linux-kernel - Re: [RFC PATCH v2 04/11] PCI/P2PDMA: Introduce pci_p2pdma_should_map_bus() and pci_p2pdma_bus

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Date:   Fri, 12 Mar 2021 18:32:20 -0800
From:   Ira Weiny <ira.weiny@...el.com>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-block@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-mm@...ck.org, iommu@...ts.linux-foundation.org,
        Stephen Bates <sbates@...thlin.com>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christian König <christian.koenig@....com>,
        Ira Weiny <iweiny@...el.com>,
        John Hubbard <jhubbard@...dia.com>,
        Don Dutile <ddutile@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Jakowski Andrzej <andrzej.jakowski@...el.com>,
        Minturn Dave B <dave.b.minturn@...el.com>,
        Jason Ekstrand <jason@...kstrand.net>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Xiong Jianxin <jianxin.xiong@...el.com>
Subject: Re: [RFC PATCH v2 04/11] PCI/P2PDMA: Introduce
 pci_p2pdma_should_map_bus() and pci_p2pdma_bus_offset()

On Thu, Mar 11, 2021 at 04:31:34PM -0700, Logan Gunthorpe wrote:
> Introduce pci_p2pdma_should_map_bus() which is meant to be called by
            ^^^^^^^^^^^^^^^^^^^^^^^^^
	    pci_p2pdma_dma_map_type() ???

FWIW I find this name confusing with pci_p2pdma_map_type() and looking at the
implementation I'm not clear why pci_p2pdma_dma_map_type() needs to exist?

Ira

[snip]

> +
> +/**
> + * pci_p2pdma_dma_map_type - determine if a DMA mapping should use the
> + *	bus address, be mapped normally or fail
> + * @dev: device doing the DMA request
> + * @pgmap: dev_pagemap structure for the mapping
> + *
> + * Returns:
> + *    1 - if the page should be mapped with a bus address,
> + *    0 - if the page should be mapped normally through an IOMMU mapping or
> + *        physical address; or
> + *   -1 - if the device should not map the pages and an error should be
> + *        returned
> + */
> +int pci_p2pdma_dma_map_type(struct device *dev, struct dev_pagemap *pgmap)
> +{
> +	struct pci_p2pdma_pagemap *p2p_pgmap = to_p2p_pgmap(pgmap);
> +	struct pci_dev *client;
> +
> +	if (!dev_is_pci(dev))
> +		return -1;
> +
> +	client = to_pci_dev(dev);
> +
> +	switch (pci_p2pdma_map_type(p2p_pgmap->provider, client)) {
> +	case PCI_P2PDMA_MAP_THRU_HOST_BRIDGE:
> +		return 0;
> +	case PCI_P2PDMA_MAP_BUS_ADDR:
> +		return 1;
> +	default:
> +		return -1;
> +	}
> +}
> +EXPORT_SYMBOL_GPL(pci_p2pdma_dma_map_type);

I guess the main point here is to export this to the DMA layer?

Ira