linux-kernel - Re: [PATCH] arm: dma-mapping: fix compilation error when CONFIG

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Date:	Mon, 27 Oct 2014 12:59:44 +0100
From:	Marek Szyprowski <m.szyprowski@...sung.com>
To:	Stefan Hengelein <stefan.hengelein@....de>,
	Laura Abbott <lauraa@...eaurora.org>
Cc:	linux-kernel@...r.kernel.org, linux@....linux.org.uk,
	akpm@...ux-foundation.org, linux-arm-kernel@...ts.infradead.org,
	Valentin Rothberg <valentinrothberg@...il.com>,
	iommu@...ts.linux-foundation.org
Subject: Re: [PATCH] arm: dma-mapping: fix compilation error when CONFIG_MMU is
 not present

Hello,

On 2014-10-24 15:15, Stefan Hengelein wrote:
> Hello,
>
> adding a dependency to CONFIG_ARM_DMA_USE_IOMMU is probably not a good
> idea anyways.
> If you read the Documentation/kbuild/kconfig-language.txt, you will see that:
> 1. in general, selected options should have no prompt and no dependencies
> 2. giving ARM_DMA_USE_IOMMU a dependency on MMU will and cannot solve
> the problem, since ARM_DMA_USE_IOMMU itself is selected by several
> sources and select will not enforce dependencies to be resolved (to be
> precise, it will simply ignore them). Thus, CONFIG_MMU will not be
> enabled (as you can see in your warning).
> If you want to ensure CONFIG_MMU is enabled when
> CONFIG_ARM_DMA_USE_IOMMU is selected, the selecting options (e.g.
> SHMOBILE_IOMMU) should have a dependency on MMU.
> However, i'm not sure if that's the right way to solve it, since it
> will hide these options when MMU is not enabled.
>
> Making the whole IOMMU subsystem depend on MMU would probably also
> solve the problem. Is there any situation where it would make sense to
> use an IOMMU without an MMU?

IMHO such configuration doesn't make sense and hiding IOMMU section
completely when non-MMU systems are enabled is the right approach.

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland

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