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

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Message-ID: <1267200044.14703.31.camel@e102109-lin.cambridge.arm.com>
Date:	Fri, 26 Feb 2010 16:00:44 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Matthew Dharm <mdharm-kernel@...-eyed-alien.net>,
	linux-usb@...r.kernel.org,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	"Mankad,Maulik Ojas" <x0082077@...com>,
	Sergei Shtylyov <sshtylyov@...mvista.com>,
	Ming Lei <tom.leiming@...il.com>,
	Sebastian Siewior <bigeasy@...utronix.de>,
	Oliver Neukum <oliver@...kum.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"Shilimkar,Santosh" <santosh.shilimkar@...com>,
	Pavel Machek <pavel@....cz>, Greg KH <greg@...ah.com>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
	James Bottomley <James.Bottomley@...senPartnership.com>
Subject: Re: USB mass storage and ARM cache coherency

On Wed, 2010-02-24 at 21:13 +0000, Benjamin Herrenschmidt wrote:
> On Wed, 2010-02-24 at 11:19 -0500, Alan Stern wrote:
> > > It is but I'm not confident the responsibility for doing that cleanup
> > > is at the HCD level. That would impact a lot of HCD activities that
> > > don't need such flushing since the use of the page is purely in-kernel.
> >
> > That's right.  The HCD merely puts data wherever it's told to.  It
> > doesn't know whether the destination is in the page cache, in
> > userspace, or anywhere else.  The same is true for usb-storage.
> 
> I'm surprised that usb-storage has an issue here. It shouldn't afaik,
> since it's just a SCSI driver (or not anymore ?) and the BIO or
> filesystems handle things there no ? I haven't seen a single call to
> flush_dcache_page() in any of drivers/scsi, drivers/ata or drivers/ide
> when I looked...

The BIO or filesystem code don't call flush_dcache_page() either (well
some do like cramfs or jffs but they decompress the data received from
the block device).

-- 
Catalin

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