linux-kernel - Re: Wait for console to become available, v3.2

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Message-ID: <20090421135034.GA30114@elte.hu>
Date:	Tue, 21 Apr 2009 15:50:34 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	David VomLehn <dvomlehn@...co.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux USB Mailing List <linux-usb@...r.kernel.org>,
	Linux Embedded Mailing List <linux-embedded@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Wait for console to become available, v3.2

* Arjan van de Ven <arjan@...radead.org> wrote:

> On Tue, 21 Apr 2009 08:43:46 +0200
> Ingo Molnar <mingo@...e.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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