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

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Message-ID: <5151C90A.5010108@ladisch.de>
Date:	Tue, 26 Mar 2013 17:12:58 +0100
From:	Clemens Ladisch <clemens@...isch.de>
To:	Peter Hurley <peter@...leysoftware.com>
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

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.

> 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.


Regards,
Clemens
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