linux-kernel - Re: [PATCH 09/16] dma-direct: Support PCI P2PDMA pages in dma-direct map

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Date:   Mon, 3 May 2021 11:06:37 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     John Hubbard <jhubbard@...dia.com>, 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
Cc:     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>,
        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>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH 09/16] dma-direct: Support PCI P2PDMA pages in dma-direct
 map_sg



On 2021-05-02 5:32 p.m., John Hubbard wrote:
> On 5/2/21 4:28 PM, John Hubbard wrote:
>> On 4/8/21 10:01 AM, Logan Gunthorpe wrote:
> ...
>>> @@ -387,19 +388,37 @@ void dma_direct_unmap_sg(struct device *dev, struct scatterlist *sgl,
>>
>> This routine now deserves a little bit of commenting, now that it is
>> doing less obvious things. How about something like this:
>>
>> /*
>> * Unmaps pages, except for PCI_P2PDMA pages, which were never mapped in the
>> * first place. Instead of unmapping PCI_P2PDMA entries, simply remove the
>> * SG_PCI_P2PDMA mark
>> */
> 
> I got that kind of wrong. They *were* mapped, but need to be left mostly
> alone...maybe you can word it better. Here's my second draft:
> 
> /*
>   * Unmaps pages, except for PCI_P2PDMA pages, which should not be unmapped at
>   * this point. Instead of unmapping PCI_P2PDMA entries, simply remove the
>   * SG_PCI_P2PDMA mark.
>   */
> 
> ...am I getting close? :)

I don't think your original comment was wrong per se. But I guess it
depends on your definition of "mapped". In dma-direct the physical
address is added to the SGL and, on some arches, the address has to be
synced on unmap. With P2PDMA, the PCI bus address is sometimes added to
the SGL and no sync is necessary at the end.

Logan