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

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Date:	Thu, 24 Apr 2008 16:12:46 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Michael Buesch <mb@...sch.de>
CC:	linux-kernel <linux-kernel@...r.kernel.org>, vojtech@...e.cz,
	muli@...ibm.com, jdmason@...zu.us, tglx@...utronix.de,
	mingo@...hat.com, davem@...emloft.net
Subject: Re: [PATCH RFC] x86: Fix 64-bit DMA masks on VIA

Michael Buesch 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 must admit my comment was slightly wrong. In some cases
it can make sense to start with a small mask and retry bigger.

> 
> I already added a workaround to the driver.
> Why do we need to workaround this in _every_ driver? 

The API was designed this way because many devices support different
hardware interfaces for different address sizes. So for example with a
32bit mask you might be able to transfer less data than with a 64bit
mask. And with the retry steps you should be able to figure out the
most efficient format for the current system.

See the discussion in Documentation/DMA-mapping.txt

cc: DaveM; I think the concept was from him originally.

-Andi
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