Message-id: <544E33B0.4020405@samsung.com>
Date: Mon, 27 Oct 2014 12:59:44 +0100
From: Marek Szyprowski <m.szyprowski@samsung.com>
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To: Stefan Hengelein <stefan.hengelein@fau.de>,
        Laura Abbott <lauraa@codeaurora.org>
Cc: linux-kernel@vger.kernel.org, linux@arm.linux.org.uk,
        akpm@linux-foundation.org, linux-arm-kernel@lists.infradead.org,
        Valentin Rothberg <valentinrothberg@gmail.com>,
        iommu@lists.linux-foundation.org
Subject: Re: [PATCH] arm: dma-mapping: fix compilation error when CONFIG_MMU is
 not present
References: <1413734340-53108-1-git-send-email-stefan.hengelein@fau.de>
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Sender: linux-kernel-owner@vger.kernel.org

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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