Re: [net-next v1 06/16] netdev: support binding dma-buf to netdevice

[David Ahern <dsahern@kernel.org>]

On 12/7/23 5:52 PM, Mina Almasry wrote:
> +
> +static int netdev_restart_rx_queue(struct net_device *dev, int rxq_idx)
> +{
> +	void *new_mem;
> +	void *old_mem;
> +	int err;
> +
> +	if (!dev || !dev->netdev_ops)
> +		return -EINVAL;
> +
> +	if (!dev->netdev_ops->ndo_queue_stop ||
> +	    !dev->netdev_ops->ndo_queue_mem_free ||
> +	    !dev->netdev_ops->ndo_queue_mem_alloc ||
> +	    !dev->netdev_ops->ndo_queue_start)
> +		return -EOPNOTSUPP;
> +
> +	new_mem = dev->netdev_ops->ndo_queue_mem_alloc(dev, rxq_idx);
> +	if (!new_mem)
> +		return -ENOMEM;
> +
> +	err = dev->netdev_ops->ndo_queue_stop(dev, rxq_idx, &old_mem);
> +	if (err)
> +		goto err_free_new_mem;
> +
> +	err = dev->netdev_ops->ndo_queue_start(dev, rxq_idx, new_mem);
> +	if (err)
> +		goto err_start_queue;
> +
> +	dev->netdev_ops->ndo_queue_mem_free(dev, old_mem);
> +
> +	return 0;
> +
> +err_start_queue:
> +	dev->netdev_ops->ndo_queue_start(dev, rxq_idx, old_mem);
> +
> +err_free_new_mem:
> +	dev->netdev_ops->ndo_queue_mem_free(dev, new_mem);
> +
> +	return err;
> +}
> +
> +/* Protected by rtnl_lock() */
> +static DEFINE_XARRAY_FLAGS(netdev_dmabuf_bindings, XA_FLAGS_ALLOC1);
> +
> +void netdev_unbind_dmabuf(struct netdev_dmabuf_binding *binding)
> +{
> +	struct netdev_rx_queue *rxq;
> +	unsigned long xa_idx;
> +	unsigned int rxq_idx;
> +
> +	if (!binding)
> +		return;
> +
> +	if (binding->list.next)
> +		list_del(&binding->list);
> +
> +	xa_for_each(&binding->bound_rxq_list, xa_idx, rxq) {
> +		if (rxq->binding == binding) {
> +			/* We hold the rtnl_lock while binding/unbinding
> +			 * dma-buf, so we can't race with another thread that
> +			 * is also modifying this value. However, the driver
> +			 * may read this config while it's creating its
> +			 * rx-queues. WRITE_ONCE() here to match the
> +			 * READ_ONCE() in the driver.
> +			 */
> +			WRITE_ONCE(rxq->binding, NULL);
> +
> +			rxq_idx = get_netdev_rx_queue_index(rxq);
> +
> +			netdev_restart_rx_queue(binding->dev, rxq_idx);

Blindly restarting a queue when a dmabuf is heavy handed. If the dmabuf
has no outstanding references (ie., no references in the RxQ), then no
restart is needed.

> +		}
> +	}
> +
> +	xa_erase(&netdev_dmabuf_bindings, binding->id);
> +
> +	netdev_dmabuf_binding_put(binding);
> +}
> +
> +int netdev_bind_dmabuf_to_queue(struct net_device *dev, u32 rxq_idx,
> +				struct netdev_dmabuf_binding *binding)
> +{
> +	struct netdev_rx_queue *rxq;
> +	u32 xa_idx;
> +	int err;
> +
> +	rxq = __netif_get_rx_queue(dev, rxq_idx);
> +
> +	if (rxq->binding)
> +		return -EEXIST;
> +
> +	err = xa_alloc(&binding->bound_rxq_list, &xa_idx, rxq, xa_limit_32b,
> +		       GFP_KERNEL);
> +	if (err)
> +		return err;
> +
> +	/* We hold the rtnl_lock while binding/unbinding dma-buf, so we can't
> +	 * race with another thread that is also modifying this value. However,
> +	 * the driver may read this config while it's creating its * rx-queues.
> +	 * WRITE_ONCE() here to match the READ_ONCE() in the driver.
> +	 */
> +	WRITE_ONCE(rxq->binding, binding);
> +
> +	err = netdev_restart_rx_queue(dev, rxq_idx);

Similarly, here binding a dmabuf to a queue. I was expecting the dmabuf
binding to add entries to the page pool for the queue. If the pool was
previously empty, then maybe the queue needs to be "started" in the
sense of creating with h/w or just pushing buffers into the queue and
moving the pidx.