
>>>>>>>>>>> dl-linux-imx <linux-imx@nxp.com>; Bartosz Golaszewski
>>>>>>>>>>> <brgl@bgdev.pl>
>>>>>>>>>>> Subject: [EXT] Re: [PATCH v13 3/4] gpio: rpmsg: add generic rpmsg
>>>>>>>>>>> GPIO driver
>>>>>>>>>>>>> struct virtio_gpio_response {
>>>>>>>>>>>>>            __u8 status;
>>>>>>>>>>>>>            __u8 value;
>>>>>>>>>>>>> };
>>>>>>>>>>>> It is the same message format. Please see the message definition
>>>>>>>>>>> (GET_DIRECTION) below:
>>>>>>>>>>>
>>>>>>>>>>>> +   +-----+-----+-----+-----+-----+----+
>>>>>>>>>>>> +   |0x00 |0x01 |0x02 |0x03 |0x04 |0x05|
>>>>>>>>>>>> +   | 1   | 2   |port |line | err | dir|
>>>>>>>>>>>> +   +-----+-----+-----+-----+-----+----+
>>>>>>>>>>> Sorry, but i don't see how two u8 vs six u8 are the same
>>>>>>>>>>> message format.
>>>>>>>>>>>
>>>>>>>>>> Some changes to the message format are necessary.
>>>>>>>>>>
>>>>>>>>>> Virtio uses two communication channels (virtqueues): one for
>>>>>>>>>> requests and
>>>>>>>>> replies, and a second one for events.
>>>>>>>>>> In contrast, rpmsg provides only a single communication
>>>>>>>>>> channel, so a
>>>>>>>>>> type field is required to distinguish between different kinds
>>>>>>>>>> of messages.
>>>>>>>>>>
>>>>>>>>>> Since rpmsg replies and events share the same message format,
>>>>>>>>>> an additional
>>>>>>>>> line is introduced to handle both cases.
>>>>>>>>>> Finally, rpmsg supports multiple GPIO controllers, so a port
>>>>>>>>>> field is added to
>>>>>>>>> uniquely identify the target controller.
>>>>>>>>>
>>>>>>>>> I have commented on this before - RPMSG is already providing
>>>>>>>>> multiplexing
>>>>>>>>> capability by way of endpoints.  There is no need for a port
>>>>>>>>> field.  One endpoint,
>>>>>>>>> one GPIO controller.
>>>>>>>>>
>>>>>>>> You still need a way to let the remote side know which port the
>>>>>>>> endpoint maps to, either
>>>>>>>> by embedding the port information in the message (the current
>>>>>>>> way), or by sending it
>>>>>>>> separately.
>>>>>>>>
>>>>>>> An endpoint is created with every namespace request.  There should be
>>>>>>> one namespace request for every GPIO controller, which yields a
>>>>>>> unique
>>>>>>> endpoint for each controller and eliminates the need for an extra
>>>>>>> field to identify them.
>>>>>>
>>>>>> Right, but this can still be done by just having one namespace
>>>>>> request.
>>>>>> We can create new endpoints bound to an existing namespace/channel by
>>>>>> invoking rpmsg_create_ept(). This is what I suggested here too:
>>>>>> https://lore.kernel.org/all/29485742-6e49-482e-
>>>>>> b73d-228295daaeec@ti.com/
>>>>>>
>>>>> I will look at your suggestion (i.e link above) later this week or
>>>>> next week.
>>>>>
>>>>>> My mental model looks like this for the complete picture:
>>>>>>
>>>>>> 1. namespace/channel#1 = rpmsg-io
>>>>>>       a. ept1 -> gpio-controller@1
>>>>>>       b. ept2 -> gpio-controller@2
>>>>>>
> If my understanding of what gpio-controller is right, than this won't
> work. We need one rpmsg channel per gpio-controller, and in most cases
> there will be only one GPIO-controller on the remote side.

Why so? In the current v13 version, the remote side already
handles 2 GPIO controllers.

> If there are
> multiple or multiple instances of same controller, than we need separate
> channel name for that controller just like we would have separate device
> on the Linux.

Why so? I think there is some confusion in the terminology:

GPIO controller = GPIO port (gpio@xyz) defined in the
Device tree = struct rpmsg_gpio_port in code

GPIO line = Individual lines within each GPIO port (max =
GPIOS_PER_PORT_DEFAULT) = struct rpmsg_gpio_line in code

>
>>>>> I've asked for one endpoint per GPIO controller since the very
>>>>> beginning.  I don't yet have a strong opinion on whether to use one
>>>>> namespace request per GPIO controller or a single request that spins
>>>>> off multiple endpoints.  I'll have to look at your link and reflect on
>>>>> that.  Regardless of how we proceed on that front, multiplexing needs
>>>>> to happen at the endpoint level rather than the packet level.  This is
>>>>> the only way this work can move forward.
>>>>>
>>>> I would be more in favor of Mathieu’s proposal: “An endpoint is
>>>> created with every namespace request.”
>>>>
>>>> If the endpoint is created only on the Linux side, how do we match
>>>> the Linux endpoint address with the local port field on the remote side?
>>>
>>> Simply by sending a message to the remote containing the newly created
>>> endpoint and the port idx. Note that is this done just one time, after
>>> this
>>> Linux need not have the port field in the message everytime its sending
>>> a message.
>>>
>>>> With a multi-namespace approach, the namespace could be rpmsg-io-
>>>> [addr], where [addr] corresponds to the GPIO controller address in
>>>> the DT. This would:
>>>
>>> You will face the same problem in this case also that you asked above:
>>> "how do we match the Linux endpoint address with the local port field
>>> on the remote side?"
>> Sorry I probably introduced confusion here
>> my sentence should be;
>>  With a multi-namespace approach, the namespace could be rpmsg-io-[port],
>>  where [port] corresponds to the GPIO controller port in the DT.
>>
>>
>> For instance:
>>
>>       rpmsg {
>>         rpmsg-io {
>>           #address-cells = <1>;
>>           #size-cells = <0>;
>>
>>           gpio@25 {
>>             compatible = "rpmsg-gpio";
>>             reg = <25>;
>>             gpio-controller;
>>             #gpio-cells = <2>;
>>             #interrupt-cells = <2>;
>>             interrupt-controller;
>>           };
>>
>>           gpio@32 {
>>             compatible = "rpmsg-gpio";
>>             reg = <32>;
>>             gpio-controller;
>>             #gpio-cells = <2>;
>>             #interrupt-cells = <2>;
>>             interrupt-controller;
>>           };
>>         };
>>       };
>>
>>  rpmsg-io-25  would match with gpio@25
>>  rpmsg-io-32  would match with gpio@32
>>
> The problem with this approach is, we will endup creating way too many
> RPMsg devices/channels. i.e. one channel per one GPIO. That limits how
> many GPIOs can be handled by remote from memory perspective. At
> somepoint we might just run-out of number ept & channels created by the
> remote. As of now, open-amp library supports 128 epts I think.

Arnaud was suggesting one channel per gpio controller,
not per line. We will not have 128 gpio controllers....

>
>>> Because the endpoint that is created on a namespace request is also
>>> dynamic in nature. How will the remote know which endpoint addr
>>> Linux allocated for a namespace that it announced?
>>>
>>> As an example/PoC, I created a firmware example which announces
>>> 2 name services to Linux, one is the standard "rpmsg_chrdev" and
>>> the other is a TI specific name service "ti.ipc4.ping-pong". You can
>>> see it created 2 different addresses (0x400 and 0x401) for each of
>>> the name service request from the same firmware:
>>>
>>> root@j784s4-evm:~# dmesg | grep virtio0 | grep -i channel
>>> [    9.290275] virtio_rpmsg_bus virtio0: creating channel
>>> ti.ipc4.ping-pong addr 0xd
>>> [    9.311230] virtio_rpmsg_bus virtio0: creating channel rpmsg_chrdev
>>> addr 0xe
>>> [    9.496645] rpmsg_chrdev virtio0.rpmsg_chrdev.-1.14: DEBUG: Channel
>>> formed from src = 0x400 to dst = 0xe
>>> [    9.707255] rpmsg_client_sample virtio0.ti.ipc4.ping-pong.-1.13:
>>> new channel: 0x401 -> 0xd!
>>>
>>> So in this case, rpmsg-io-1 can have different ept addr than rpmsg-io-2
>>> Back to same problem. Simple solution is to reply to remote with the
>>> created ept addr and the index.
>> That why I would like to suggest to use the name service field to
>> identify the port/controller, instead of the endpoint address.
>>>
>>>> - match the RPMsg probe with the DT,
>>>
>>> We can probe from all controllers with a single name service
>>> announcement too.
>>>
>>>> - provide a simple mapping between the port and the endpoint on both
>>>> sides,
>>>
>>> We are trying to get rid of this mapping from Linux side to adapt
>>> the gpio-virtio design.
>>>
>>>> - allow multiple endpoints on the remote side,
>>>
>>> We can support this as well with single nameservice model.
>>> There is no limitation. Remote has to send a message with
>>> its newly created ept that's all.
>>>
>>>> - provide a simple discovery mechanism for remote capabilities.
>>>
>>> A single announcement: "rpmsg-io" is also discovery mechanism.
>>>
>>> Feel free to let me know if you have concerns with any of the
>>> suggestions!
>> My only concern, whatever the solution, is that we find a smart
>> solution to associate the correct endpoint with the correct GPIO
>> port/controller defined in the DT.
>>
>> I may have misunderstood your solution. Could you please help me
>> understand your proposal by explaining how you would handle three
>> GPIO ports defined in the DT, considering that the endpoint
>> addresses on the Linux side can be random?
>> If I assume there is a unique endpoint on the remote side,
>> I do not understand how you can match, on the firmware side,
>> the Linux endpoint address to the GPIO port.
>>
>> Thanks and Regards,Arnaud
>>
>>> Thanks,
>>> Beleswar
>>>
>>>> Regards,
>>>> Arnaud
>>>>
>>>>>> 2. namespace/channel#2 = rpmsg-i2c
>>>>>>       a. ept1 -> i2c@1
>>>>>>       b. ept2 -> i2c@2
>>>>>>       c. ept3 -> i2c@3
>>>>>>
>>>>>> etc...
>>>>>>
> Just want to clear-up few terms before I jump to the solution:
>
> **RPMsg channel/device**:
> - These are devices announced by the remote processor, and created by
> linux. They are created at: /sys/bus/rpmsg/devices
> - The channel format: <name>.<src ept>.<dst ept>
>
> **RPMsg endpoint**:
> - Endpoint is differnt than channel. Single channel can have multiple
> endpoints, and represented in the linux with: /dev/rpmsg? devices.
>
> To create endpoint device, we have rpmsg_create_ept API, which takes
> channel information as input, which has src-ept, dst-ept.
>
> Following is proposed solution:
>
> 1) Assign RPMsg channel/device per rpmsg-gpio controller (Not per GPIO
> pin/port).

One channel per pin was not suggested earlier...

> - In our case that would be, single rpmsg-io node. (That makes me
> question if bindings are correct or not).
>
> 2) Assign GPIO number as src ept.
>
> i.e. *rpmsg-io.<GPIO number>.<dst ept>*. Do not randomly assign src
> endpoint.
>
> Now, RPMSG channel by spec reserves first 1024 endpoints [1], so we can
> add 1024 offset to the GPIO number:
>
> so, when calling rpmsg_create_ept() API, we assing src_endpoint as:
> (GPIO_NUMBER + RPMSG_RESERVED_ADDRESSES)
>
> Now on the remote side, there is single channel and only single-endpoint
> is needed that is mapped to the rpmsg-io channel callback.
>
> That callback will receive all the payloads from the Linux, which will
> have src-ept i.e. (RPMSG_RESERVED_ADDRESSES + GPIO_NUMBER).
>
> It can retrieve GPIO_NUMBER easily, and convert to appropriate pin based
> on platform specific logic.
>
> This doesn't need PORT information at all. Also it makes sure that
> remote is using only single-endpoint so not much memory is used.
>
> *Example*:
> If only rpmsg-gpio channel is created by the remote side, than following
> is the representation of the devices when GPIO 25, 26, 27 is assigned to
> the rpmsg-io controller:
>
> Linux Remote
>
> rpmsg-channel: rpmsg-gpio.0x400.0x400
>
> /dev/rpmsg0 - GPIO25 ept (rpmsg-gpio.0x419.0x400)-|
> |
> /dev/rpmsg1 - GPIO26 ept (rpmsg-gpio.0x41a.0x400)-|-> rpmsg-gpio.*.0x400
> |
> /dev/rpmsg2 - GPIO27 ept (rpmsg-gpio.0x41b.0x400)-| 0x400 ept callback.
>
>
> *On remote side*:
>
> ept_0x400_callback(..., int src_ept, ...,)
> {
> int gpio_num = src_ept - RPMSG_RESERVED_ADDRESSES;
> // platform specific logic to convert gpio num to proper pin,
> // just like you would convert gpio num to pin on a linux gpio controller.
> }
>
> My question on the binding:
>
> Why each GPIO is represented with the separate node? I think rpmsg-gpio
> can be represented just any other GPIO controller? Please let me know if
> I am missing something.

These are separate GPIO controllers, not separate pins within
the same GPIO controller. Could you revisit your solution with
this update.

Thanks for your time,
Beleswar