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

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Message-ID: <4A94C5B6.3060907@gmail.com>
Date:	Tue, 25 Aug 2009 23:18:46 -0600
From:	Robert Hancock <hancockrwd@...il.com>
To:	Brice Goglin <Brice.Goglin@...ia.fr>
CC:	Dan Williams <dan.j.williams@...el.com>,
	Maciej Sosnowski <maciej.sosnowski@...el.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: how to get a DMA channel near a process?

On 08/25/2009 07:05 AM, Brice Goglin wrote:
> 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

Don't think that DMA channel is the term you are looking for, maybe PCI 
Express lanes? DMA channels are an ancient ISA thing.

> 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.

Presumably incorrect information in the BIOS tables (SLIT or MADT, maybe?)

>
> 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

Not sure quite what you are trying to say here. Normally when memory is 
allocated for DMA use for a specific device I believe the kernel tries 
to allocate it on the same node that the device is attached to.

> 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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