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

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Date:	Tue, 02 Feb 2010 17:20:11 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Oliver Neukum <oliver@...kum.org>,
	Matthew Dharm <mdharm-kernel@...-eyed-alien.net>,
	Sergei Shtylyov <sshtylyov@...mvista.com>,
	Ming Lei <tom.leiming@...il.com>, linux-usb@...r.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Sebastian Siewior <bigeasy@...utronix.de>,
	Greg KH <greg@...ah.com>
Subject: Re: USB mass storage and ARM cache coherency

On Tue, 2010-02-02 at 17:11 +0000, Alan Stern wrote:
> On Tue, 2 Feb 2010, Oliver Neukum wrote:
> 
> > Am Dienstag, 2. Februar 2010 13:39:35 schrieb Catalin Marinas:
> > > > For storage that is correct. But what about other sources of pages,
> > > > for example iSCSI?
> > >
> > > In the iSCSI case, does the HCD driver write directly to a page cache
> > > page? Or it just fills in network packets that are copied to page cache
> > > pages by the iSCSI code (sorry, I'm not familiar with this part of the
> > > kernel). If the latter, the cache flushing in the HCD driver would not
> > > help and it needs to be done in the iSCSI code.
> >
> > As far as I can tell iSCSI does a private copy. But I don't know how
> > many methods to transfer code pages over USB exist. I'd say the
> > conservative solution is to flush for everything but control transfers.
> 
> This doesn't make any sense.  Nobody would ever use isochronous
> transfers to store data into a code page because isochronous is
> unreliable.  (Audio isn't a counterexample -- audio data may be mapped
> to userspace, but only to data pages, not code pages.  And the problem
> here is to maintain consistency between the D and I caches.)

My issues is with both I-D coherency and D-cache aliasing caused by
pages mapped in both user and kernel space (with different colours). The
flush_dcache_page() call should target both cases.

-- 
Catalin

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