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

On Tue, Jul 21, 2026 at 11:02:13AM -0500, Shah, Tanmay wrote:
> 
> 
> On 7/21/2026 10:50 AM, Mathieu Poirier wrote:
> > 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 <[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?
> >>>
> >>
> >> 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.
> > 
> 
> I think I am missing something. Are you saying that buffer size can not
> be configured greater than 512 bytes?

I am not.

What I am saying is that if alignment is important to a remote processor, it
should choose the buffer size accordingly.  rpmsg_get_mtu() should return the
value of the buffer size, exactly the way it is today. 

> 
> If the higher level protocol (not RPMsg) wants to use 4030 bytes for
> single packet payload then that is what the MTU size should be. And so
> the firmware will configure 4030 bytes as single buffer size in the vdev
> config space. That is why alignment should be treated separately.
> Because it is not equal to payload size needed by higher level protocol.

In that case and assuming alignment is required, the buffer size should be 4096
and rpmsg_get_mtu() should also return 4096.  How a higher protocol uses the
buffer space is none of our concern.

Currently, the buffer size is set to 512 and users don't always fill the entire
buffer.  I don't see why things should be different with a configurable buffer
size.   

> 
> >>
> >> 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
> >>>>>>
> >>>>
> >>
>