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

On Wed, 15 Jul 2026 at 11:28, Shah, Tanmay <[email protected]> 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 <[email protected]>
> >> ---
> >> 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?

> 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?


> 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
> >>
>