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SubjectRe: [patch] x86: allow ZONE_DMA to be configurable
On Wed, 13 Oct 2010, H. Peter Anvin wrote:

> And the value of those additional options is what? I'd consider adding
> this to the sewer pit called CONFIG_EMBEDDED (with a BUG_ON, not a
> warning... sheesh)

BUG_ON() could panic the machine which would be rather unfortunate if we
simply tried to load a driver that the kernel no longer supports because
it doesn't have DMA. A WARN_ON() seems much more appropriate to identify
what the problem was. It's not a fatal condition.

> but only if there is any demonstrable value other
> than a trivial amount of code (kilobytes?) in exchange for a bunch of
> crap #ifdef.
>

The data savings is about 1% and the text savings is about 0.1% with all
three options disabled:

7922297 1245500 989600 10157397 9afd55 vmlinux.before
7914674 1232700 989472 10136846 9aad0e vmlinux.after

This is the only #ifdef necessary to make CONFIG_ZONE_DMA=n compile and
CONFIG_GENERIC_ISA_DMA=n would require two additional #ifdefs
(CONFIG_ISA_DMA_API=n would require none). We carry this patch
internally, so it would be trivial to send follow-up patches that do that
if this patch is merged.


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