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

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Date:	Mon, 28 Apr 2008 18:53:52 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	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, jbarnes@...tuousgeek.org
Subject: Re: [PATCH RFC] x86: Fix 64-bit DMA masks on VIA


* 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)

	Ingo
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