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

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Date:	Wed, 13 Oct 2010 18:48:21 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	"H. Peter Anvin" <hpa@...or.com>
cc:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Casey Dahlin <cdahlin@...hat.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [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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