From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Raj, Ashok" <ashok.raj@intel.com>
Subject: Re: [PATCH] iommu/vt-d: Fix scatterlist offset handling
Date: Thu, 28 Sep 2017 08:43:46 -0700
Message-ID: <20170928154345.GA101272@otc-nc-03>
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Cc: Casey Leedom <leedom@chelsio.com>,
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To: Robin Murphy <robin.murphy@arm.com>
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Hi Robin

thanks.. i have no idea.. i see all the other patches from you :-)
my email has decided to play games with me i suppose :-)

On Thu, Sep 28, 2017 at 05:59:11PM +0100, Robin Murphy wrote:
> I hope our email server hasn't got blacklisted again... Said patch is
> the top of this very thread we're replying on[1] - you were definitely
> on cc :(

        (sg->dma_address + sg->dma_len) ----+
        sg->dma_address ---------+          |
        iov_pfn------+           |          |
                     |           |          |
                     v           v          v
    iova:   a        b        c        d        e        f
            |--------|--------|--------|--------|--------|
                              <...calculated....>
                     [_____mapped______]
    pfn:    0        1        2        3        4        5
            |--------|--------|--------|--------|--------|
                     ^           ^          ^
                     |           |          |
        sg->page ----+           |          |
        sg->offset --------------+          |
        (sg->offset + sg->length) ----------+

The picture seems right. Looking at the code i'm not sure if i understand
it correctly.

pgoff = sg->offset & ~PAGE_MASK;

this gets the offset past the start of page.

sg->dma_address = ((dma_addr_t)iov_pfn << VTD_PAGE_SHIFT) + pgoff;

this would set dma_address at b+off instead of starting at c+off correct?

> 
> Robin
> 
> [1]:https://lists.linuxfoundation.org/pipermail/iommu/2017-September/024371.html