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

[Shenwei Wang]

> -----Original Message-----
> From: Padhi, Beleswar <b-padhi at ti.com>
> Sent: Tuesday, April 28, 2026 10:53 AM
> To: Shenwei Wang <shenwei.wang at nxp.com>; Linus Walleij
> <linusw at kernel.org>; Bartosz Golaszewski <brgl at kernel.org>; Jonathan Corbet
> <corbet at lwn.net>; Rob Herring <robh at kernel.org>; Krzysztof Kozlowski
> <krzk+dt at kernel.org>; Conor Dooley <conor+dt at kernel.org>; Bjorn Andersson
> <andersson at kernel.org>; Mathieu Poirier <mathieu.poirier at linaro.org>; Frank Li
> <frank.li at nxp.com>; Sascha Hauer <s.hauer at pengutronix.de>
> Cc: Shuah Khan <skhan at linuxfoundation.org>; linux-gpio at vger.kernel.org; linux-
> doc at vger.kernel.org; linux-kernel at vger.kernel.org; Pengutronix Kernel Team
> <kernel at pengutronix.de>; Fabio Estevam <festevam at gmail.com>; Peng Fan
> <peng.fan at nxp.com>; devicetree at vger.kernel.org; linux-
> remoteproc at vger.kernel.org; imx at lists.linux.dev; linux-arm-
> kernel at lists.infradead.org; dl-linux-imx <linux-imx at nxp.com>; Bartosz
> Golaszewski <brgl at bgdev.pl>; Andrew Lunn <andrew at lunn.ch>
> Subject: [EXT] Re: [PATCH v13 3/4] gpio: rpmsg: add generic rpmsg GPIO driver
> >>
> >> Nothing extra in my opinion. rpmsg_create_ept() just creates a
> >> dynamic local endpoint address for Linux's usage. The firmware just
> >> has to make sure to reply to the same endpoint address where it
> >> received the message. This should already be in place IMO, because
> >> currently you are sending all messages in the default
> > Since rpmsg_create_ept creates a new local endpoint address on the
> > Linux side, how is the remote system expected to learn and use this
> > new address for communication if no additional logic is added on the remote
> side?
> 
> 
> Remote side learns the endpoint when it receives any message from Linux from
> the dynamic endpoint.
> 
> Lets say rpmsg_create_ept() allocates a dynamic local ept of 1026. When you
> send the message from this endpoint, the standard rpmsg header would have:
> 
>      85 struct rpmsg_hdr {
>      86         __rpmsg32 src; // 1026
>      87         __rpmsg32 dst; // rpdev->dst (e.g. 400)
>      88         __rpmsg32 reserved;
>      89         __rpmsg16 len;
>      90         __rpmsg16 flags;
>      91         u8 data[];
>      92 } __packed;
> 
> Remote side tracks the dynamic endpoint by reading src = 1026. And while
> sending the response it fills the header as:
> 
>      85 struct rpmsg_hdr {
>      86         __rpmsg32 src; // 400
>      87         __rpmsg32 dst; // 1026
>      88         __rpmsg32 reserved;
>      89         __rpmsg16 len;
>      90         __rpmsg16 flags;
>      91         u8 data[];
>      92 } __packed;
> 

This explains how reply messages work in this scenario: the remote side can simply send 
the response back to the source address of the incoming message.

How does this work for notification messages initiated by the remote side? Should the remote 
system need to add additional logic to track the source address based on the GPIO instance?

Thanks,
Shenwei

> Note: Remote firmware can also send messages from dynamically created
> endpoints on its side, and Linux can learn those in the same manner. The dynamic
> endpoint address is passed to the callback as 'u32 src'. So you could pass on the
> 'src' from rpmsg_gpio_channel_callback() to
> rpmsg_gpio_send_message() as 'dst' and call rpmsg_sendto(port->ept, msg,
> sizeof(*msg), dst) to reply to the dynamic endpoint on firmware's side.
> 
> Thanks,
> Beleswar
> 
> >
> > Is this handled automatically by the rpmsg stack software, or does it
> > require explicit support on the remote system to exchange and track endpoint
> addresses?
> >
> > Thanks,
> > Shenwei
> >
> >> endpoint (rpdev->ept) which is also dynamic[1] and is created when
> >> the channel is created. And you receive the responses correctly.
> >> (Unless you have hard-coded the default ept address in the firmware)
> >>
> >> [1]:
> >> https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgit
> >>
> hub.co%2F&data=05%7C02%7Cshenwei.wang%40nxp.com%7C057bf7f0976749
> 5bcc3
> >>
> 108dea53e43f7%7C686ea1d3bc2b4c6fa92cd99c5c301635%7C0%7C0%7C63912
> 98840
> >>
> 26686251%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiI
> wLjAu
> >>
> MDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C
> %7C
> >>
> &sdata=iSxGWEvYSyu29loP9b1R2bw8bvwR7pbzQ7D%2FGeB%2BUYE%3D&reser
> ved=0
> >>
> m%2Ftorvalds%2Flinux%2Fblob%2Fmaster%2Fdrivers%2Frpmsg%2Frpmsg_core.
> >>
> c%23L480&data=05%7C02%7Cshenwei.wang%40nxp.com%7C4ec06bf01bb14dd
> >>
> 2625708dea5387471%7C686ea1d3bc2b4c6fa92cd99c5c301635%7C0%7C0%7C6
> >>
> 39129859078622527%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRyd
> >>
> WUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%
> >>
> 3D%7C0%7C%7C%7C&sdata=OUmjqOdWqiXSTjPDv1TUvrjKP1YTx9ji44SdGlIR2n
> >> Q%3D&reserved=0
> >> (chinfo.src is RPMSG_ADDR_ANY)
> >>
> >> Thanks,
> >> Beleswar
> >>
> >>> If the remote side does not need any extra support, this would be an
> >>> excellent
> >> solution.
> >>> Thanks,
> >>> Shenwei
> >>>
> >>>> 3. Send msgs from local ept in rpmsg_gpio_send_message() by:
> >>>>       rpmsg_send(port->ept, msg, sizeof(*msg));
> >>>>
> >>>> 4. Get the port info in rpmsg_gpio_channel_callback() by:
> >>>>       struct rpmsg_gpio_port *port = priv;
> >>>>
> >>>> Which also eliminates the need for struct rpdev_drvdata as you can
> >>>> just do
> >>>> rpmsg_get_rproc_node_name(rpdev) from rpmsg_gpiochip_register().
> >>>>