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

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Date:	Thu, 4 Feb 2010 13:39:23 +0900
From:	Paul Mundt <lethal@...ux-sh.org>
To:	George Spelvin <linux@...izon.com>
Cc:	catalin.marinas@....com, linux-kernel@...r.kernel.org
Subject: Re: USB mass storage and ARM cache coherency

On Wed, Feb 03, 2010 at 06:56:44PM -0500, George Spelvin wrote:
> > Apart from that, flush_dcache_page() doesn't have any data flow
> > information. Optimisations could be done on ARM if we know that the
> > kernel only intends to read from a page (no flushing necessary with a
> > non-aliasing D-cache).
> 
> Already done in flush_dcache_page().  If possible (uniprocessor), it just
> flags the page as PG_dcache_dirty, and defers the actual flush operation
> until it's mapped somewhere else (either a virtual alias or executable).
> 
Try reading the thread again, as you seem to have missed the point
completely. The issue isn't with lazy dcache writeback, the issue is that
flush_dcache_page() is a bit of a sledgehammer for cases when directional
information is available. The DMA mapping operations conversely are aware
of data flow and optimize accordingly.

Additionally, with something like a flush_dcache_range() it's possible
to optimize for large ranges as opposed to page-at-a-time looping for
anything that needs to flag PG_dcache_dirty on a bulk group of pages.
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