linux-kernel - Re: [PATCH 4 of 4] x86-64: Calgary IOMMU: Fix off by one when calculating register space

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Date:	Sat, 30 Sep 2006 10:55:18 -0400
From:	Valdis.Kletnieks@...edu
To:	Muli Ben-Yehuda <muli@...ibm.com>
Cc:	ak@...e.de, jdmason@...zu.us, linux-kernel@...r.kernel.org,
	discuss@...-64.org
Subject: Re: [PATCH 4 of 4] x86-64: Calgary IOMMU: Fix off by one when calculating register space

On Sat, 30 Sep 2006 11:43:32 +0300, Muli Ben-Yehuda said:

> +	 * Each Calgary has four busses. The first four busses (first Calgary)
> +	 * have RIO node ID 2, then the next four (second Calgary) have RIO
> +	 * node ID 3, the next four (third Calgary) have node ID 2 again, etc.
> +	 * We use a gross hack - relying on the dev->bus->number ordering,
> +	 * modulo 14 - to decide which Calgary a given bus is on. Busses 0, 1,
> +	 * 2 and 4 are on the first Calgary (id 2), 6, 8, a and c are on the
> +	 * second (id 3), and then it repeats modulo 14.
> + 	 */
> +	rionodeid = (dev->bus->number % 14 > 4) ? 3 : 2;

A quick perusal of the pci-calgary.c in 2.6.18-mm2 doesn't find a single
comment explaining where "6, 8, a, c" comes from, which makes that 14 seem
"magical" - are the 3rd Calgary's busses e,f, 10, and 12? It makes me wonder
why they didn't just blow 2 "reserved" numbers to make it mod 16 and an easier
decode. ;)

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