Re: [PATCH v4 3/5] rpmsg: virtio_rpmsg_bus: get buffer size from config space

[Arnaud POULIQUEN <arnaud.pouliquen@foss.st.com>]

Hi Tanmay,

On 6/15/26 22:20, 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 v4: squash to virtio rpmsg config patch
>    - 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   | 129 ++++++++++++++++++++++++-----
>   include/linux/rpmsg/virtio_rpmsg.h |  50 +++++++++++
>   2 files changed, 160 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 99df1ae07055..a59925f870a4 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 +41,8 @@
>    * @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
>    * @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 +62,8 @@ 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;
>   	int last_tx_buf;
>   	dma_addr_t bufs_dma;
>   	struct mutex tx_lock;
> @@ -68,9 +72,6 @@ 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 +129,7 @@ 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.
> @@ -444,7 +445,7 @@ 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 * vrp->last_tx_buf++;
>   	/* or recycle a used one */
>   	else
>   		ret = virtqueue_get_buf(vrp->svq, &len);
> @@ -514,7 +515,7 @@ 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;
>   	}
> @@ -647,7 +648,7 @@ 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,
> @@ -673,7 +674,7 @@ 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;
> @@ -706,7 +707,7 @@ 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);
> @@ -820,10 +821,13 @@ 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)
> @@ -855,9 +859,90 @@ 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 two\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;
> +		}
> +
> +		/*
> +		 * The buffer size must be aligned to the provided alignment for
> +		 * so that the start address of tx bufs can be aligned.
> +		 */

'tx' to remove as  it also concerns Rx buffers


What about removing this check to manage alignment during buffer allocation?

For example, if the alignment is on a 64-bit address and the tx_buffer 
and rx_buffer sizes are 40 bytes, 48 bytes can be allocated in memory 
for each buffer, and the virtio descriptor can be filled with aligned 
addresses.

In other words, the rpmsg_buf_align field contains the alignment 
constraint from the remote processor. If the Linux kernel wants to 
impose another alignment constraint, it must test or update 
rpmsg_buf_align, but it must not impose alignment on the buffer size.


> +		if (rpmsg_buf_align &&
> +		    (!IS_ALIGNED(vrp->rx_buf_size, rpmsg_buf_align) ||
> +		     !IS_ALIGNED(vrp->tx_buf_size, rpmsg_buf_align))) {
> +			dev_err(&vdev->dev,
> +				"bad vdev config: buf sizes (rx %u, tx %u) not aligned to %u\n",
> +				vrp->rx_buf_size, vrp->tx_buf_size,
> +				rpmsg_buf_align);
> +			err = -EINVAL;
> +			goto vqs_del;
> +		}
> +
> +		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;
> +	}
> +
> +	total_buf_space = (vrp->num_rx_buf * vrp->rx_buf_size) +
> +			  (vrp->num_tx_buf * vrp->tx_buf_size);
>   
>   	/* allocate coherent memory for the buffers */
>   	bufs_va = dma_alloc_coherent(vdev->dev.parent,
> @@ -874,15 +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);
>   
>   	/* 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;
>   
> -		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);
> @@ -965,8 +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) +
> +				 (vrp->num_tx_buf * vrp->tx_buf_size);
>   	int ret;
>   
>   	virtio_reset_device(vdev);
> @@ -992,6 +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..7e14da68fd17
> --- /dev/null
> +++ b/include/linux/rpmsg/virtio_rpmsg.h
> @@ -0,0 +1,50 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Copyright (C) Pinecone Inc. 2019
> + * Copyright (C) Xiang Xiao <xiaoxiang@pinecone.net>
> + * 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.
> + * @reserved:	reserved for padding, must be zero.
> + * @size:	size of this structure in bytes.
> + * @rpmsg_buf_align:	required alignment in bytes for each buffer. Must be a
> + *		power of two so that both the buffer sizes and the TX buffer
> + *		base address can be aligned (e.g. to a cache line).
> + * @reserved1:	reserved for padding, must be zero. Keeps the following 32-bit
> + *		fields naturally aligned.
> + * @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;
> +	u8 reserved;

Why about defining the version type to u16 to avoid the reserved field?

> +	__virtio16 size;
> +	__virtio16 rpmsg_buf_align;
> +	__virtio16 reserved1;

Seems useless if __packed prevents the compiler from inserting extra padding
bytes between fields,

> +	/* The tx/rx individual buffer size (if VIRTIO_RPMSG_F_BUFSZ) */
> +	__virtio32 txbuf_size;
> +	__virtio32 rxbuf_size;
> +} __packed;

proposal

+struct virtio_rpmsg_config {
+	__virtio16 version;
+	__virtio16 size;
+	/* The tx/rx individual buffer size (if VIRTIO_RPMSG_F_BUFSZ) */
+	__virtio32 txbuf_size;
+	__virtio32 rxbuf_size;
+	__virtio16 rpmsg_buf_align;
+} __packed;
+

Regards,
Arnaud

> +
> +#endif /* _LINUX_VIRTIO_RPMSG_H */