Date: Tue, 21 Apr 2009 15:50:34 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Arjan van de Ven <arjan@infradead.org>
Cc: David VomLehn <dvomlehn@cisco.com>, "H. Peter Anvin" <hpa@zytor.com>,
       Thomas Gleixner <tglx@linutronix.de>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Linux USB Mailing List <linux-usb@vger.kernel.org>,
       Linux Embedded Mailing List <linux-embedded@vger.kernel.org>,
       Andrew Morton <akpm@linux-foundation.org>
Subject: Re: Wait for console to become available, v3.2
Message-ID: <20090421135034.GA30114@elte.hu>
References: <20090420234006.GA1958@cuplxvomd02.corp.sa.net> <20090421064346.GB8020@elte.hu> <20090421063549.3b71881d@infradead.org>
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* Arjan van de Ven <arjan@infradead.org> wrote:

> On Tue, 21 Apr 2009 08:43:46 +0200
> Ingo Molnar <mingo@elte.hu> wrote:
> 
> > The proper approach would be to use one of the async_synchronize*() 
> > facilities in kernel/async.c to properly order the opening of the 
> > console with device init.
> 
> Unfortunately this is not the answer.
> 
> First of all, USB does not use the async_* infrastructure, so it does
> not do anything.

[ Of course it does not. Two kernel cycles ago nothing had any async 
  infrastructure ;-) We barely have any use of it today. But cases 
  like this are the way to extend its use. ]

> But more importantly... USB *CANNOT* do this fundamental 
> operation. USB does not have the capability to know when you have 
> seen all devices that are connected. Devices just show up a random 
> amount of time after you turn the power on for the bus.... there's 
> no "and now we've seen all" operation.

Yes - and this is fundamentally true of any hotplug bus design.

Nevertheless the wish has been expressed to wait for such a device 
to become available, in this very thread we are discussing.

What i'm saying is: instead of "wait 2000 msecs, maybe it works out" 
hack, there should be a proper sleep+event based approach to the 
same. With perhaps a _timeout_ for the "no console arrived" negative 
case as well. (which timeout can be set to zero in the "I _know_ 
there's no console around".)

Do you see the fundamental design difference between the two 
solutions?

	Ingo
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