Re: [PATCH v2 1/3] rpmsg: virtio_rpmsg_bus: allow different size of tx and rx bufs

[Mathieu Poirier]

Hi Tanmay

(Apologies for the late review)

On Wed, Apr 29, 2026 at 09:10:51AM -0700, Tanmay Shah wrote:
> Current design allocates memory for tx and rx buffers equally. The
> throughput can be increased if the user is allowed to configure number
> of tx and rx buffers as required. Hence, do not split number of tx & rx
> buffers into half, but decide based on respective vring size.
> 
> Signed-off-by: Tanmay Shah <tanmay.shah@xxxxxxx>
> ---
> 
> Test performed:
>   - Test this patch with existing firmware as it is, rpmsg working.
> 
> Changes in v2:
>   - Change author
>   - fix commit message with better explanation
>   - %s/sbuf/tx_buf
>   - %s/rbuf/rx_buf
>   - %s/num_rbuf/num_rx_buf/
>   - %s/num_sbuf/num_tx_buf/

Please split this patch in two parts - one to do the refactoring of the
tx/rx_buf and another one for the varying size.

> 
>  drivers/rpmsg/virtio_rpmsg_bus.c | 68 ++++++++++++++++----------------
>  1 file changed, 34 insertions(+), 34 deletions(-)
> 
> diff --git a/drivers/rpmsg/virtio_rpmsg_bus.c b/drivers/rpmsg/virtio_rpmsg_bus.c
> index 5ae15111fb4f..e59d8cf9b975 100644
> --- a/drivers/rpmsg/virtio_rpmsg_bus.c
> +++ b/drivers/rpmsg/virtio_rpmsg_bus.c
> @@ -35,13 +35,14 @@
>   * @vdev:	the virtio device
>   * @rvq:	rx virtqueue
>   * @svq:	tx virtqueue
> - * @rbufs:	kernel address of rx buffers
> - * @sbufs:	kernel address of tx buffers
> - * @num_bufs:	total number of buffers for rx and tx
> - * @buf_size:   size of one rx or tx buffer
> + * @rx_bufs:	kernel address of rx buffers
> + * @tx_bufs:	kernel address of tx buffers
> + * @num_rx_buf:	total number of buffers for rx
> + * @num_tx_buf:	total number of buffers for tx
> + * @buf_size:	size of one rx or tx buffer
>   * @last_sbuf:	index of last tx buffer used
>   * @bufs_dma:	dma base addr of the buffers
> - * @tx_lock:	protects svq and sbufs, to allow concurrent senders.
> + * @tx_lock:	protects svq and tx_bufs, to allow concurrent senders.
>   *		sending a message might require waking up a dozing remote
>   *		processor, which involves sleeping, hence the mutex.
>   * @endpoints:	idr of local endpoints, allows fast retrieval
> @@ -55,8 +56,9 @@
>  struct virtproc_info {
>  	struct virtio_device *vdev;
>  	struct virtqueue *rvq, *svq;
> -	void *rbufs, *sbufs;
> -	unsigned int num_bufs;
> +	void *rx_bufs, *tx_bufs;
> +	unsigned int num_rx_buf;
> +	unsigned int num_tx_buf;
>  	unsigned int buf_size;
>  	int last_sbuf;
>  	dma_addr_t bufs_dma;
> @@ -110,7 +112,7 @@ struct virtio_rpmsg_channel {
>  /*
>   * We're allocating buffers of 512 bytes each for communications. The
>   * number of buffers will be computed from the number of buffers supported
> - * by the vring, upto a maximum of 512 buffers (256 in each direction).
> + * by the vring, up to a maximum of 256 in each direction.
>   *
>   * Each buffer will have 16 bytes for the msg header and 496 bytes for
>   * the payload.
> @@ -125,7 +127,7 @@ struct virtio_rpmsg_channel {
>   * can change this without changing anything in the firmware of the remote
>   * processor.
>   */
> -#define MAX_RPMSG_NUM_BUFS	(512)
> +#define MAX_RPMSG_NUM_BUFS	(256)
>  #define MAX_RPMSG_BUF_SIZE	(512)
>  
>  /*
> @@ -440,12 +442,9 @@ static void *get_a_tx_buf(struct virtproc_info *vrp)
>  
>  	mutex_lock(&vrp->tx_lock);
>  
> -	/*
> -	 * either pick the next unused tx buffer
> -	 * (half of our buffers are used for sending messages)
> -	 */
> -	if (vrp->last_sbuf < vrp->num_bufs / 2)
> -		ret = vrp->sbufs + vrp->buf_size * vrp->last_sbuf++;
> +	/* either pick the next unused tx buffer */
> +	if (vrp->last_sbuf < vrp->num_tx_buf)
> +		ret = vrp->tx_bufs + vrp->buf_size * vrp->last_sbuf++;
>  	/* or recycle a used one */
>  	else
>  		ret = virtqueue_get_buf(vrp->svq, &len);
> @@ -631,11 +630,10 @@ static __poll_t virtio_rpmsg_poll(struct rpmsg_endpoint *ept, struct file *filp,
>  
>  	/*
>  	 * check for a free buffer, either:
> -	 * - we haven't used all of the available transmit buffers (half of the
> -	 *   allocated buffers are used for transmit, hence num_bufs / 2), or,
> +	 * - we haven't used all of the available transmit buffers or,
>  	 * - we ask the virtqueue if there's a buffer available
>  	 */
> -	if (vrp->last_sbuf < vrp->num_bufs / 2 ||
> +	if (vrp->last_sbuf < vrp->num_tx_buf ||
>  	    !virtqueue_enable_cb(vrp->svq))
>  		mask |= EPOLLOUT;
>  
> @@ -846,19 +844,20 @@ static int rpmsg_probe(struct virtio_device *vdev)
>  	vrp->rvq = vqs[0];
>  	vrp->svq = vqs[1];
>  
> -	/* we expect symmetric tx/rx vrings */
> -	WARN_ON(virtqueue_get_vring_size(vrp->rvq) !=
> -		virtqueue_get_vring_size(vrp->svq));
> -
>  	/* we need less buffers if vrings are small */
> -	if (virtqueue_get_vring_size(vrp->rvq) < MAX_RPMSG_NUM_BUFS / 2)
> -		vrp->num_bufs = virtqueue_get_vring_size(vrp->rvq) * 2;
> +	if (virtqueue_get_vring_size(vrp->rvq) < MAX_RPMSG_NUM_BUFS)
> +		vrp->num_rx_buf = virtqueue_get_vring_size(vrp->rvq);
> +	else
> +		vrp->num_rx_buf = MAX_RPMSG_NUM_BUFS;
> +
> +	if (virtqueue_get_vring_size(vrp->svq) < MAX_RPMSG_NUM_BUFS)
> +		vrp->num_tx_buf = virtqueue_get_vring_size(vrp->svq);
>  	else
> -		vrp->num_bufs = MAX_RPMSG_NUM_BUFS;
> +		vrp->num_tx_buf = MAX_RPMSG_NUM_BUFS;
>  
>  	vrp->buf_size = MAX_RPMSG_BUF_SIZE;
>  
> -	total_buf_space = vrp->num_bufs * vrp->buf_size;
> +	total_buf_space = (vrp->num_rx_buf + vrp->num_tx_buf) * vrp->buf_size;
>  
>  	/* allocate coherent memory for the buffers */
>  	bufs_va = dma_alloc_coherent(vdev->dev.parent,
> @@ -872,16 +871,16 @@ static int rpmsg_probe(struct virtio_device *vdev)
>  	dev_dbg(&vdev->dev, "buffers: va %p, dma %pad\n",
>  		bufs_va, &vrp->bufs_dma);
>  
> -	/* half of the buffers is dedicated for RX */
> -	vrp->rbufs = bufs_va;
> +	/* first part of the buffers is dedicated for RX */
> +	vrp->rx_bufs = bufs_va;
>  
> -	/* and half is dedicated for TX */
> -	vrp->sbufs = bufs_va + total_buf_space / 2;
> +	/* and second part is dedicated for TX */
> +	vrp->tx_bufs = bufs_va + vrp->num_rx_buf * vrp->buf_size;
>  
>  	/* set up the receive buffers */
> -	for (i = 0; i < vrp->num_bufs / 2; i++) {
> +	for (i = 0; i < vrp->num_rx_buf; i++) {
>  		struct scatterlist sg;
> -		void *cpu_addr = vrp->rbufs + i * vrp->buf_size;
> +		void *cpu_addr = vrp->rx_bufs + i * vrp->buf_size;
>  
>  		rpmsg_sg_init(&sg, cpu_addr, vrp->buf_size);
>  
> @@ -966,7 +965,8 @@ static int rpmsg_remove_device(struct device *dev, void *data)
>  static void rpmsg_remove(struct virtio_device *vdev)
>  {
>  	struct virtproc_info *vrp = vdev->priv;
> -	size_t total_buf_space = vrp->num_bufs * vrp->buf_size;
> +	unsigned int num_bufs = vrp->num_rx_buf + vrp->num_tx_buf;
> +	size_t total_buf_space = num_bufs * vrp->buf_size;
>  	int ret;
>  
>  	virtio_reset_device(vdev);
> @@ -980,7 +980,7 @@ static void rpmsg_remove(struct virtio_device *vdev)
>  	vdev->config->del_vqs(vrp->vdev);
>  
>  	dma_free_coherent(vdev->dev.parent, total_buf_space,
> -			  vrp->rbufs, vrp->bufs_dma);
> +			  vrp->rx_bufs, vrp->bufs_dma);
>  
>  	kfree(vrp);
>  }
> -- 
> 2.34.1
>