linux-kernel - Re: [PATCH/RFC] arm64: defconfig: Enlarge CMA alignment to 2 MiB

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Message-ID: <a4e171b0-6403-9d88-3b8f-e63f3f050689@arm.com>
Date:   Wed, 1 Feb 2017 14:06:45 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Magnus Damm <magnus.damm@...il.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [PATCH/RFC] arm64: defconfig: Enlarge CMA alignment to 2 MiB

On 01/02/17 13:45, Magnus Damm wrote:
> Hi Geert,
> 
> On Sat, Jan 28, 2017 at 1:03 AM, Geert Uytterhoeven
> <geert+renesas@...der.be> wrote:
>> Some IOMMUs (e.g. Renesas IPMMU/VMSA) support only page sizes of 4 KiB,
>> 2 MiB, and 1 GiB.
>>
>> With the default setting of CONFIG_CMA_ALIGNMENT = 8, allocations larger
>> than 1 MiB are aligned to a 1 MiB boundary only.  Hence a 2 MiB
>> allocation may not be aligned, leading to a mapping of 512 4 KiB pages.
>>
>> Increase CONFIG_CMA_ALIGNMENT to allow mapping a 2 MiB buffer using a
>> single PTE, decreasing memory usage and TLB pressure.
>>
>> Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>
>> ---
>> Is this useful?
>>
>> Should there instead be different defaults in Kconfig, depending on
>> enabled platform support?
> 
> I think there is a dependency on the kernel page size configuration as
> well. In case of 16 KiB or 64 KiB page size configuration other large
> page sizes may be required.

I don't see the original patch (linux-arm-kernel doesn't look to have it
archived either), but I think bumping the default up to 2MB to match our
normal section size sounds generally reasonable - users can still
override it manually in their config, right? For 16KB and 64KB granules,
it is at least the size covered by contiguous-hinted pages; the actual
section sizes there are far too big to be practical for alignment
purposes anyway (32MB and 512MB respectively).

Robin.

> 
> Thanks,
> 
> / magnus
> 
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> linux-arm-kernel@...ts.infradead.org
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> 

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