Re: [PATCH 2/4] rpmsg: char: Add support to use rpmsg_rx_done

[Mathieu Poirier]

On Tue, Jun 07, 2022 at 06:16:43PM -0700, Chris Lew wrote:
> Add support into the rpmsg char driver to skip copying the data into an
> skb if the endpoint supports rpmsg_rx_done. If the endpoint supports
> the rx_done operation, allocate a zero sized skb and set the data to
> the buffer returned in the rx callback. When the packet is read from
> the character device, release the memory by calling rpmsg_rx_done().
> 
> Signed-off-by: Chris Lew <quic_clew@xxxxxxxxxxx>
> ---
>  drivers/rpmsg/rpmsg_char.c | 50 ++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 48 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/rpmsg/rpmsg_char.c b/drivers/rpmsg/rpmsg_char.c
> index b6183d4f62a2..be62ddcf356c 100644
> --- a/drivers/rpmsg/rpmsg_char.c
> +++ b/drivers/rpmsg/rpmsg_char.c
> @@ -91,8 +91,8 @@ int rpmsg_chrdev_eptdev_destroy(struct device *dev, void *data)
>  }
>  EXPORT_SYMBOL(rpmsg_chrdev_eptdev_destroy);
>  
> -static int rpmsg_ept_cb(struct rpmsg_device *rpdev, void *buf, int len,
> -			void *priv, u32 addr)
> +static int rpmsg_ept_copy_cb(struct rpmsg_device *rpdev, void *buf, int len,
> +			     void *priv, u32 addr)
>  {
>  	struct rpmsg_eptdev *eptdev = priv;
>  	struct sk_buff *skb;
> @@ -113,6 +113,43 @@ static int rpmsg_ept_cb(struct rpmsg_device *rpdev, void *buf, int len,
>  	return 0;
>  }
>  
> +static int rpmsg_ept_no_copy_cb(struct rpmsg_device *rpdev, void *buf, int len,
> +				void *priv, u32 addr)
> +{
> +	struct rpmsg_eptdev *eptdev = priv;
> +	struct sk_buff *skb;
> +
> +	skb = alloc_skb(0, GFP_ATOMIC);
> +	if (!skb)
> +		return -ENOMEM;
> +
> +	skb->head = buf;
> +	skb->data = buf;
> +	skb_reset_tail_pointer(skb);
> +	skb_set_end_offset(skb, len);
> +	skb_put(skb, len);
> +

I was worried about all that open ended code but looking at the sk_buff API I
don't think it is possible to do otherwise.  As such:

Reviewed-by: Mathieu Poirier <mathieu.poirier@xxxxxxxxxx>


> +	spin_lock(&eptdev->queue_lock);
> +	skb_queue_tail(&eptdev->queue, skb);
> +	spin_unlock(&eptdev->queue_lock);
> +
> +	/* wake up any blocking processes, waiting for new data */
> +	wake_up_interruptible(&eptdev->readq);
> +
> +	return RPMSG_DEFER;
> +}
> +
> +static int rpmsg_ept_cb(struct rpmsg_device *rpdev, void *buf, int len,
> +			void *priv, u32 addr)
> +{
> +	struct rpmsg_eptdev *eptdev = priv;
> +	rpmsg_rx_cb_t cb;
> +
> +	cb = (eptdev->ept->rx_done) ? rpmsg_ept_no_copy_cb : rpmsg_ept_copy_cb;
> +
> +	return cb(rpdev, buf, len, priv, addr);
> +}
> +
>  static int rpmsg_eptdev_open(struct inode *inode, struct file *filp)
>  {
>  	struct rpmsg_eptdev *eptdev = cdev_to_eptdev(inode->i_cdev);
> @@ -210,6 +247,15 @@ static ssize_t rpmsg_eptdev_read_iter(struct kiocb *iocb, struct iov_iter *to)
>  	if (copy_to_iter(skb->data, use, to) != use)
>  		use = -EFAULT;
>  
> +	if (eptdev->ept->rx_done) {
> +		rpmsg_rx_done(eptdev->ept, skb->data);
> +		/*
> +		 * Data memory is freed by rpmsg_rx_done(), reset the skb data
> +		 * pointers so kfree_skb() does not try to free a second time.
> +		 */
> +		skb->head = NULL;
> +		skb->data = NULL;
> +	}
>  	kfree_skb(skb);
>  
>  	return use;
> -- 
> 2.7.4
>