linux-kernel - Re: [PATCH] arch/tile: Add driver to enable access to the user dynamic network.

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Date:	Mon, 28 Jun 2010 11:23:16 -0400
From:	Chris Metcalf <cmetcalf@...era.com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arch/tile: Add driver to enable access to the user dynamic
 network.

On 6/28/2010 7:12 AM, Arnd Bergmann wrote:
> On Sunday 27 June 2010, Chris Metcalf wrote:
>   
>> On 6/26/2010 7:16 AM, Arnd Bergmann wrote:
>>     
>>> It seems strange that you need this. Why does linux/list.h
>>> depend on asm/processor.h?
>>>       
>> <linux/list.h> -> <linux/poison.h> -> <linux/prefetch.h> ->
>> <asm/processor.h>.  There doesn't seem to be any good way around this. 
>> I could, I suppose, use an opaque "struct list_head;" declaration in
>> <asm/processor.h>, then create one with kmalloc on demand, but that
>> seemed like overkill, so I embed the made-up version here, then validate
>> it as a BUILD_BUG_ON() to be the same size as a real list_head.  I never
>> actually use the "hardwall_list" structure directly.
>>     
> We could break the dependency by turning prefetch_range into a macro
> or an extern function. There is only one user, and it's in a staging
> driver, so the impact would be minimal.
>   

I don't think so.  The problem is that users of <linux/list.h> expect to
be able to #include that one header, then use things like
list_for_each() (which uses prefetch, as defined in <asm/processor.h>),
but without also being required to #include <asm/processor.h> themselves
explicitly.

I think the only "true" fix would be to have a new <linux/list_types.h>
header that provides list_head (and presumably hlist_head and
hlist_node), which <linux/list.h> would include, as would our
<asm/processor.h>.  This is certainly in line with recent
header-separation changes (e.g. mm_types.h).  Would there be interest in
a change like this?  I implemented it in my tree, and if it sounds
plausible to you, I'll send out a git diff, but it looks pretty much
exactly like this description :-)

-- 
Chris Metcalf, Tilera Corp.
http://www.tilera.com

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