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From: Arnd Bergmann <arnd@arndb.de>
To: Leo Li <pku.leo@gmail.com>
Cc: Felipe Balbi <balbi@kernel.org>,
        Russell King - ARM Linux <linux@arm.linux.org.uk>,
        Grygorii Strashko <grygorii.strashko@ti.com>,
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        <linux-arm-kernel@lists.infradead.org>,
        Stuart Yoder <stuart.yoder@nxp.com>, Scott Wood <oss@buserror.net>
Subject: Re: [PATCH] usb: dwc3: host: inherit dma configuration from parent dev
Date: Fri, 02 Sep 2016 12:43:39 +0200
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On Thursday, September 1, 2016 5:14:28 PM CEST Leo Li wrote:
> 
> Hi Felipe and Arnd,
> 
> It has been a while since the last response to this discussion, but we
> haven't reached an agreement yet!  Can we get to a conclusion on if it
> is valid to create child platform device for abstraction purpose?  If
> yes, can this child device do DMA by itself?

I'd say it's no problem for a driver to create child devices in order
to represent different aspects of a device, but you should not rely on
those devices working when used with the dma-mapping interfaces.

This used to be simpler back when we could configure the kernel for
only one SoC platform at a time, and the platforms could provide their
own overrides for the dma-mapping interfaces. These days, we rely on
firmware or bootloader to describe various aspects of how DMA is done,
so you can't assume that passing a device without an of_node pointer
or ACPI data into those functions will do the right thing.

	Arnd