Re: [PATCH v6 4/5] docs: rpmsg: add virtio config space details

[Mathieu Poirier <mathieu.poirier@linaro.org>]

On Thu, Aug 13, 2026 at 09:42:37PM -0700, Tanmay Shah wrote:
> The virtio config space can provide single rpmsg buffer size in each
> direction. Document details of the configurable buffer size via virtio
> config space.
> 
> Signed-off-by: Tanmay Shah <tanmay.shah@amd.com>
> ---
> 
> Changes in v6:
>   - rpmsg.rst: modify alignment related documentation
> 
> Changes in v5:
>   - fix documentation of buffer alignment
> 
>  Documentation/staging/rpmsg.rst | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/Documentation/staging/rpmsg.rst b/Documentation/staging/rpmsg.rst
> index 63612b7ee120..cabaf3d11ae7 100644
> --- a/Documentation/staging/rpmsg.rst
> +++ b/Documentation/staging/rpmsg.rst
> @@ -299,3 +299,20 @@ by the bus, and can then start sending messages to the remote service.
>  
>  The plan is also to add static creation of rpmsg channels via the virtio
>  config space, but it's not implemented yet.
> +
> +Configurable buffer sizes
> +=========================
> +
> +By default each rpmsg buffer is 512 bytes, and the same size is used for both
> +the receive and transmit directions. Remote processors that need a different
> +size (for example a larger MTU, or different RX and TX sizes) can advertise it

"... or differnt RX and TX queue sizes)...

> +through the virtio device config space by setting the VIRTIO_RPMSG_F_BUFSZ
> +feature bit.
> +
> +When this feature is negotiated, the driver reads struct virtio_rpmsg_config
> +from the config space. This structure provides the size of a single buffer for
> +each direction (TX and RX). The device should advertise TX/RX buffer sizes
> +that already satisfy its own alignment constraints.
> +
> +If the feature is not negotiated, the default 512-byte buffers are used for
> +both directions.
> -- 
> 2.43.0
>