linux-kernel - Re: [PATCH resend 5/9] MIPS: sync after cacheflush

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Message-ID: <20101018191936.GH27377@linux-mips.org>
Date:	Mon, 18 Oct 2010 20:19:36 +0100
From:	Ralf Baechle <ralf@...ux-mips.org>
To:	Shinya Kuribayashi <skuribay@...ox.com>
Cc:	Kevin Cernekee <cernekee@...il.com>, linux-mips@...ux-mips.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH resend 5/9] MIPS: sync after cacheflush

On Mon, Oct 18, 2010 at 10:44:46PM +0900, Shinya Kuribayashi wrote:

> I suspect that SYNC insn alone is still not enough, insn't it?  In
> such systems with that 'deep' write buffer and data incoherency is
> visibly observed, there sill may be data write transactions floating
> in the internal bus system.

A SYNC in theory should ensure global visibilty of preceding writes and
completion of earlier reads.  That usually works between CPUs but not
all I/O systems fully participate in that "consistency domain" so more
or less arbitary shaking of the I/O system may still be required to to
achieve consistency.

> To make sure that all data (data inside processor's write buffer and
> data floating in the internal bus system), we need the following
> three steps:
> 
> 1. Flush data cache
> 2. Uncached, dummy load operation from _DRAM_ (not somewhere else)
> 3. then SYNC instruction
> 
> With these steps, data in write buffer will be pushed out of the
> processor's write buffer, wait for uncached load operation to be
> completed, and then finally the pipeline gets cleared.  Thoughts?

I'm trying to get a statement from the MIPS architecture guys if the
necessity to do anything beyond a cache flush is an architecture violation.

Don't worry, I'm not going to refuse patches for something just because
it's not complying to a piece of paper as long as the silicon is in the
wild.

  Ralf
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