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

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Date:	Tue, 21 Apr 2009 08:43:46 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	David VomLehn <dvomlehn@...co.com>,
	Arjan van de Ven <arjan@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	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


* David VomLehn <dvomlehn@...co.com> wrote:

> Parallelization to improve boot times has been successful enough 
> that race conditions now exist between the init_post() open of 
> /dev/console and initialization of the console device. When this 
> occurs, opening /dev/console fails and any applications inherited 
> from init have no standard in/out/error devices. This is expected 
> behavior if no console device is available, but quite unfortunate 
> in the case where the console is just a bit slow waking up.
> 
> Some buses, such as USB, offer no guarantees about how long it 
> takes to discover devices, so there is no reliable way to 
> distinguish between a missing console and a slow one.  The 
> pragmatic approach taken in this patch is to wait for a while to 
> see if a console shows up, and just go on if it doesn't. The 
> default delay is 1000 msec (1 second). This value is conjured out 
> of thing air; any suggestions for a value that more closely 
> approximates the effective delays from the olden days before USB 
> consoles starting failing are more than welcome.

hm, this really seems like a bad hack and a workaround to me and as 
such it is not really an acceptable solution.

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.

Certain subsystems like storage (SCSI, libata, mount code and 
modules) has already been extended to this scheme.

So i think the right approach, if you want to speed up bootup, would 
be to extend the same concepts to console discovery, init and open 
methods.

	Ingo
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