linux-kernel - Re: USB mass storage and ARM cache coherency

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Date:	Wed, 17 Feb 2010 20:40:09 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Oliver Neukum <oliver@...kum.org>
Cc:	"Shilimkar, Santosh" <santosh.shilimkar@...com>,
	Matthew Dharm <mdharm-kernel@...-eyed-alien.net>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Ming Lei <tom.leiming@...il.com>,
	"Mankad, Maulik Ojas" <x0082077@...com>,
	Sergei Shtylyov <sshtylyov@...mvista.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Sebastian Siewior <bigeasy@...utronix.de>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Pavel Machek <pavel@....cz>, Greg KH <greg@...ah.com>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: USB mass storage and ARM cache coherency

On Wed, 2010-02-17 at 10:15 +0100, Oliver Neukum wrote:
> We should have changed the subject line.
> 
> There's a second problem. It turns out that on ARM
> mapping for DMA must not be done if PIO will be used. Some HCDs
> use PIO for some transfers but DMA for others. The generic layer
> must learn about this. 

Ah, that makes a lot of sense and the same problem would happen on
any non-DMA coherent architecture, including some embedded ppc's.

I can see why the dma unmap would invalidate the dcache and blow
away the PIO.

What bugs me here is that the dma_map_* operation should always
be done at the lowest level, ie, the actual HCD driver, and thus
it should be up to the HCD to decide whether to dma_map or not
depending on whether it's going to do DMA or not. I haven't
scrutinized USB lately but if that isn't the case and the dma_map_*
operations are done behind your back by the USB core then that needs to
be changed in a way or another, or hooked at least.

Cheers,
Ben.

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