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

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Date:	Mon, 28 Apr 2008 10:04:07 -0700
From:	Jesse Barnes <jbarnes@...tuousgeek.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, Michael Buesch <mb@...sch.de>,
	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 Monday, April 28, 2008 9:53 am Ingo Molnar wrote:
> * Alan Cox <alan@...rguk.ukuu.org.uk> wrote:
> > > > > 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.
>
> yes, and i suspect Michael is correct in suggesting that the majority of
> drivers would use that interface and would let the PCI layer handle the
> probing/fallback details. (Jesse Cc:-ed)

With an implied fallback to 32 bits?  Michael's right (at least I think 
Michael's the one being quoted there) that "try 64 then fallback to 32 on 
error" is a pretty common sight, so having a hint that says you'd like 64 but 
don't really care would be a win for drivers.

Michael, want to hack something up?

Thanks,
Jesse


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