linux-kernel - Re: [RFC 7/7] iommu/core: split mapping to page sizes as supported by the hardware

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Message-ID: <CAHQjnOMT3UOFMi=JbSXf4-s_BbZ+fSGbOEjxwT7t7N-zd+9-zg@mail.gmail.com>
Date:	Wed, 7 Sep 2011 10:30:33 +0900
From:	KyongHo Cho <pullip.cho@...sung.com>
To:	Ohad Ben-Cohen <ohad@...ery.com>
Cc:	iommu@...ts.linux-foundation.org, linux-omap@...r.kernel.org,
	Hiroshi DOYU <Hiroshi.DOYU@...ia.com>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Joerg Roedel <Joerg.Roedel@....com>,
	David Woodhouse <dwmw2@...radead.org>,
	linux-arm-kernel@...ts.infradead.org,
	David Brown <davidb@...eaurora.org>,
	Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org
Subject: Re: [RFC 7/7] iommu/core: split mapping to page sizes as supported by
 the hardware

Hi

On Sat, Sep 3, 2011 at 2:32 AM, Ohad Ben-Cohen <ohad@...ery.com> wrote:
> When mapping a memory region, split it to page sizes as supported
> by the iommu hardware. Always prefer bigger pages, when possible,
> in order to reduce the TLB pressure.
>
True. It is important for the peripheral devices that works with IOMMU.

> This allows a more lenient granularity of mappings: traditionally the
> IOMMU API took 'order' (of a page) as a mapping size, and directly let
> the low level iommu drivers handle the mapping. Now that the IOMMU
> core can split arbitrary memory regions into pages, we can remove this
> limitation, so users don't have to split those regions by themselves.
>

Please find the following link that I submitted for our IOMMU.
https://lkml.org/lkml/2011/7/3/152

s5p_iommu_map/unmap accepts any order of physical address and iova
without support of your suggestion if the order is not less than PAGE_SHIFT

Of course, an IO virtual memory manager must be careful when it allocates
IO virtual memory to lead best performance.
But I think IOMMU API must not expect that the caller of iommu_map() knows
about the restriction of IOMMU API implementation.

Regards,
Cho KyongHo.
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