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From: Will Deacon <will.deacon@arm.com>
To: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Robin Murphy <robin.murphy@arm.com>,
	Magnus Damm <magnus.damm@gmail.com>,
	linux-arm-kernel@lists.infradead.org,
	linux-renesas-soc@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH/RESEND] arm64: defconfig: Enlarge CMA alignment to 2 MiB
Date: Mon, 13 Feb 2017 11:36:24 +0000	[thread overview]
Message-ID: <20170213113624.GI1512@arm.com> (raw)
In-Reply-To: <1486983858-11362-1-git-send-email-geert+renesas@glider.be>

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@glider.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

WARNING: multiple messages have this Message-ID (diff)
From: will.deacon@arm.com (Will Deacon)
To: linux-arm-kernel@lists.infradead.org
Subject: [PATCH/RESEND] arm64: defconfig: Enlarge CMA alignment to 2 MiB
Date: Mon, 13 Feb 2017 11:36:24 +0000	[thread overview]
Message-ID: <20170213113624.GI1512@arm.com> (raw)
In-Reply-To: <1486983858-11362-1-git-send-email-geert+renesas@glider.be>

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@glider.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

  reply	other threads:[~2017-02-13 12:39 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-02-13 11:04 [PATCH/RESEND] arm64: defconfig: Enlarge CMA alignment to 2 MiB Geert Uytterhoeven
2017-02-13 11:04 ` Geert Uytterhoeven
2017-02-13 11:36 ` Will Deacon [this message]
2017-02-13 11:36   ` Will Deacon

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