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

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Message-Id: <201002161446.59920.oliver@neukum.org>
Date:	Tue, 16 Feb 2010 14:46:59 +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 14:40:45 schrieb Shilimkar, Santosh:
> > > If the CPU writes to a DMA_FROM_DEVICE buffer between map and unmap, the
> > > writes can sit in the cache, and on unmap, they will be discarded.
> > >
> > > Cleaning the cache on unmap is not an option; that too can lead to DMA
> > > buffer corruption in the DMA case.
> > 
> > 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?

	Regards
		Oliver
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