linux-kernel - Re: Problem with DMA on x86

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Message-ID: <45632AB0.60507@lwfinger.net>
Date:	Tue, 21 Nov 2006 10:34:56 -0600
From:	Larry Finger <Larry.Finger@...inger.net>
To:	Alan <alan@...rguk.ukuu.org.uk>
CC:	Ray Lee <ray-lk@...rabbit.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: Problem with DMA on x86_64 with 3 GB RAM

Alan wrote:
>> 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". 

I agree that pci_set_dma_mask should at least return an error. That will protect the ignorant!

Thanks for the pointer to the b44 code. I'll try to fix bcm43xx the same way.

Larry
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