Hi Tanmay,
On 5/4/26 21:19, 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 <[email protected]> wrote:
Hi Mathieu,
On 4/29/2026 11:03 PM, Mathieu Poirier wrote:
On Wed, 29 Apr 2026 at 10:53, Shenwei Wang <[email protected]>
wrote:
-----Original Message-----
From: Mathieu Poirier <[email protected]>
Sent: Wednesday, April 29, 2026 10:42 AM
To: Shenwei Wang <[email protected]>
Cc: Andrew Lunn <[email protected]>; Padhi, Beleswar <b-
[email protected]>; Linus
Walleij <[email protected]>; Bartosz Golaszewski
<[email protected]>; Jonathan
Corbet <[email protected]>; Rob Herring <[email protected]>;
Krzysztof Kozlowski
<[email protected]>; Conor Dooley <[email protected]>; Bjorn
Andersson
<[email protected]>; Frank Li <[email protected]>; Sascha Hauer
<[email protected]>; Shuah Khan
<[email protected]>; linux-
[email protected]; [email protected]; linux-
[email protected];
Pengutronix Kernel Team <[email protected]>; Fabio Estevam
<[email protected]>; Peng Fan <[email protected]>;
[email protected]; [email protected];
[email protected]; [email protected]; dl-
linux-imx <linux-
[email protected]>; Bartosz Golaszewski <[email protected]>
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 <[email protected]>
Sent: Monday, April 27, 2026 3:49 PM
To: Shenwei Wang <[email protected]>
Cc: Padhi, Beleswar <[email protected]>; Linus Walleij
<[email protected]>; Bartosz Golaszewski <[email protected]>;
Jonathan
Corbet <[email protected]>; Rob Herring <[email protected]>; Krzysztof
Kozlowski <[email protected]>; Conor Dooley
<[email protected]>;
Bjorn Andersson <[email protected]>; Mathieu Poirier
<[email protected]>; Frank Li <[email protected]>; Sascha
Hauer <[email protected]>; Shuah Khan
<[email protected]>; [email protected]; linux-
[email protected]; [email protected]; Pengutronix
Kernel Team <[email protected]>; Fabio Estevam
<[email protected]>; Peng Fan <[email protected]>;
[email protected]; linux- [email protected];
[email protected]; linux-arm- [email protected];
dl-linux-imx <[email protected]>; Bartosz Golaszewski
<[email protected]>
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-
[email protected]/
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. 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.
As done in ehe rpmsg_tty driver it could be instantiated several times with
the same channel/service name. This would imply a specific rpmsg to retreive
the gpio controller index from the remote side.
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.
Right, I proposed a solution in my previous answer to Beleswar who has
the same concern.
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).
- 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).
Interesting approach. I also tried to find a similar solution.
The question here is: how can we guarantee continuous addresses? Given
the static and dynamic allocation of endpoint addresses that are
implemented, my conclusion was that it is not reliable enough.
but perhaps I missed something...
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. So rpmsg channel/rpmsg device is not created per
GPIO, but per controller. GPIO number multiplexing should be done with
rpmsg src ept, that removes the need of having each GPIO as a separate node.
rpmsg_gpio: rpmsg-gpio@0 {
compatible = "rpmsg-gpio";
reg = <0>;
gpio-controller;
#gpio-cells = <2>;
#interrupt-cells = <2>;
interrupt-controller;
};
Then in DT, use like regular GPIO, but with the rpmsg-gpio controller:
rpmsg-gpios = <&rpmsg_gpio (GPIO NUM) (flags)>;
If the intent to create separate gpio nodes was only for the channel
creation, then it's not really needed.
[1]
https://github.com/torvalds/linux/blob/6d35786de28116ecf78797a62b84e6bf3c45aa5a/drivers/rpmsg/virtio_rpmsg_bus.c#L136
It is already the case. bindings declare GPIO controllers, not directly
GPIOs in:
[PATCH v13 2/4] dt-bindings: remoteproc: imx_rproc: Add "rpmsg" subnode
support
The discussion is around having an unique RPmsg endpoint for all
GPIO controller or one RPmsg endpoint per GPIO controller.
Or did I misunderstand your questions?
Thanks,
Arnaud
This way device groups are isolated with each channel/namespace, and
instances within each device groups are also respected with specific
endpoints.
Thanks,
Beleswar
