linux-kernel - how to get a DMA channel near a process?

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Date:	Tue, 25 Aug 2009 15:05:22 +0200
From:	Brice Goglin <Brice.Goglin@...ia.fr>
To:	Dan Williams <dan.j.williams@...el.com>,
	Maciej Sosnowski <maciej.sosnowski@...el.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: how to get a DMA channel near a process?

Hello,

I am playing with DMA engine on a Nehalem box with two X58 chipsets
(Supermicro X8DAH). My understanding is that there are 8 DMA channels on
each chipset, so 8 channels near each processor. Unfortunately, my BIOS
and 2.6.31-rc still wrongly reports the physical location of my devices
(it claims all PCI devices are near the first processor) but I worked
around the problem manually.

The offloaded copy performance changes a lot depending on whether the
process memory is allocated near the DMA device. So first I would like
to know if DMA channels are allocated near the requesting
processor/process. Then I guess it's possible to read the cpu mask near
a given chan by following chan->dev.device up to the pci device, right?
But is there any way to request a DMA channel near a specific socket or
NUMA node or cpu mask?

Thanks,
Brice

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