Re: [patch v2] x86: allow ZONE_DMA to be configurable

From: H. Peter Anvin
Date: Fri Apr 15 2011 - 16:09:11 EST

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.


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