linux-kernel - Re: [PATCH 11/14] PCI/P2PDMA: dma_map P2PDMA map requests that traverse the host bridge

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Date:   Wed, 24 Jul 2019 08:32:32 +0200
From:   Christoph Hellwig <hch@....de>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-rdma@...r.kernel.org,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Christoph Hellwig <hch@....de>,
        Christian Koenig <Christian.Koenig@....com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...com>,
        Dan Williams <dan.j.williams@...el.com>,
        Eric Pilmore <epilmore@...aio.com>,
        Stephen Bates <sbates@...thlin.com>
Subject: Re: [PATCH 11/14] PCI/P2PDMA: dma_map P2PDMA map requests that
 traverse the host bridge

>  	struct dev_pagemap *pgmap = sg_page(sg)->pgmap;
> +	struct pci_dev *client;
> +	int dist;
> +
> +	client = find_parent_pci_dev(dev);
> +	if (WARN_ON_ONCE(!client))
> +		return 0;
>  
> +	dist = upstream_bridge_distance(pgmap->pci_p2pdma_provider,
> +					client, NULL);

Doing this on every mapping call sounds expensive..

> +	if (WARN_ON_ONCE(dist & P2PDMA_NOT_SUPPORTED))
> +		return 0;
> +
> +	if (dist & P2PDMA_THRU_HOST_BRIDGE)
> +		return dma_map_sg_attrs(dev, sg, nents, dir, attrs);
> +	else
> +		return __pci_p2pdma_map_sg(pgmap, dev, sg, nents);

Can't we organize the values so that we can switch on the return
value instead of doing flag checks?

>  }
>  EXPORT_SYMBOL_GPL(pci_p2pdma_map_sg_attrs);
>  
> @@ -847,6 +861,21 @@ EXPORT_SYMBOL_GPL(pci_p2pdma_map_sg_attrs);
>  void pci_p2pdma_unmap_sg_attrs(struct device *dev, struct scatterlist *sg,
>  		int nents, enum dma_data_direction dir, unsigned long attrs)
>  {
> +	struct dev_pagemap *pgmap = sg_page(sg)->pgmap;
> +	struct pci_dev *client;
> +	int dist;
> +
> +	client = find_parent_pci_dev(dev);
> +	if (!client)
> +		return;
> +
> +	dist = upstream_bridge_distance(pgmap->pci_p2pdma_provider,
> +					client, NULL);

And then we do it for every unmap again..