linux-kernel - Re: [Patch] Wait for console to become available, ver 3

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Message-Id: <20090420155033.a89ad4a0.akpm@linux-foundation.org>
Date:	Mon, 20 Apr 2009 15:50:33 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David VomLehn <dvomlehn@...co.com>
Cc:	stern@...land.harvard.edu, linux-kernel@...r.kernel.org,
	linux-embedded@...r.kernel.org, linux-usb@...r.kernel.org
Subject: Re: [Patch] Wait for console to become available, ver 3

On Mon, 20 Apr 2009 15:35:00 -0700
David VomLehn <dvomlehn@...co.com> wrote:

> On Mon, Apr 20, 2009 at 03:14:00PM -0700, Andrew Morton wrote:
> > On Mon, 20 Apr 2009 17:51:16 -0400 (EDT)
> > Alan Stern <stern@...land.harvard.edu> wrote:
> ...
> > > What if a subsystem simply doesn't know in advance whether or not it's 
> > > going to register a console?  Or doesn't know when it has finished 
> > > probing all devices (since a new device could be plugged in at any 
> > > time)?
> > 
> > Fix it.  It's trivial to make a sub-driver call back into a higher
> > layer to tell it that it registered a console.  Or just do the
> > i_will_be_adding_a_console_soon()/oops_im_not_adding_a_console_after_all()
> > calls from the layer which _does_ know.
> 
> In the case of the console, we already have register_console(), which is
> what I'm using. I think your proposal will require adding code all over
> the place. And buses such as USB simply have no way of knowing whether they
> are done enumerating devices. A new device could take hours to come on line.

Add a timeout parameter to i_will_be_adding_a_console_soon().  (This
means that the how-long-to-wait-for policy is probably ahrd-coded into
the kernel which might be a problem).

> > Yes, a boot parameter is "simple" to inplement.  But it's ghastly from
> > a usability POV.  Especially if you care about boot times.  For how
> > long do you delay?  The user has to experiment with different delays
> > until he finds the magic number.  Then he adds 10% and waits for the
> > inevitable failure reports to come in.
> > 
> > It's much better to just get it right, even if that makes it more
> > "complex".
> 
> With USB, you just can't *ever* get it right. There is no limit on how
> long a device has to tell you its there. I wish this weren't the case,
> but our good friends in the USB world tell us that we have been lucky
> to have had USB consoles work as long as they have.

Sigh, OK, I appreciate the problem better.  But the proposed "solution"
is really quite fragile.  I expect that it will only prove usable in
highly controlled hardware setups.

Is my option a) any use?
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