linux-kernel - Re: [PATCH V4 1/3] x86: introduce CONFIG_X86_DEV_DMA

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Message-ID: <20120412203220.GA27629@mail.gnudd.com>
Date:	Thu, 12 Apr 2012 22:32:20 +0200
From:	Alessandro Rubini <rubini@...dd.com>
To:	hpa@...or.com
Cc:	mingo@...e.hu, akpm@...ux-foundation.org, jbarnes@...tuousgeek.org,
	fujita.tomonori@....ntt.co.jp, linux-kernel@...r.kernel.org,
	tglx@...utronix.de, mingo@...hat.com, x86@...nel.org,
	giancarlo.asnaghi@...com, alan@...ux.intel.com
Subject: Re: [PATCH V4 1/3] x86: introduce CONFIG_X86_DEV_DMA_OPS

>>   config MAX_DMA_PADDR
>>           int
>>           default 536870912 if MAX_DMA_PADDR_512M
>>           default 16777216
>> 
>>   config MAX_DMA_PADDR_512M
>>           bool
>> 
>> (actually, we'd benefit from being able to use hex in defaults)
>> 
>> Is this worth exploring, to possibly submit a patch in this direction?
>> 
> 
> Not as a compile-time patch.

Why not? CONFIG_ISA is a compile-time choice. I thought that if you
compile for non-ISA systems you may reuse the zone mechanism with a
different split.

But if not a compile-time thing, would you accept it at run time?
I feel it's not a beautiful approach, because it must clearly be
masked by #ifndef CONFIG_ISA or such. Worse, you must know the zone
split at the beginning, before you scan the PCI bus and find your
devices.

thanks
/alessandro
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