Re: [PATCH v5 13/13] nvmet: Optionally use PCI P2P memory

[Sagi Grimberg]

> +/*
> + * If allow_p2pmem is set, we will try to use P2P memory for the SGL lists for
> + * Î/O commands. This requires the PCI p2p device to be compatible with the
> + * backing device for every namespace on this controller.
> + */
> +static void nvmet_setup_p2pmem(struct nvmet_ctrl *ctrl, struct nvmet_req *req)
> +{
> +	struct nvmet_ns *ns;
> +	int ret;
> +
> +	if (!req->port->use_p2pmem || !req->p2p_client)
> +		return;
> +
> +	mutex_lock(&ctrl->subsys->lock);
> +
> +	ret = pci_p2pdma_add_client(&ctrl->p2p_clients, req->p2p_client);
> +	if (ret) {
> +		pr_err("failed adding peer-to-peer DMA client %s: %d\n",
> +		       dev_name(req->p2p_client), ret);
> +		goto free_devices;
> +	}
> +
> +	list_for_each_entry_rcu(ns, &ctrl->subsys->namespaces, dev_link) {
> +		ret = nvmet_p2pdma_add_client(ctrl, ns);
> +		if (ret)
> +			goto free_devices;
> +	}
> +
> +	if (req->port->p2p_dev) {
> +		if (!pci_p2pdma_assign_provider(req->port->p2p_dev,
> +						&ctrl->p2p_clients)) {
> +			pr_info("peer-to-peer memory on %s is not supported\n",
> +				pci_name(req->port->p2p_dev));
> +			goto free_devices;
> +		}
> +		ctrl->p2p_dev = pci_dev_get(req->port->p2p_dev);
> +	} else {

When is port->p2p_dev == NULL? a little more documentation would help 
here...