linux-kernel - Re: [PATCH v11 20/27] iommu/exynos: allow having multiple System MMUs for a master H/W

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Date:	Fri, 14 Mar 2014 17:12:03 +0100
From:	Tomasz Figa <t.figa@...sung.com>
To:	Cho KyongHo <pullip.cho@...sung.com>,
	Linux ARM Kernel <linux-arm-kernel@...ts.infradead.org>,
	Linux DeviceTree <devicetree@...r.kernel.org>,
	Linux IOMMU <iommu@...ts.linux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux Samsung SOC <linux-samsung-soc@...r.kernel.org>
Cc:	Antonios Motakis <a.motakis@...tualopensystems.com>,
	Grant Grundler <grundler@...omium.org>,
	Joerg Roedel <joro@...tes.org>,
	Kukjin Kim <kgene.kim@...sung.com>,
	Prathyush <prathyush.k@...sung.com>,
	Rahul Sharma <rahul.sharma@...sung.com>,
	Sachin Kamat <sachin.kamat@...aro.org>,
	Sylwester Nawrocki <s.nawrocki@...sung.com>,
	Varun Sethi <Varun.Sethi@...escale.com>
Subject: Re: [PATCH v11 20/27] iommu/exynos: allow having multiple System MMUs
 for a master H/W

Hi KyongHo,

On 14.03.2014 06:10, Cho KyongHo wrote:
> Some master device descriptor like fimc-is which is an abstraction
> of very complex H/W may have multiple System MMUs. For those devices,
> the design of the link between System MMU and its master H/W is needed
> to be reconsidered.
>
> A link structure, sysmmu_list_data is introduced that provides a link
> to master H/W and that has a pointer to the device descriptor of a
> System MMU. Given a device descriptor of a master H/W, it is possible
> to traverse all System MMUs that must be controlled along with the
> master H/W.

NAK.

A device driver should handle particular hardware instances separately, 
without abstracting a virtual hardware instance consisting of multiple 
physical ones.

If such abstraction is needed, it should be done above the exynos-iommu 
driver, e.g. by something like iommu-composite driver that would 
aggregate several IOMMUs. Keep in mind that such IOMMUs in a group could 
be different, e.g. different Exynos SysMMU versions or even completely 
different IPs handled by different drivers.

Still, I don't think there is a real need for such abstraction. Instead, 
related drivers shall be fixed to properly handle multiple memory 
masters and their IOMMUs.

Best regards,
Tomasz
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