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

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Date:	Thu, 04 Mar 2010 18:07:29 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Paul Mundt <lethal@...ux-sh.org>
Cc:	James Bottomley <James.Bottomley@...senPartnership.com>,
	Pavel Machek <pavel@....cz>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	benh@...nel.crashing.org, linux@....linux.org.uk,
	mdharm-kernel@...-eyed-alien.net, linux-usb@...r.kernel.org,
	x0082077@...com, sshtylyov@...mvista.com, tom.leiming@...il.com,
	bigeasy@...utronix.de, oliver@...kum.org,
	linux-kernel@...r.kernel.org, santosh.shilimkar@...com,
	greg@...ah.com, linux-arm-kernel@...ts.infradead.org
Subject: Re: USB mass storage and ARM cache coherency

On Thu, 2010-03-04 at 15:41 +0000, Paul Mundt wrote:
> On Thu, Mar 04, 2010 at 03:29:38PM +0000, Catalin Marinas wrote:
> > On Thu, 2010-03-04 at 14:21 +0000, James Bottomley wrote:
> > > The thing which was discovered in this thread is basically that ARM is
> > > handling deferred flushing (for D/I coherency) in a slightly different
> > > way from everyone else ...
> >
> > Doing a grep for PG_dcache_dirty defined in terms of PG_arch_1 reveals
> > that MIPS, Parisc, Score, SH and SPARC do similar things to ARM. PowerPC
> > and IA-64 use PG_arch_1 as a clean rather than dirty bit.
> 
> SH used to use it as a PG_mapped which was roughly similar to the
> PG_dcache_clean approach, at which point things like flushing for the PIO
> case in the HCD wasn't necessary. It did result in rather aggressive over
> flushing though, which is one of the reasons we elected to switch to
> PG_dcache_dirty.

Are you more in favour if a PIO kmap API than inverting the meaning of
PG_arch_1? 

I'm not familiar with SH but for PIO devices the flushing shouldn't be
more aggressive. For the DMA devices, Russell suggested that we mark the
page as clean (set PG_dcache_clean) in the DMA API to avoid the default
flushing.

> Note that the PG_dcache_dirty semantics are also outlined in
> Documentation/cachetlb.txt for PG_arch_1 usage, so it's hardly esoteric.

Yes, but the flush_dcache_page() semantics outlined in the same file
aren't followed by all the PIO drivers in the kernel.

-- 
Catalin

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