linux-kernel - Re: [PATCH 02/18] Blackfin Serial Driver: use barrier instead of cpu

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Message-ID: <8bd0f97a0903100325k6cbcf73ek8e5f3723b2c4bb0a@mail.gmail.com>
Date:	Tue, 10 Mar 2009 06:25:08 -0400
From:	Mike Frysinger <vapier.adi@...il.com>
To:	gyang <graf.yang@...log.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Bryan Wu <cooloney@...nel.org>, alan@...rguk.ukuu.org.uk,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 02/18] Blackfin Serial Driver: use barrier instead of 
	cpu_relax for Blackfin SMP like patch

On Tue, Mar 10, 2009 at 06:07, gyang wrote:
> On Fri, 2009-03-06 at 14:37 -0800, Andrew Morton wrote:
>> On Fri,  6 Mar 2009 14:42:44 +0800
>> Bryan Wu <cooloney@...nel.org> wrote:
>>
>> > From: Graf Yang <graf.yang@...log.com>
>> >
>> > We are making a SMP like patch to blackfin, cpu_relax() is replaced by a
>> > data cache flush function which will count it to a per-cpu counter.
>> > If this serial function is called too early, the per-cpu data area have
>> > not been initialized, this call will cause crash.
>>
>> That's a bug in blackfin architecture support.  The kernel should be
>> able to call cpu_relax() at any time, surely.  It's a very low-level
>> and simple thing.
>>
>> > So we'd like to use barrier() instead of cpu_relax().
>> >
>>
>> barrier() is purely a compiler concept.  We might as well just remove
>> the cpu_relax() altogether.
>
> Do you mean remove cpu_relax(), and either not add barrier() here?

afaik, early printk all runs before SMP is setup, so having it be a
100% busy wait is fine
-mike
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