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

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Date:   Tue, 24 Oct 2017 14:08:12 -0400
From:   Jim Quinlan <jim2101024@...il.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     Christoph Hellwig <hch@....de>,
        Robin Murphy <robin.murphy@....com>,
        "linux-kernel@...r.kernel.org" <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" <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" 
        <linux-arm-kernel@...ts.infradead.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>
Subject: Re: [PATCH 5/9] PCI: host: brcmstb: add dma-ranges for inbound traffic

On Mon, Oct 23, 2017 at 5:06 AM, David Laight <David.Laight@...lab.com> wrote:
> From: Jim Quinlan
>> Sent: 20 October 2017 16:28
>> On Fri, Oct 20, 2017 at 10:57 AM, Christoph Hellwig <hch@....de> wrote:
>> > On Fri, Oct 20, 2017 at 10:41:56AM -0400, Jim Quinlan wrote:
>> >> 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?
>> >
>> > What prevents a merge from say the regions of
>> > 0....3fffffff and 40000000....7fffffff?
>>
>> Huh? [0x3ffff000...x3ffffff] is not available to be used. Drawing from
>> the original example, we now have to tell Linux that these are now our
>> effective memory regions:
>>
>>       memc0-a@[        0....3fffefff] <=> pci@[        0....3fffefff]
>>       memc0-b@[100000000...13fffefff] <=> pci@[ 40000000....7fffefff]
>>       memc1-a@[ 40000000....7fffefff] <=> pci@[ 80000000....bfffefff]
>>       memc1-b@[300000000...33fffefff] <=> pci@[ c0000000....ffffefff]
>>       memc2-a@[ 80000000....bfffefff] <=> pci@[100000000...13fffefff]
>>       memc2-b@[c00000000...c3fffffff] <=> pci@[140000000...17fffffff]
>>
>> This leaves a one-page gap between phsyical memory regions which would
>> normally be contiguous. One cannot have a dma alloc that spans any two
>> regions.  This is a drastic step, but I don't see an alternative.
>> Perhaps  I may be missing what you are saying...
>
> Isn't this all unnecessary?
> Both kmalloc() and dma_alloc() are constrained to allocate memory
> that doesn't cross an address boundary that is larger than the size.
> So if you allocate 16k it won't cross a 16k physical address boundary.
>
>         David
>
Hi David,  Christoph was also concerned about this:

"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."

Do you consider this a non-issue as well or can this happen and span
memory regions?