linux-kernel - Re: [PATCH v6 6/7] PCI: update dma configuration from DT

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Date:	Wed, 25 Feb 2015 15:45:01 -0500
From:	Murali Karicheri <m-karicheri2@...com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	Bjorn Helgaas <bhelgaas@...gle.com>,
	<linux-arm-kernel@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>, <iommu@...ts.linux-foundation.org>,
	<devicetree@...r.kernel.org>, <linux-pci@...r.kernel.org>,
	Joerg Roedel <joro@...tes.org>,
	Grant Likely <grant.likely@...aro.org>,
	Rob Herring <robh+dt@...nel.org>,
	Will Deacon <will.deacon@....com>,
	Russell King <linux@....linux.org.uk>,
	Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>
Subject: Re: [PATCH v6 6/7] PCI: update dma configuration from DT

On 02/25/2015 11:09 AM, Arnd Bergmann wrote:
> On Wednesday 25 February 2015 11:03:02 Murali Karicheri wrote:
>>
>>> (I don't know exactly how these patches all fit together, so that's
>>> probably not accurate, but that's the *sort* of thing I'd like to include.)
>>>
>>> If that actually *is* what's going on, I have to wonder why this isn't
>>> implemented as a very simple IOMMU instead of adding dma_pfn_offset,
>>> which is present on all arches but only used on ARM.  In some sense that
>>> offset is parallel but incompatible with an IOMMU: they both translate DMA
>>> addresses into system RAM addresses.
>>
>> I don't have much history on any previous discussion on the subject you
>> are referring to. I assume it would have happened when
>> of_dma_configure() was first introduced. On Keystone, we don't have
>> IOMMU support and dma_pfn_offset is needed to translate DMA address to
>> System RAM address and vice-versa. So this has to be supported for
>> Keystone. There can be enhancement for IOMMU with out impacting this
>> feature for Keystone.
>
> The direction we are taking with IOMMU in general is opposite to what Bjorn
> is suggesting: I believe what he wants to say is that we should use the
> traditional approach of having a specialized dma_map_ops implementation
> for this, just like we do for IOMMU implementations on x86, IA64 or PowerPC.
>
> However, with the recent explosion of IOMMU implementations on ARM, we
> are moving towards having a single dma_map_ops structure for all of them,
> and that structure does not work with the keystone hardware. Instead,
> the normal ARM dma_map_ops have been changed to handle the offset,
> which is the same thing we do on PowerPC.
>
Arnd,

Thanks for the clarification.

Murali

> 	Arnd


-- 
Murali Karicheri
Linux Kernel, Texas Instruments
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