Re: [PATCH v5 3/5] rpmsg: virtio_rpmsg_bus: get buffer size from config space

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

On Thu, Jul 16, 2026 at 11:12:55AM -0500, Shah, Tanmay wrote:
> 
> 
> On 7/16/2026 10:48 AM, Mathieu Poirier wrote:
> > On Wed, 15 Jul 2026 at 11:28, Shah, Tanmay <tanmays@amd.com> wrote:
> >>
> >> Hi,
> >>
> >> Please find my response below:
> >>
> >> On 7/15/2026 11:24 AM, Mathieu Poirier wrote:
> >>> On Fri, Jul 10, 2026 at 12:28:29PM -0700, Tanmay Shah wrote:
> >>>> 512 bytes isn't always suitable for all case, let firmware
> >>>> maker decide the best value from resource table.
> >>>> enable by VIRTIO_RPMSG_F_BUFSZ feature bit.
> >>>>
> >>>> Signed-off-by: Tanmay Shah <tanmay.shah@amd.com>
> >>>> ---
> >>>> Changes in v5:
> >>>>
> >>>>   - fix documentation about alignment of the buffer size
> >>>>   - change version field from u16 to u8
> >>>>   - remove buffer alignment check
> >>>>   - Separate buffer alignment vs MTU of a single buffer
> >>>>   - Use buffer alignment only to get next buffer address at alignment
> >>>>     boundary
> >>>>
> >>
> >> [...]
> >>
> >>>> +#ifndef _LINUX_VIRTIO_RPMSG_H
> >>>> +#define _LINUX_VIRTIO_RPMSG_H
> >>>> +
> >>>> +#include <linux/types.h>
> >>>> +#include <linux/virtio_types.h>
> >>>> +
> >>>> +/* The feature bitmap for virtio rpmsg */
> >>>> +#define VIRTIO_RPMSG_F_NS   0 /* RP supports name service notifications */
> >>>> +#define VIRTIO_RPMSG_F_BUFSZ        1 /* RP get buffer size from config space */
> >>>> +
> >>>> +/* Version of struct virtio_rpmsg_config understood by this driver */
> >>>> +#define RPMSG_VDEV_CONFIG_V1        1
> >>>> +
> >>>> +/**
> >>>> + * struct virtio_rpmsg_config - config space for rpmsg virtio device
> >>>> + *
> >>>> + * @version:        version of this structure, currently %RPMSG_VDEV_CONFIG_V1.
> >>>> + * @size:   size of this structure in bytes.
> >>>> + * @rpmsg_buf_align: alignment in bytes for each buffer. Must be a power of
> >>>> + *               two. If 0 then no alignment will be done. This alignment
> >>>> + *               will not decide actual size of the buffer but will be
> >>>> + *               used to decided the start address of the buffer. The
> >>>> + *               actual size of the buffer can be different than the
> >>>> + *               aligned size of the buffer.
> >>>
> >>> Is there really a need to have a buffer size different from its alignment?  It's
> >>> not like the (small) delta between the buffer size and its alignment will be
> >>> used for something else.  I'm fine with a buffer alignment requirement but in
> >>> those cases, the firmware should set the size of the buffer in accordance with
> >>> its alignment requirement.  Otherwise, the complexity needed to manage the
> >>> discrpancy between the two yields a driver that is hard to maintain and prone to
> >>> bugs.
> >>>
> >>
> >> I had the same concern before. However, following example changed my mind:
> >>
> >> So, a single buffer size is the MTU size of a packet for the protocol
> >> supported by the firmware. Now that can be different than the aligned
> >> size of the buffer.
> >>
> >> For example, the higher level protocol (not rpmsg) has 430 bytes as the
> >> max size of a payload. However, cache line alignment is 64-bytes. Then
> >> in that case, the aligned buffer size is 448 bytes. But, that doesn't
> >> mean we can say protocol's MTU size is 448 bytes. If user end up
> >> treating MTU size 448 bytes and use space beyond 430 bytes, then the
> >> higher level apps might discard that data and communication may fail.
> >>
> > 
> > How is that scenario different from today's 512 byte buffer size?
> > Most users don't use all 512 bytes and we don't run in the problem
> > described above?
> > 
> 
> 512 buffer size is hardcoded, so it is enforced on the protocol by the
> framework. But by allowing the configuration of the buffer size we are
> allowing the protocol to decide what the buffer size should be. So, when
> user request the buffer via rpmsg_get_mtu() API, then that should be the
> original buffer size which is expected by the protocol, which may not be
> same as the aligned buffer size.

Regardless of the buffer size, whether it is set to 512 byte or some arbitrary
value by the remote processsor's firmware, there is a possibility of a
discrepancy with what is expected by the protocol.  Right now rpmsg_get_mtu()
returns 512 regardless of what a protocol uses.  The only thing that should be
important to the protocol is not to exceed that limit.

> 
> If for internal management we want to treat buffer size = aligned buffer
> size, I am okay. But rpmsg_get_mtu() must give unaligned buffer size
> which is expected by the protocol.

I agree with the first sentence but not the second.  The only thing protocols
should care about is the start address of a buffer and that its size is
sufficient for what it needs.

> 
> Thanks,
> Tanmay
> 
> >> The alignment field is used only to decide where the next buffer start
> >> address is to ease cache operations.
> >>
> >> Sure, we need to maintain this complexity, but I think it's worth it.
> >>
> > 
> > The same as in my previous email to Arnaud applies here - is this an
> > immediate requirement of something we think may be happening in the
> > future?
> > 
> 
> IMHO, vendors will use it if the feature is available, otherwise the
> need to optimize alignment is not easily encountered.
> 
> > 
> >> Thanks,
> >> Tanmay
> >>
> >>>> + * @txbuf_size:     Tx buf size from remote's view. For Linux this is rx buf size.
> >>>> + * @rxbuf_size:     Rx buf size from remote's view. For Linux this is tx buf size.
> >>>> + *
> >>>> + * This is the configuration structure shared by the device and the driver,
> >>>> + * read when %VIRTIO_RPMSG_F_BUFSZ is negotiated. The fields are laid out so
> >>>> + * the structure is naturally 32-bit aligned.
> >>>> + */
> >>>> +struct virtio_rpmsg_config {
> >>>> +    u8 version;
> >>>> +    __virtio16 size;
> >>>> +    __virtio16 rpmsg_buf_align;
> >>>> +    /* The tx/rx individual buffer size (if VIRTIO_RPMSG_F_BUFSZ) */
> >>>> +    __virtio32 txbuf_size;
> >>>> +    __virtio32 rxbuf_size;
> >>>> +} __packed;
> >>>> +
> >>>> +#endif /* _LINUX_VIRTIO_RPMSG_H */
> >>>> --
> >>>> 2.34.1
> >>>>
> >>
>