linux-kernel - Re: [PATCH 5/9] PCI: host: brcmstb: add dma-ranges for inbound traffic

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Date:   Fri, 20 Oct 2017 10:41:56 -0400
From:   Jim Quinlan <jim2101024@...il.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Robin Murphy <robin.murphy@....com>, linux-kernel@...r.kernel.org,
        Mark Rutland <mark.rutland@....com>,
        Linux-MIPS <linux-mips@...ux-mips.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        devicetree@...r.kernel.org, linux-pci <linux-pci@...r.kernel.org>,
        Kevin Cernekee <cernekee@...il.com>,
        Will Deacon <will.deacon@....com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Rob Herring <robh+dt@...nel.org>,
        bcm-kernel-feedback-list <bcm-kernel-feedback-list@...adcom.com>,
        Gregory Fong <gregory.0xf0@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Brian Norris <computersforpeace@...il.com>,
        linux-arm-kernel@...ts.infradead.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        iommu@...ts.linux-foundation.org
Subject: Re: [PATCH 5/9] PCI: host: brcmstb: add dma-ranges for inbound traffic

On Fri, Oct 20, 2017 at 3:37 AM, Christoph Hellwig <hch@....de> wrote:
> On Thu, Oct 19, 2017 at 06:47:45PM -0400, Jim Quinlan wrote:
>> The only way to prevent this is to reserve a single page at the end of
>> the first memory region of any pair that are adjacent in physical
>> memory.  A hack, yes, but I don't see an easier way out of this.  Many
>> if not most of our boards do not have adjacent regions and would not
>> need this.
>
> dma mappings can be much larger than a single page.  For the block
> world take a look at __blk_segment_map_sg which does the merging
> of contiguous pages into a single SG segment.  You'd have to override
> BIOVEC_PHYS_MERGEABLE to prevent this from happening in your supported
> architectures for the block layer.

I am not sure I understand your comment -- the size of the request
shouldn't be a factor.  Let's look at your example of the DMA request
of 3fffff00 to 4000000f (physical memory).  Lets say it is for 15
pages.  If we block out  the last page [0x3ffff000..0x3fffffff] from
what is available, there is no 15 page span that can happen across the
0x40000000 boundary.  For SG, there can be no merge that connects a
page from one region to another region.  Can you give an example of
the scenario you are thinking of?