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

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Date:   Mon, 13 Feb 2017 11:36:24 +0000
From:   Will Deacon <will.deacon@....com>
To:     Geert Uytterhoeven <geert+renesas@...der.be>
Cc:     Marek Szyprowski <m.szyprowski@...sung.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Robin Murphy <robin.murphy@....com>,
        Magnus Damm <magnus.damm@...il.com>,
        linux-arm-kernel@...ts.infradead.org,
        linux-renesas-soc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH/RESEND] arm64: defconfig: Enlarge CMA alignment to 2 MiB

On Mon, Feb 13, 2017 at 12:04:18PM +0100, Geert Uytterhoeven 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?

I assume you're proposing it because you see an improvement? :)

> Should there instead be different defaults in Kconfig, depending on
> enabled platform support?

I don't object to updating defconfig as a quick hack, but the right solution
is probably to make the core Kconfig default value overridable by the
architecture.

Will