linux-kernel - Re: [patch v2] x86: allow ZONE

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Message-ID: <alpine.DEB.2.00.1104201929170.15661@chino.kir.corp.google.com>
Date:	Wed, 20 Apr 2011 19:31:57 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	"H. Peter Anvin" <hpa@...or.com>
cc:	Clemens Ladisch <clemens@...isch.de>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch v2] x86: allow ZONE_DMA to be configurable

On Fri, 15 Apr 2011, H. Peter Anvin wrote:

> On 04/15/2011 04:42 AM, Clemens Ladisch wrote:
> > David Rientjes wrote:
> >> ZONE_DMA is unnecessary for a large number of machines that do not
> >> require addressing in the lower 16MB of memory because they do not use
> >> ISA devices with 16-bit address registers (plus one page byte register).
> >>
> >> This patch allows users to disable ZONE_DMA for x86 if they know they
> >> will not be using such devices with their kernel.
> >> ...
> >>  config ZONE_DMA
> >> -	def_bool y
> >> +	bool "DMA memory allocation support" if EXPERT
> > 
> > Better call it "ISA/LPC DMA memory allocation support".
> > 
> >> +	default y
> > 
> > Does it make sense to have this without CONFIG_ISA_DMA_API?
> > 
> >> +	help
> >> +	  DMA memory allocation support allows old ISA devices with 16-bit
> >> +	  address registers to allocate memory within the first 16MB of
> >> +	  address space.  Disable if no such devices will be used.
> > 
> > "Old ISA devices" might be misleading because ISA does not imply actual
> > ISA slots; please mention LPC devices like the floppy controller and the
> > parallel port in ECP mode.
> > 
> 
> This isn't really a very good edit, as ZONE_DMA isn't inherently limited
> to ISA/LPC devices (and LPC devices which do LPC-native bus mastering
> DMA are not affected.)  ZONE_DMA is used even for some PCI devices which
> have restrictions on the DMA address mask.
> 

Peter, do you think we can merge this patch now?  The discussion from 
October resulted in the decision to rename CONFIG_EMBEDDED to 
CONFIG_EXPERT and make CONFIG_GENERIC_ISA_DMA and CONFIG_ISA_DMA_API 
configurable for x86.  That's all been done via the -mm tree and is in the 
latest git.

We've run without ZONE_DMA for a few years without issues (and, obviously, 
this patch) on x86.
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