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

On Thu, 16 Jul 2026 at 02:19, Arnaud POULIQUEN
<[email protected]> wrote:
>
> Hello Mathieu,
>
> On 7/15/26 18:24, 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
> >>
> >> Changes in v4:
> >>
> >>    - Introduce new patch to modify rpmsg.rst documentation
> >>    - check version is always 1.
> >>    - check size field is same as size of struct virtio_rpmsg_config
> >>    - introduce alignment field
> >>    - check alignment field is power of 2
> >>    - check tx and rx buf size is aligned with alignment passed in the
> >>      structure
> >>
> >> Changes in v3:
> >>
> >>    - change version field from u16 to u8
> >>    - introduce size field in the rpmsg_virtio_config structure
> >>    - check version field is set to any non-zero value.
> >>    - check size field is not 0.
> >>    - Remove field for private config, as not needed for now.
> >>    - add documentation of rpmsg_virtio_config structure
> >>
> >>   drivers/rpmsg/virtio_rpmsg_bus.c   | 130 ++++++++++++++++++++++++-----
> >>   include/linux/rpmsg/virtio_rpmsg.h |  48 +++++++++++
> >>   2 files changed, 159 insertions(+), 19 deletions(-)
> >>   create mode 100644 include/linux/rpmsg/virtio_rpmsg.h
> >>
> >> diff --git a/drivers/rpmsg/virtio_rpmsg_bus.c b/drivers/rpmsg/virtio_rpmsg_bus.c
> >> index c84b6cec9caf..5e5473f4adeb 100644
> >> --- a/drivers/rpmsg/virtio_rpmsg_bus.c
> >> +++ b/drivers/rpmsg/virtio_rpmsg_bus.c
> >> @@ -15,11 +15,13 @@
> >>   #include <linux/idr.h>
> >>   #include <linux/jiffies.h>
> >>   #include <linux/kernel.h>
> >> +#include <linux/log2.h>
> >>   #include <linux/module.h>
> >>   #include <linux/mutex.h>
> >>   #include <linux/rpmsg.h>
> >>   #include <linux/rpmsg/byteorder.h>
> >>   #include <linux/rpmsg/ns.h>
> >> +#include <linux/rpmsg/virtio_rpmsg.h>
> >>   #include <linux/scatterlist.h>
> >>   #include <linux/slab.h>
> >>   #include <linux/sched.h>
> >> @@ -39,7 +41,10 @@
> >>    * @tx_bufs:       kernel address of tx buffers
> >>    * @num_rx_buf: total number of rx buffers
> >>    * @num_tx_buf: total number of tx buffers
> >> - * @buf_size:   size of one rx or tx buffer
> >> + * @rx_buf_size: size of one rx buffer
> >> + * @tx_buf_size: size of one tx buffer
> >> + * @rx_buf_size_aligned: aligned size of one rx buffer
> >> + * @tx_buf_size_aligned: aligned size of one tx buffer
> >>    * @last_tx_buf: index of last tx buffer used
> >>    * @bufs_dma:      dma base addr of the buffers
> >>    * @tx_lock:       protects svq and tx_bufs, to allow concurrent senders.
> >> @@ -59,7 +64,10 @@ struct virtproc_info {
> >>      void *rx_bufs, *tx_bufs;
> >>      unsigned int num_rx_buf;
> >>      unsigned int num_tx_buf;
> >> -    unsigned int buf_size;
> >> +    unsigned int rx_buf_size;
> >> +    unsigned int tx_buf_size;
> >> +    unsigned int rx_buf_size_aligned;
> >> +    unsigned int tx_buf_size_aligned;
> >>      int last_tx_buf;
> >>      dma_addr_t bufs_dma;
> >>      struct mutex tx_lock;
> >> @@ -68,9 +76,6 @@ struct virtproc_info {
> >>      wait_queue_head_t sendq;
> >>   };
> >>
> >> -/* The feature bitmap for virtio rpmsg */
> >> -#define VIRTIO_RPMSG_F_NS   0 /* RP supports name service notifications */
> >> -
> >>   /**
> >>    * struct rpmsg_hdr - common header for all rpmsg messages
> >>    * @src: source address
> >> @@ -128,7 +133,7 @@ struct virtio_rpmsg_channel {
> >>    * processor.
> >>    */
> >>   #define MAX_RPMSG_NUM_BUFS (256)
> >> -#define MAX_RPMSG_BUF_SIZE  (512)
> >> +#define DEFAULT_RPMSG_BUF_SIZE      (512)
> >>
> >>   /*
> >>    * Local addresses are dynamically allocated on-demand.
> >> @@ -443,7 +448,7 @@ static void *get_a_tx_buf(struct virtproc_info *vrp)
> >>
> >>      /* either pick the next unused tx buffer */
> >>      if (vrp->last_tx_buf < vrp->num_tx_buf)
> >> -            ret = vrp->tx_bufs + vrp->buf_size * vrp->last_tx_buf++;
> >> +            ret = vrp->tx_bufs + vrp->tx_buf_size_aligned * vrp->last_tx_buf++;
> >>      /* or recycle a used one */
> >>      else
> >>              ret = virtqueue_get_buf(vrp->svq, &len);
> >> @@ -513,7 +518,7 @@ static int rpmsg_send_offchannel_raw(struct rpmsg_device *rpdev,
> >>       * messaging), or to improve the buffer allocator, to support
> >>       * variable-length buffer sizes.
> >>       */
> >> -    if (len > vrp->buf_size - sizeof(struct rpmsg_hdr)) {
> >> +    if (len > vrp->tx_buf_size - sizeof(struct rpmsg_hdr)) {
> >>              dev_err(dev, "message is too big (%d)\n", len);
> >>              return -EMSGSIZE;
> >>      }
> >> @@ -646,7 +651,7 @@ static ssize_t virtio_rpmsg_get_mtu(struct rpmsg_endpoint *ept)
> >>      struct rpmsg_device *rpdev = ept->rpdev;
> >>      struct virtio_rpmsg_channel *vch = to_virtio_rpmsg_channel(rpdev);
> >>
> >> -    return vch->vrp->buf_size - sizeof(struct rpmsg_hdr);
> >> +    return vch->vrp->tx_buf_size - sizeof(struct rpmsg_hdr);
> >>   }
> >>
> >>   static int rpmsg_recv_single(struct virtproc_info *vrp, struct device *dev,
> >> @@ -672,7 +677,7 @@ static int rpmsg_recv_single(struct virtproc_info *vrp, struct device *dev,
> >>       * We currently use fixed-sized buffers, so trivially sanitize
> >>       * the reported payload length.
> >>       */
> >> -    if (len > vrp->buf_size ||
> >> +    if (len > vrp->rx_buf_size ||
> >>          msg_len > (len - sizeof(struct rpmsg_hdr))) {
> >>              dev_warn(dev, "inbound msg too big: (%d, %d)\n", len, msg_len);
> >>              return -EINVAL;
> >> @@ -705,7 +710,7 @@ static int rpmsg_recv_single(struct virtproc_info *vrp, struct device *dev,
> >>              dev_warn_ratelimited(dev, "msg received with no recipient\n");
> >>
> >>      /* publish the real size of the buffer */
> >> -    rpmsg_sg_init(&sg, msg, vrp->buf_size);
> >> +    rpmsg_sg_init(&sg, msg, vrp->rx_buf_size);
> >>
> >>      /* add the buffer back to the remote processor's virtqueue */
> >>      err = virtqueue_add_inbuf(vrp->rvq, &sg, 1, msg, GFP_KERNEL);
> >> @@ -819,10 +824,13 @@ static int rpmsg_probe(struct virtio_device *vdev)
> >>      struct virtproc_info *vrp;
> >>      struct virtio_rpmsg_channel *vch = NULL;
> >>      struct rpmsg_device *rpdev_ns, *rpdev_ctrl;
> >> +    u16 rpmsg_buf_align = 0;
> >>      void *bufs_va;
> >>      int err = 0, i;
> >>      size_t total_buf_space;
> >>      bool notify;
> >> +    u8 version;
> >> +    u16 size;
> >>
> >>      vrp = kzalloc_obj(*vrp);
> >>      if (!vrp)
> >> @@ -854,9 +862,87 @@ static int rpmsg_probe(struct virtio_device *vdev)
> >>      else
> >>              vrp->num_tx_buf = MAX_RPMSG_NUM_BUFS;
> >>
> >> -    vrp->buf_size = MAX_RPMSG_BUF_SIZE;
> >> +    /*
> >> +     * If VIRTIO_RPMSG_F_BUFSZ feature is supported, then configure buf
> >> +     * size from virtio device config space from the resource table.
> >> +     * If the feature is not supported, then assign default buf size.
> >> +     */
> >> +    if (virtio_has_feature(vdev, VIRTIO_RPMSG_F_BUFSZ)) {
> >> +            virtio_cread(vdev, struct virtio_rpmsg_config,
> >> +                         version, &version);
> >> +
> >> +            /* for now we support only v1 */
> >> +            if (version != RPMSG_VDEV_CONFIG_V1) {
> >> +                    dev_err(&vdev->dev,
> >> +                            "unsupported vdev config version %u\n", version);
> >> +                    err = -EINVAL;
> >> +                    goto vqs_del;
> >> +            }
> >> +
> >> +            /* size of the config space must match */
> >> +            virtio_cread(vdev, struct virtio_rpmsg_config,
> >> +                         size, &size);
> >> +            if (size != sizeof(struct virtio_rpmsg_config)) {
> >> +                    dev_err(&vdev->dev, "invalid size of vdev config %u\n",
> >> +                            size);
> >> +                    err = -EINVAL;
> >> +                    goto vqs_del;
> >> +            }
> >>
> >> -    total_buf_space = (vrp->num_rx_buf + vrp->num_tx_buf) * vrp->buf_size;
> >> +            /*
> >> +             * Optional alignment applied to each buffer size and to the TX
> >> +             * buffer base address (e.g. to align buffers on a cache line).
> >> +             * It must be a power of two; zero means no extra alignment.
> >> +             */
> >> +            virtio_cread(vdev, struct virtio_rpmsg_config,
> >> +                         rpmsg_buf_align, &rpmsg_buf_align);
> >> +            if (rpmsg_buf_align && !is_power_of_2(rpmsg_buf_align)) {
> >> +                    dev_err(&vdev->dev,
> >> +                            "bad vdev config: rpmsg_buf_align %u is not a power of 2\n",
> >> +                            rpmsg_buf_align);
> >> +                    err = -EINVAL;
> >> +                    goto vqs_del;
> >> +            }
> >> +
> >> +            /* note: tx and rx are defined from remote view */
> >> +            virtio_cread(vdev, struct virtio_rpmsg_config,
> >> +                         txbuf_size, &vrp->rx_buf_size);
> >> +            virtio_cread(vdev, struct virtio_rpmsg_config,
> >> +                         rxbuf_size, &vrp->tx_buf_size);
> >> +
> >> +            /* The buffers must hold at least the rpmsg header */
> >> +            if (vrp->rx_buf_size < sizeof(struct rpmsg_hdr) ||
> >> +                vrp->tx_buf_size < sizeof(struct rpmsg_hdr)) {
> >> +                    dev_err(&vdev->dev,
> >> +                            "bad vdev config: rx buf sz = %u, tx buf sz = %u\n",
> >> +                            vrp->rx_buf_size, vrp->tx_buf_size);
> >> +                    err = -EINVAL;
> >> +                    goto vqs_del;
> >> +            }
> >> +
> >> +            if (rpmsg_buf_align) {
> >> +                    vrp->rx_buf_size_aligned = ALIGN(vrp->rx_buf_size,
> >> +                                                     rpmsg_buf_align);
> >> +                    vrp->tx_buf_size_aligned = ALIGN(vrp->tx_buf_size,
> >> +                                                     rpmsg_buf_align);
> >> +            } else {
> >> +                    vrp->rx_buf_size_aligned = vrp->rx_buf_size;
> >> +                    vrp->tx_buf_size_aligned = vrp->tx_buf_size;
> >> +            }
> >> +
> >> +            dev_dbg(&vdev->dev,
> >> +                    "vdev config: ver=%u, align=0x%x, rx sz = 0x%x, tx sz = 0x%x\n",
> >> +                    version, rpmsg_buf_align, vrp->rx_buf_size,
> >> +                    vrp->tx_buf_size);
> >> +    } else {
> >> +            vrp->rx_buf_size = DEFAULT_RPMSG_BUF_SIZE;
> >> +            vrp->tx_buf_size = DEFAULT_RPMSG_BUF_SIZE;
> >> +            vrp->rx_buf_size_aligned = vrp->rx_buf_size;
> >> +            vrp->tx_buf_size_aligned = vrp->tx_buf_size;
> >> +    }
> >> +
> >> +    total_buf_space = (vrp->num_rx_buf * vrp->rx_buf_size_aligned) +
> >> +                      (vrp->num_tx_buf * vrp->tx_buf_size_aligned);
> >>
> >>      /* allocate coherent memory for the buffers */
> >>      bufs_va = dma_alloc_coherent(vdev->dev.parent,
> >> @@ -873,15 +959,20 @@ static int rpmsg_probe(struct virtio_device *vdev)
> >>      /* first part of the buffers is dedicated for RX */
> >>      vrp->rx_bufs = bufs_va;
> >>
> >> -    /* and second part is dedicated for TX */
> >> -    vrp->tx_bufs = bufs_va + vrp->num_rx_buf * vrp->buf_size;
> >> +    /*
> >> +     * Here buf_va is aligned to a page. Also rx buf size is aligned with
> >> +     * cache line alignment provided by the firmware, so tx buf's start
> >> +     * address is guranteed to be aligned with the alignment provided by
> >> +     * the firmware.
> >> +     */
> >> +    vrp->tx_bufs = bufs_va + (vrp->num_rx_buf * vrp->rx_buf_size_aligned);
> >>
> >>      /* set up the receive buffers */
> >>      for (i = 0; i < vrp->num_rx_buf; i++) {
> >>              struct scatterlist sg;
> >> -            void *cpu_addr = vrp->rx_bufs + i * vrp->buf_size;
> >> +            void *cpu_addr = vrp->rx_bufs + i * vrp->rx_buf_size_aligned;
> >>
> >> -            rpmsg_sg_init(&sg, cpu_addr, vrp->buf_size);
> >> +            rpmsg_sg_init(&sg, cpu_addr, vrp->rx_buf_size);
> >>
> >>              err = virtqueue_add_inbuf(vrp->rvq, &sg, 1, cpu_addr,
> >>                                        GFP_KERNEL);
> >> @@ -964,8 +1055,8 @@ static int rpmsg_remove_device(struct device *dev, void *data)
> >>   static void rpmsg_remove(struct virtio_device *vdev)
> >>   {
> >>      struct virtproc_info *vrp = vdev->priv;
> >> -    unsigned int num_bufs = vrp->num_rx_buf + vrp->num_tx_buf;
> >> -    size_t total_buf_space = num_bufs * vrp->buf_size;
> >> +    size_t total_buf_space = (vrp->num_rx_buf * vrp->rx_buf_size_aligned) +
> >> +                             (vrp->num_tx_buf * vrp->tx_buf_size_aligned);
> >>      int ret;
> >>
> >>      virtio_reset_device(vdev);
> >> @@ -991,6 +1082,7 @@ static struct virtio_device_id id_table[] = {
> >>
> >>   static unsigned int features[] = {
> >>      VIRTIO_RPMSG_F_NS,
> >> +    VIRTIO_RPMSG_F_BUFSZ,
> >>   };
> >>
> >>   static struct virtio_driver virtio_ipc_driver = {
> >> diff --git a/include/linux/rpmsg/virtio_rpmsg.h b/include/linux/rpmsg/virtio_rpmsg.h
> >> new file mode 100644
> >> index 000000000000..735a947d5582
> >> --- /dev/null
> >> +++ b/include/linux/rpmsg/virtio_rpmsg.h
> >> @@ -0,0 +1,48 @@
> >> +/* SPDX-License-Identifier: GPL-2.0 */
> >> +/*
> >> + * Copyright (C) Pinecone Inc. 2019
> >> + * Copyright (C) Xiang Xiao <[email protected]>
> >> + * Copyright (C) Advanced Micro Devices, Inc. 2026
> >> + */
> >> +
> >> +#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 would be in favor to keep it.
>
> The remote processor could expose some alignment constraints. The Linux
> kernel
> or another main processor OS can have its own constraints. As it
> allocates the
> buffers, it can decide the final alignment independently of the buffer
> sizes.
>
> An example is a remote processor without a cache and a main processor with a
> cache. rpmsg_buf_align could be set to 0, but the main processor could
> decide to
> align buffer addresses on a cache line.

But is there an immediate need for that?  I'm all for it if there is a
legitimate use case where we know exactly what the constraints are.
If not we are adding complexity that may, or may not, be able to
address hypothetical constraints.

> Thanks,
> Arnaud
>
> >
> >> + * @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
> >>
> >
>