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

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Date:	Fri, 15 Apr 2011 13:08:46 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Clemens Ladisch <clemens@...isch.de>
CC:	David Rientjes <rientjes@...gle.com>,
	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 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.

	-hpa


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