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

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Date:   Mon, 23 Oct 2017 09:06:03 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Jim Quinlan' <jim2101024@...il.com>,
        Christoph Hellwig <hch@....de>
CC:     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

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