Message-ID: <324fdbe2-037a-4daa-84de-8b63dbac8117@foss.st.com>
Date: Fri, 5 Dec 2025 17:01:17 +0100
From: Arnaud POULIQUEN <arnaud.pouliquen@...s.st.com>
To: <tanmay.shah@....com>, Zhongqiu Han <zhongqiu.han@....qualcomm.com>,
        <andersson@...nel.org>, <mathieu.poirier@...aro.org>, <mst@...hat.com>,
        <jasowang@...hat.com>, <xuanzhuo@...ux.alibaba.com>,
        <eperezma@...hat.com>
CC: <linux-kernel@...r.kernel.org>, <linux-remoteproc@...r.kernel.org>,
        <virtualization@...ts.linux.dev>, <xiaoxiang@...omi.com>,
        Xiang Xiao
	<xiaoxiang781216@...il.com>
Subject: Re: [RFC PATCH 2/2] rpmsg: virtio_rpmsg_bus: get buffer size from
 config space

Hi,

On 12/3/25 19:12, Tanmay Shah wrote:
> Hello,
> 
> Thanks for your reviews. Please find the response below.
> 
> On 11/22/25 6:05 AM, Zhongqiu Han wrote:
>> On 11/15/2025 2:46 AM, Tanmay Shah wrote:
>>> From: Xiang Xiao <xiaoxiang781216@...il.com>
>>>
>>> 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: Xiang Xiao <xiaoxiang@...omi.com>
>>> Signed-off-by: Tanmay Shah <tanmay.shah@....com>
>>> ---
>>>   drivers/rpmsg/virtio_rpmsg_bus.c | 68 +++++++++++++++++++++++---------
>>>   include/linux/virtio_rpmsg.h     | 24 +++++++++++
>>>   2 files changed, 74 insertions(+), 18 deletions(-)
>>>   create mode 100644 include/linux/virtio_rpmsg.h
>>>
>>> diff --git a/drivers/rpmsg/virtio_rpmsg_bus.c b/drivers/rpmsg/ 
>>> virtio_rpmsg_bus.c
>>> index cc26dfcc3e29..03dd5535880a 100644
>>> --- a/drivers/rpmsg/virtio_rpmsg_bus.c
>>> +++ b/drivers/rpmsg/virtio_rpmsg_bus.c
>>> @@ -25,6 +25,7 @@
>>>   #include <linux/sched.h>
>>>   #include <linux/virtio.h>
>>>   #include <linux/virtio_ids.h>
>>> +#include <linux/virtio_rpmsg.h>
>>>   #include <linux/virtio_config.h>
>>>   #include <linux/wait.h>
>>> @@ -39,7 +40,8 @@
>>>    * @sbufs:    kernel address of tx buffers
>>>    * @num_rbufs:    total number of buffers for rx
>>>    * @num_sbufs:    total number of buffers for tx
>>> - * @buf_size:    size of one rx or tx buffer
>>> + * @rbuf_size:    size of one rx buffer
>>> + * @sbuf_size:    size of one tx buffer
>>>    * @last_sbuf:    index of last tx buffer used
>>>    * @bufs_dma:    dma base addr of the buffers
>>>    * @tx_lock:    protects svq, sbufs and sleepers, to allow 
>>> concurrent senders.
>>> @@ -60,7 +62,8 @@ struct virtproc_info {
>>>       void *rbufs, *sbufs;
>>>       unsigned int num_rbufs;
>>>       unsigned int num_sbufs;
>>> -    unsigned int buf_size;
>>> +    unsigned int rbuf_size;
>>> +    unsigned int sbuf_size;
>>>       int last_sbuf;
>>>       dma_addr_t bufs_dma;
>>>       struct mutex tx_lock;
>>> @@ -70,9 +73,6 @@ struct virtproc_info {
>>>       atomic_t sleepers;
>>>   };
>>> -/* 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
>>> @@ -130,7 +130,7 @@ 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 +443,7 @@ static void *get_a_tx_buf(struct virtproc_info *vrp)
>>>       /* either pick the next unused tx buffer */
>>>       if (vrp->last_sbuf < vrp->num_sbufs)
>>> -        ret = vrp->sbufs + vrp->buf_size * vrp->last_sbuf++;
>>> +        ret = vrp->sbufs + vrp->sbuf_size * vrp->last_sbuf++;
>>>       /* or recycle a used one */
>>>       else
>>>           ret = virtqueue_get_buf(vrp->svq, &len);
>>> @@ -569,7 +569,7 @@ 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->sbuf_size - sizeof(struct rpmsg_hdr)) {
>>>           dev_err(dev, "message is too big (%d)\n", len);
>>>           return -EMSGSIZE;
>>>       }
>>> @@ -680,7 +680,7 @@ 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->sbuf_size - sizeof(struct rpmsg_hdr);
>>>   }
>>>   static int rpmsg_recv_single(struct virtproc_info *vrp, struct 
>>> device *dev,
>>> @@ -706,7 +706,7 @@ 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->rbuf_size ||
>>>           msg_len > (len - sizeof(struct rpmsg_hdr))) {
>>>           dev_warn(dev, "inbound msg too big: (%d, %d)\n", len, 
>>> msg_len);
>>>           return -EINVAL;
>>> @@ -739,7 +739,7 @@ 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->rbuf_size);
>>>       /* add the buffer back to the remote processor's virtqueue */
>>>       err = virtqueue_add_inbuf(vrp->rvq, &sg, 1, msg, GFP_KERNEL);
>>> @@ -888,9 +888,39 @@ static int rpmsg_probe(struct virtio_device *vdev)
>>>       else
>>>           vrp->num_sbufs = 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)) {
>>> +        /* note: virtio_rpmsg_config is defined from remote view */
>>> +        virtio_cread(vdev, struct virtio_rpmsg_config,
>>> +                 txbuf_size, &vrp->rbuf_size);
>>> +        virtio_cread(vdev, struct virtio_rpmsg_config,
>>> +                 rxbuf_size, &vrp->sbuf_size);
>>> +
>>> +        /* The buffers must hold rpmsg header atleast */
>>> +        if (vrp->rbuf_size < sizeof(struct rpmsg_hdr) ||
>>> +            vrp->sbuf_size < sizeof(struct rpmsg_hdr)) {
>>
>>
>> Hello Tanmay,
>>
>> May I know if the omission of = here is to accommodate the ping/pong/ack
>> scenarios? mtu will 0
>>
>>
> 
> Yes. At minimum RPMsg header is needed to ping the correct endpoint. We 
> don't need to have any payload attached to the packet. MTU will be 
> sizeof rpmsg_hdr I think.
> 
>>> +            dev_err(&vdev->dev,
>>> +                "vdev config: rx buf sz = %d, tx buf sz = %d\n",
>>> +                vrp->rbuf_size, vrp->sbuf_size);
>>> +            err = -EINVAL;
>>> +            goto vqs_del;
>>> +        }
>>> +
>>> +        dev_dbg(&vdev->dev,
>>> +            "vdev config: rx buf sz = 0x%x, tx buf sz = 0x%x\n",
>>> +            vrp->rbuf_size, vrp->sbuf_size);
>>> +    } else {
>>> +        vrp->rbuf_size = DEFAULT_RPMSG_BUF_SIZE;
>>> +        vrp->sbuf_size = DEFAULT_RPMSG_BUF_SIZE;
>>> +    }
>>> -    total_buf_space = (vrp->num_rbufs + vrp->num_sbufs) * vrp- 
>>> >buf_size;
>>> +    total_buf_space = (vrp->num_rbufs * vrp->rbuf_size) +
>>> +              (vrp->num_sbufs * vrp->sbuf_size);
>>> +    total_buf_space = ALIGN(total_buf_space, PAGE_SIZE);
>>>       /* allocate coherent memory for the buffers */
>>>       bufs_va = dma_alloc_coherent(vdev->dev.parent,
>>> @@ -908,14 +938,14 @@ static int rpmsg_probe(struct virtio_device *vdev)
>>>       vrp->rbufs = bufs_va;
>>>       /* and second part is dedicated for TX */
>>> -    vrp->sbufs = bufs_va + vrp->num_rbufs * vrp->buf_size;
>>> +    vrp->sbufs = bufs_va + (vrp->num_rbufs * vrp->rbuf_size);
>>>       /* set up the receive buffers */
>>>       for (i = 0; i < vrp->num_rbufs; i++) {
>>>           struct scatterlist sg;
>>> -        void *cpu_addr = vrp->rbufs + i * vrp->buf_size;
>>> +        void *cpu_addr = vrp->rbufs + i * vrp->rbuf_size;
>>> -        rpmsg_sg_init(&sg, cpu_addr, vrp->buf_size);
>>> +        rpmsg_sg_init(&sg, cpu_addr, vrp->rbuf_size);
>>>           err = virtqueue_add_inbuf(vrp->rvq, &sg, 1, cpu_addr,
>>>                         GFP_KERNEL);
>>> @@ -1001,8 +1031,8 @@ 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_rbufs + vrp->num_sbufs;
>>> -    size_t total_buf_space = num_bufs * vrp->buf_size;
>>> +    size_t total_buf_space = (vrp->num_rbufs * vrp->rbuf_size) +
>>> +                 (vrp->num_sbufs * vrp->sbuf_size);
>>>       int ret;
>>>       virtio_reset_device(vdev);
>>> @@ -1015,6 +1045,7 @@ static void rpmsg_remove(struct virtio_device 
>>> *vdev)
>>>       vdev->config->del_vqs(vrp->vdev);
>>> +    total_buf_space = ALIGN(total_buf_space, PAGE_SIZE);
>>>       dma_free_coherent(vdev->dev.parent, total_buf_space,
>>>                 vrp->rbufs, vrp->bufs_dma);
>>> @@ -1028,6 +1059,7 @@ 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/virtio_rpmsg.h b/include/linux/virtio_rpmsg.h
>>> new file mode 100644
>>> index 000000000000..6406bc505383
>>> --- /dev/null
>>> +++ b/include/linux/virtio_rpmsg.h
>>> @@ -0,0 +1,24 @@
>>> +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
>>
>>
>> Echo Arnaud's comments. If it is intended for UAPI, please keep it in
>> include/uapi/linux
>>
>>
> 
> It's not intended for UAPI. I need to fix the license. I will check 
> other virtio headers in the same directory and fix the license accordingly.
> 
>>> +/*
>>> + * Copyright (C) Pinecone Inc. 2019
>>> + * Copyright (C) Xiang Xiao <xiaoxiang@...econe.net>
>>> + */
>>> +
>>> +#ifndef _LINUX_VIRTIO_RPMSG_H
>>> +#define _LINUX_VIRTIO_RPMSG_H
>>> +
>>> +#include <linux/types.h>
>>> +
>>> +/* The feature bitmap for virtio rpmsg */
>>> +#define VIRTIO_RPMSG_F_NS    0 /* RP supports name service 
>>> notifications */
>>> +#define VIRTIO_RPMSG_F_BUFSZ    2 /* RP get buffer size from config 
>>> space */
>>
>> May I know why skip bit 1?
>>
>>
> 
> Thanks, that's a good question. I keept id 2 unmodified from the 
> original series. I don't know why ID 2 was chosen in the original 
> series. I will have to discuss this with the linux remoteproc/rpmsg 
> maintainers and choose the correct ID.
> 
> I don't see any problem choosing ID 1, but for some reason if ID 1 was 
> assigned and deprecated (I don't think that is the case) then only we 
> should use ID 2.


The ID 1 was proposed in an openamp PR [1]. If we
enter VIRTIO_RPMSG_F_BUFSZ first it makes sense to set its ID to 1.

[1]https://github.com/OpenAMP/open-amp/pull/160/commits/d4a13128f94e46180285c05a20da78fdca54f7d7


Regards,
Arnaud

> 
> 
> Arnaud, Mathieu, Bjorn any input here?
> 
>>> +
>>> +struct virtio_rpmsg_config {
>>> +    /* The tx/rx individual buffer size(if VIRTIO_RPMSG_F_BUFSZ) */
>>> +    __u32 txbuf_size;
>>> +    __u32 rxbuf_size;
>>> +    __u32 reserved[14]; /* Reserve for the future use */
>>
>> Should we use __virtio32 instead of __u32 to avoid endianness issues?
>>
>>
> 
> Sure, if that is the standard in other virtio headers I will modify it.
> 
> Thanks,
> Tanmay
> 
>>> +    /* Put the customize config here */
>>> +} __attribute__((packed));
>>> +
>>> +#endif /* _LINUX_VIRTIO_RPMSG_H */
>>
>>
> 

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