linux-kernel - Re: [PATCH RFC] x86: Fix 64-bit DMA masks on VIA

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Date:	Thu, 24 Apr 2008 17:19:55 +0200
From:	Michael Buesch <mb@...sch.de>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Andi Kleen <andi@...stfloor.org>,
	"linux-kernel" <linux-kernel@...r.kernel.org>, vojtech@...e.cz,
	muli@...ibm.com, jdmason@...zu.us, tglx@...utronix.de,
	mingo@...hat.com
Subject: Re: [PATCH RFC] x86: Fix 64-bit DMA masks on VIA

On Thursday 24 April 2008 16:32:40 Alan Cox wrote:
> On Thu, 24 Apr 2008 16:06:00 +0200
> Michael Buesch <mb@...sch.de> wrote:
> 
> > On Thursday 24 April 2008 15:43:50 Andi Kleen wrote:
> > > Michael Buesch <mb@...sch.de> writes:
> > > 
> > > > This untested patch is supposed to fix DMAing on some VIA boards.
> > > > Currently the DMA subsystem returns an error, if the driver does
> > > > tell that it supports a 64bit DMA mask. So the driver probing
> > > > would fail in that case. 
> > > 
> > > The driver is broken then. It is supposed to retry with a small
> > > mask on an error. Please fix the driver.
> > 
> > I already added a workaround to the driver.
> > Why do we need to workaround this in _every_ driver? (Note that _every_
> > driver supporting a 64bit mask is affected). Why not fix it in the DMA layer?
> 
> Some hardware wants to know it can get a given DMA mask or failure. I
> agree however that a "pci_prefer_64bit_dma(pdev)" function would be a
> good patch for someone to submit tot he PCI layer code.

Yeah well. I see the issue. However, I think the actual probing should
be done in the DMA layer. We could pass dma_set_mask() the mask as a pointer
and modify the mask value to what is actually used. So the driver would
know what mask we felt back to. That was actually my first idea,
but I preferred to submit a more simple solution without an API change
to the list.

However, in the end I don't care too much, as our driver is fixed.
I just think that we will have more of these bugs in the future. Especially,
as this bug won't hit on the majority of platforms.

-- 
Greetings Michael.
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