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

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Message-ID: <CAK=WgbZPFryzCFLa5AhAYDmH1S8c3x-OsSTyxYeq0WLAe8VyTg@mail.gmail.com>
Date:	Wed, 7 Sep 2011 09:01:26 +0300
From:	Ohad Ben-Cohen <ohad@...ery.com>
To:	KyongHo Cho <pullip.cho@...sung.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 Cho,

On Wed, Sep 7, 2011 at 4:30 AM, KyongHo Cho <pullip.cho@...sung.com> wrote:
> 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

That's exactly what I'm trying to prevent; there's no reason to
duplicate the exact same logic in every iommu driver.

As a result, your map function is quite big (it even duplicates the
same logic for every page size it tries).

Try to rebase your patch on the "iommu/core: split mapping to page
sizes as supported by the hardware" patch I've just sent, and see how
significantly simpler your code becomes.

Relying on a single implementation, provided by the IOMMU core, means
less code to maintain (and debug) and a consistent behavior (across
all supported hardware) exposed to users of the IOMMU API.

> But I think IOMMU API must not expect that the caller of iommu_map() knows
> about the restriction of IOMMU API implementation.

Right. But I don't think there's any disagreement here ?

If any, then I think that s5p_iommu_map() is more limited than what I
propose: if it is given a 64KB region aligned only to a 4KB address,
it will BUG_ON() it. While not ideal, I don't think there's any reason
not to map it using 4KB pages (which is exactly what the new
iommu_map() I propose will do).

Thanks,
Ohad.
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