linux-kernel - Re: [PATCH 2.6.22.y] ieee1394: revert "sbp2: enforce 32bit DMA mapping"

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Date:	Tue, 07 Aug 2007 00:22:29 +0200
From:	Stefan Richter <stefanr@...6.in-berlin.de>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
CC:	Olaf Hering <olh@...e.de>, Robert Hancock <hancockr@...w.ca>,
	linuxppc-dev@...abs.org, stable@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2.6.22.y] ieee1394: revert "sbp2: enforce 32bit	DMA	mapping"

Benjamin Herrenschmidt wrote:
> Oh and, don't do the set_dma_mask() in sbp2, it has nothing to do there.
> It should be in the ohci1394 driver.

That's not quite right.  OHCI-1394 implementations can go beyond 4GB bus
address space.  (Although I don't know if there are such implementations
available.  At least there are two implementations which can set the
so-called Physical Range bigger than 4GB.)

Sbp2 however requires that everything which it DMA-maps resides in the
Physical Range of the controller.  This way the CPU is not involved in
most of the data transfers.  The OHCI-1394 controller acts as bus bridge
between IEEE 1394 bus and local bus, with a 1:1 mapping of IEEE 1394 bus
addresses to and from local bus addresses --- but not in the whole 48
bits white IEEE 1394 bus address range, only in the
implementation-dependent Physical Range.  The minimum Physical Range
that all OHCI-1394 implementations guarantee is 4GB.  I could actually
have set a bigger mask in sbp2 when the controller supports a
programmable bigger range.

So that's the story why that dma_set_mask went into sbp2:  Sbp2 wants
mappings in a _subset_ of the OHCI-1394 controllers DMA range.

Anyway.  For now I will simply go with what 2.6.23-rc has and what
2.6.21 had:  No dma_set_mask anywhere in the 1394 subsystem.  We can
revisit this whenever an actual need arises.
-- 
Stefan Richter
-=====-=-=== =--- --==-
http://arcgraph.de/sr/
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