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

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Message-Id: <201002161522.38467.oliver@neukum.org>
Date:	Tue, 16 Feb 2010 15:22:38 +0100
From:	Oliver Neukum <oliver@...kum.org>
To:	"Shilimkar, Santosh" <santosh.shilimkar@...com>
Cc:	"Russell King - ARM Linux" <linux@....linux.org.uk>,
	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

Am Dienstag, 16. Februar 2010 15:12:45 schrieb Shilimkar, Santosh:
> > > > I am afraid for these controllers the controller driver must be responsible
> > > > for all DMA and cache issues. Indicating the exact requirements to the
> > > > upper layer would be a battle already lost.
> > > > so the safe choice is not to set has_dma and the generic layer will leave
> > > > the issue to the lower level.
> > > This means don't use dma at all which will almost kill the performance.
> > 
> > Why would you be unable to map a buffer in the hcd driver when you know
> > that you'll use DMA?
> Probably it can be. The USB stack has the dma maintenance code at common 
> place for all controllers and hence we were just trying to see if there is 
> way to handle that way.

This is true. If you can find a clean way to describe your requirements
to the generic layer, that would be better. The problem is that we must
not end up with a dozen flags.

Your original patch however kills ehci, ohci and uhci on some architectures.

	Regards
		Oliver
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