linux-kernel - Re: [PATCH v3 0/4] PCI: get DMA configuration from parent device

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Message-ID: <2785994.gTZ9chGdKf@wuerfel>
Date:	Thu, 08 Jan 2015 23:27:14 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Murali Karicheri <m-karicheri2@...com>
Cc:	bhelgaas@...gle.com, linux-arm-kernel@...ts.infradead.org,
	joro@...tes.org, grant.likely@...aro.org, robh+dt@...nel.org,
	iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	devicetree@...r.kernel.org, linux-pci@...r.kernel.org,
	will.deacon@....com, linux@....linux.org.uk
Subject: Re: [PATCH v3 0/4] PCI: get DMA configuration from parent device

On Thursday 08 January 2015 09:56:39 Arnd Bergmann wrote:
> There is another interesting case, which is a USB host controller or
> something similar behind a PCI bus. These are quite common and also
> need to be handled in some form. Let's do just PCI first for now, but
> be aware that this will come next. Should we assume that the ID that
> is required for a device is either known from the device node, or
> that it comes from a PCI device? That means for the USB case, we will
> likely need to have some custom logic. There seems to be an implicit
> assumption all over the kernel that all devices have the same IOMMU
> instance, but we can't really rely on that here.
> 

Update: I've checked the USB implementation and we are lucky because
these interactions are all done by the USB host controller, and a
USB device is not itself DMA capable. We don't have to do anything
for USB.

	Arnd
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