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:   Tue, 16 Mar 2021 09:14:16 +0100
From:   Christoph Hellwig <hch@....de>
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
> DMA map functions to determine how to map a given p2pdma page.
> 
> pci_p2pdma_bus_offset() is also added to allow callers to get the bus
> offset if they need to map the bus address.
> 
> Signed-off-by: Logan Gunthorpe <logang@...tatee.com>
> ---
>  drivers/pci/p2pdma.c       | 50 ++++++++++++++++++++++++++++++++++++++
>  include/linux/pci-p2pdma.h | 11 +++++++++
>  2 files changed, 61 insertions(+)
> 
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index 7f6836732bce..66d16b7eb668 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -912,6 +912,56 @@ void pci_p2pdma_unmap_sg_attrs(struct device *dev, struct scatterlist *sg,
>  }
>  EXPORT_SYMBOL_GPL(pci_p2pdma_unmap_sg_attrs);
>  
> +/**
> + * pci_p2pdma_bus_offset - returns the bus offset for a given page
> + * @page: page to get the offset for
> + *
> + * Must be passed a PCI p2pdma page.
> + */
> +u64 pci_p2pdma_bus_offset(struct page *page)
> +{
> +	struct pci_p2pdma_pagemap *p2p_pgmap = to_p2p_pgmap(page->pgmap);
> +
> +	WARN_ON(!is_pci_p2pdma_page(page));
> +
> +	return p2p_pgmap->bus_offset;
> +}
> +EXPORT_SYMBOL_GPL(pci_p2pdma_bus_offset);

I don't see why this would be exported.  

> +EXPORT_SYMBOL_GPL(pci_p2pdma_dma_map_type);

Same here.  Also the two helpers don't really look very related.

I suspect you really want to move the p2p handling bits currenly added
to kernel/dma/direct.c into drivers/pci/p2pdma.c instead, as that will
allow direct access to the pci_p2pdma_pagemap and should thus simplify
things quite a bit.