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

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Date:   Sun, 2 May 2021 16:32:50 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Logan Gunthorpe <logang@...tatee.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 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? :)

thanks,
-- 
John Hubbard
NVIDIA