linux-kernel - Re: Problem with DMA on x86

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Message-ID: <20061121112829.19a9c043@localhost.localdomain>
Date:	Tue, 21 Nov 2006 11:28:29 +0000
From:	Alan <alan@...rguk.ukuu.org.uk>
To:	Ray Lee <ray-lk@...rabbit.org>
Cc:	Larry Finger <Larry.Finger@...inger.net>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Problem with DMA on x86_64 with 3 GB RAM

> Confused. As in, once the bcm43xx module initcall happens? Or without bcm43xx
> at all? If the former, is the behavior different when built as a module versus
> built-in? (ie, are there ordering problems.)

The pci_dma code on the x86_64 platform is broken for the case of PCI
devices with < 32bit DMA. Has been forever, this is a problem with
various devices, although most of the others are obsolete except for the
bcm43xx and b44 (the latter has hacks to work around the x86-64
brokenness).

At the very least the pci_set_dma_mask should error in this situation or
switch to using GFP_DMA (24bit) memory spaces. Having it error isn't the
whole solution as you still need some way to handle the "what do I do
next". 

Alan
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