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

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Date:	Wed, 03 Mar 2010 10:40:17 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	mdharm-kernel@...-eyed-alien.net, oliver@...kum.org,
	linux@....linux.org.uk, greg@...ah.com, x0082077@...com,
	sshtylyov@...mvista.com, bigeasy@...utronix.de,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	James.Bottomley@...senPartnership.com, santosh.shilimkar@...com,
	pavel@....cz, tom.leiming@...il.com,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: USB mass storage and ARM cache coherency

On Tue, 2010-03-02 at 23:29 +0000, Benjamin Herrenschmidt wrote:
> On Tue, 2010-03-02 at 17:05 +0000, Catalin Marinas wrote:
> 
> > The viable solutions so far:
> >
> >      1. Implement a PIO mapping API similar to the DMA API which takes
> >         care of the D-cache flushing. This means that PIO drivers would
> >         need to be modified to use an API like pio_kmap()/pio_kunmap()
> >         before writing to a page cache page.
> >      2. Invert the meaning of PG_arch_1 to denote a clean page. This
> >         means that by default newly allocated page cache pages are
> >         considered dirty and even if there isn't a call to
> >         flush_dcache_page(), update_mmu_cache() would flush the D-cache.
> >         This is the PowerPC approach.
[...]
> > Option 2 above looks pretty appealing to me since it can be done in the
> > ARM code exclusively. I've done some tests and it indeed solves the
> > cache coherency with a rootfs on a USB stick. As Russell suggested, it
> > can be optimised to mark a page as clean when the DMA API is involved to
> > avoid duplicate flushing.
> 
> That wouldn't solve the need for invalidating the I-cache... Unless we
> use another bit.

Indeed. We currently always invalidate the I-cache when the page is
mapped. With PG_arch_2, we could optimise this but I'm not sure it is
worth since I think we only get an update_mmu_cache() call for a page
(unless it is unmapped and re-mapped again).

-- 
Catalin

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