Re: [PATCH] rpmsg: virtio: EPOLLOUT support

[Mathieu Poirier <mathieu.poirier@linaro.org>]

Hi Patrick,

I started reviewing this patch.  Despite the short amount of code, it will take
me a few days to understand the ramifications of your proposal.

Thanks,
Mathieu

On Wed, Oct 22, 2025 at 02:28:17PM +1100, patrick.oppenlander@gmail.com wrote:
> From: Patrick Oppenlander <patrick.oppenlander@gmail.com>
> 
> Previously, polling an rpmsg endpoint (e.g. /dev/ttyRPMSGx) would
> generate EPOLLIN events but no EPOLLOUT events.
> 
> Unfortunately, poll support means that we can no longer disable
> tx-complete interrupts as there is no way to know whether a poller is
> waiting in sendq, so we always need notifications.
> 
> Signed-off-by: Patrick Oppenlander <patrick.oppenlander@gmail.com>
> ---
>  drivers/rpmsg/virtio_rpmsg_bus.c | 101 ++++++++++---------------------
>  1 file changed, 32 insertions(+), 69 deletions(-)
> 
> diff --git a/drivers/rpmsg/virtio_rpmsg_bus.c b/drivers/rpmsg/virtio_rpmsg_bus.c
> index 484890b4a6a74..79d983055b4d6 100644
> --- a/drivers/rpmsg/virtio_rpmsg_bus.c
> +++ b/drivers/rpmsg/virtio_rpmsg_bus.c
> @@ -41,13 +41,12 @@
>   * @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, sbufs and sleepers, to allow concurrent senders.
> + * @tx_lock:	protects svq and sbufs, 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
>   * @endpoints_lock: lock of the endpoints set
>   * @sendq:	wait queue of sending contexts waiting for a tx buffers
> - * @sleepers:	number of senders that are waiting for a tx buffer
>   *
>   * This structure stores the rpmsg state of a given virtio remote processor
>   * device (there might be several virtio proc devices for each physical
> @@ -65,7 +64,6 @@ struct virtproc_info {
>  	struct idr endpoints;
>  	struct mutex endpoints_lock;
>  	wait_queue_head_t sendq;
> -	atomic_t sleepers;
>  };
>  
>  /* The feature bitmap for virtio rpmsg */
> @@ -144,6 +142,8 @@ static int virtio_rpmsg_sendto(struct rpmsg_endpoint *ept, void *data, int len,
>  static int virtio_rpmsg_trysend(struct rpmsg_endpoint *ept, void *data, int len);
>  static int virtio_rpmsg_trysendto(struct rpmsg_endpoint *ept, void *data,
>  				  int len, u32 dst);
> +static __poll_t virtio_rpmsg_poll(struct rpmsg_endpoint *ept, struct file *filp,
> +				  poll_table *wait);
>  static ssize_t virtio_rpmsg_get_mtu(struct rpmsg_endpoint *ept);
>  static struct rpmsg_device *__rpmsg_create_channel(struct virtproc_info *vrp,
>  						   struct rpmsg_channel_info *chinfo);
> @@ -154,6 +154,7 @@ static const struct rpmsg_endpoint_ops virtio_endpoint_ops = {
>  	.sendto = virtio_rpmsg_sendto,
>  	.trysend = virtio_rpmsg_trysend,
>  	.trysendto = virtio_rpmsg_trysendto,
> +	.poll = virtio_rpmsg_poll,
>  	.get_mtu = virtio_rpmsg_get_mtu,
>  };
>  
> @@ -436,7 +437,6 @@ static void *get_a_tx_buf(struct virtproc_info *vrp)
>  	unsigned int len;
>  	void *ret;
>  
> -	/* support multiple concurrent senders */
>  	mutex_lock(&vrp->tx_lock);
>  
>  	/*
> @@ -454,62 +454,6 @@ static void *get_a_tx_buf(struct virtproc_info *vrp)
>  	return ret;
>  }
>  
> -/**
> - * rpmsg_upref_sleepers() - enable "tx-complete" interrupts, if needed
> - * @vrp: virtual remote processor state
> - *
> - * This function is called before a sender is blocked, waiting for
> - * a tx buffer to become available.
> - *
> - * If we already have blocking senders, this function merely increases
> - * the "sleepers" reference count, and exits.
> - *
> - * Otherwise, if this is the first sender to block, we also enable
> - * virtio's tx callbacks, so we'd be immediately notified when a tx
> - * buffer is consumed (we rely on virtio's tx callback in order
> - * to wake up sleeping senders as soon as a tx buffer is used by the
> - * remote processor).
> - */
> -static void rpmsg_upref_sleepers(struct virtproc_info *vrp)
> -{
> -	/* support multiple concurrent senders */
> -	mutex_lock(&vrp->tx_lock);
> -
> -	/* are we the first sleeping context waiting for tx buffers ? */
> -	if (atomic_inc_return(&vrp->sleepers) == 1)
> -		/* enable "tx-complete" interrupts before dozing off */
> -		virtqueue_enable_cb(vrp->svq);
> -
> -	mutex_unlock(&vrp->tx_lock);
> -}
> -
> -/**
> - * rpmsg_downref_sleepers() - disable "tx-complete" interrupts, if needed
> - * @vrp: virtual remote processor state
> - *
> - * This function is called after a sender, that waited for a tx buffer
> - * to become available, is unblocked.
> - *
> - * If we still have blocking senders, this function merely decreases
> - * the "sleepers" reference count, and exits.
> - *
> - * Otherwise, if there are no more blocking senders, we also disable
> - * virtio's tx callbacks, to avoid the overhead incurred with handling
> - * those (now redundant) interrupts.
> - */
> -static void rpmsg_downref_sleepers(struct virtproc_info *vrp)
> -{
> -	/* support multiple concurrent senders */
> -	mutex_lock(&vrp->tx_lock);
> -
> -	/* are we the last sleeping context waiting for tx buffers ? */
> -	if (atomic_dec_and_test(&vrp->sleepers))
> -		/* disable "tx-complete" interrupts */
> -		virtqueue_disable_cb(vrp->svq);
> -
> -	mutex_unlock(&vrp->tx_lock);
> -}
> -
>  /**
>   * rpmsg_send_offchannel_raw() - send a message across to the remote processor
>   * @rpdev: the rpmsg channel
> @@ -582,9 +526,6 @@ static int rpmsg_send_offchannel_raw(struct rpmsg_device *rpdev,
>  
>  	/* no free buffer ? wait for one (but bail after 15 seconds) */
>  	while (!msg) {
> -		/* enable "tx-complete" interrupts, if not already enabled */
> -		rpmsg_upref_sleepers(vrp);
> -
>  		/*
>  		 * sleep until a free buffer is available or 15 secs elapse.
>  		 * the timeout period is not configurable because there's
> @@ -595,9 +536,6 @@ static int rpmsg_send_offchannel_raw(struct rpmsg_device *rpdev,
>  					(msg = get_a_tx_buf(vrp)),
>  					msecs_to_jiffies(15000));
>  
> -		/* disable "tx-complete" interrupts if we're the last sleeper */
> -		rpmsg_downref_sleepers(vrp);
> -
>  		/* timeout ? */
>  		if (!err) {
>  			dev_err(dev, "timeout waiting for a tx buffer\n");
> @@ -676,6 +614,34 @@ static int virtio_rpmsg_trysendto(struct rpmsg_endpoint *ept, void *data,
>  	return rpmsg_send_offchannel_raw(rpdev, src, dst, data, len, false);
>  }
>  
> +static __poll_t virtio_rpmsg_poll(struct rpmsg_endpoint *ept, struct file *filp,
> +				  poll_table *wait)
> +{
> +	struct rpmsg_device *rpdev = ept->rpdev;
> +	struct virtio_rpmsg_channel *vch = to_virtio_rpmsg_channel(rpdev);
> +	struct virtproc_info *vrp = vch->vrp;
> +	__poll_t mask = 0;
> +
> +	poll_wait(filp, &vrp->sendq, wait);
> +
> +	/* support multiple concurrent senders */
> +	mutex_lock(&vrp->tx_lock);
> +
> +	/*
> +	 * 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 ask the virtqueue if there's a buffer available
> +	 */
> +	if (vrp->last_sbuf < vrp->num_bufs / 2 ||
> +	    !virtqueue_enable_cb(vrp->svq))
> +		mask |= EPOLLOUT;
> +
> +	mutex_unlock(&vrp->tx_lock);
> +
> +	return mask;
> +}
> +
>  static ssize_t virtio_rpmsg_get_mtu(struct rpmsg_endpoint *ept)
>  {
>  	struct rpmsg_device *rpdev = ept->rpdev;
> @@ -922,9 +888,6 @@ static int rpmsg_probe(struct virtio_device *vdev)
>  		WARN_ON(err); /* sanity check; this can't really happen */
>  	}
>  
> -	/* suppress "tx-complete" interrupts */
> -	virtqueue_disable_cb(vrp->svq);
> -
>  	vdev->priv = vrp;
>  
>  	rpdev_ctrl = rpmsg_virtio_add_ctrl_dev(vdev);
> -- 
> 2.51.1.dirty
>