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

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Date:	Sat, 20 Feb 2010 00:21:48 -0700
From:	Pete Zaitcev <zaitcev@...hat.com>
To:	"Gadiyar, Anand" <gadiyar@...com>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	"Shilimkar, Santosh" <santosh.shilimkar@...com>,
	Catalin Marinas <catalin.marinas@....com>,
	Pavel Machek <pavel@....cz>, Greg KH <greg@...ah.com>,
	Matthew Dharm <mdharm-kernel@...-eyed-alien.net>,
	Sergei Shtylyov <sshtylyov@...mvista.com>,
	Ming Lei <tom.leiming@...il.com>,
	Sebastian Siewior <bigeasy@...utronix.de>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
	"Mankad, Maulik Ojas" <x0082077@...com>
Subject: Re: USB mass storage and ARM cache coherency

On Tue, 16 Feb 2010 14:21:48 +0530
"Gadiyar, Anand" <gadiyar@...com> wrote:

> >         hcd->self.uses_dma = (dev->dma_mask != NULL);
> > 
> > Is it easier to make sure that PIO devices don't have dev->dma_mask set?
> 
> Not really. For instance, in the case of the DMA engine in the MUSB
> controller in OMAP3, we can only use DMA with endpoints other than
> EP0, and EP0 is what is used for control transfers.
> 
> It's not PIO for all the endpoints or DMA for all of them.

The HC driver does not have to be 100% truthful here. If the system
is not HIGHMEM, HCD can easily set uses_dma to false yet use DMA
by mapping buffers itself, without relying on the quoted code.

On a HIGHMEM system, block layer will bounce-buffer data in such case.
Hopefuly not a problem for ARM?

All network stack drivers work that way, BTW.

-- Pete
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