linux-kernel - Re: [PATCH] firewire: Enable physical DMA above 4GB

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Date:	Tue, 26 Mar 2013 13:04:36 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Clemens Ladisch <clemens@...isch.de>
Cc:	Stefan Richter <stefanr@...6.in-berlin.de>,
	linux1394-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] firewire: Enable physical DMA above 4GB

On Tue, 2013-03-26 at 17:12 +0100, Clemens Ladisch wrote:
> Peter Hurley wrote:
> > Quadlet reads to memory above 4GB is painfully slow when serviced
> > by the AR DMA context. In addition, the CPU(s) may be locked-up,
> > preventing any transfer at all.
> 
> Using physical DMA prevents the use of that address space for software
> address handlers, so you have adjust the low_memory_region start in
> core-transaction.c.

Right, thanks for pointing that out.

> > Write the PhyUpperBound register with the end-of-memory value. If
> > end-of-memory is beyond the OHCI limit of 0x0000ffff00000000,
> > clamp to that value.
> 
> You will have to lower this limit; there are protcols that assume that
> addresses like 0xecc000000000 are available for software.

Maybe 0x0000e80000000000 is a better maximum upper bound?

Regards,
Peter Hurley

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