netdev - Re: [PATCH 1/5] initdev:kernel: Asynchronously-discovered device synchronization, v5

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Date:	Sun, 3 May 2009 16:21:15 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	James Bottomley <James.Bottomley@...senPartnership.com>,
	David VomLehn <dvomlehn@...co.com>,
	<linux-kernel@...r.kernel.org>, <akpm@...ux-foundation.org>,
	<linux-usb@...r.kernel.org>, <greg@...ah.com>,
	<linux-scsi@...r.kernel.org>, <netdev@...r.kernel.org>
Subject: Re: [PATCH 1/5] initdev:kernel: Asynchronously-discovered device
 synchronization, v5

On Sat, 2 May 2009 13:55:45 -0400 (EDT)
Alan Stern <stern@...land.harvard.edu> wrote:

> On Sat, 2 May 2009, James Bottomley wrote:
> 
> > OK, so in your scheme, I get why console devices: they need to be
> > present early before we start dumping console output otherwise it
> > can get lost.
> > 
> > However, I don't see the need for either network or block.
> > 
> > For network, the only early discovery use is net root (which can be
> > done fully asynchronously)
> 
> Are you referring to the "rootwait" kernel parameter?  Is there a 
> reason why this is a boot-time parameter instead of always being
> set? I mean, under what circumstances would you _not_ want to wait
> until the root device is present?

if you have an initrd ;-)
> 
> Perhaps with some enterprise systems, it is preferred to have the 
> system fail with an explicit error message rather than wait 
> indefinitely...

actually, in an enterprise system, you want to reboot.
The bootloader might boot a different kernel the next time
that is known to work.
(for example, the current kernel might have been booted with the "once"
grub option)
> 
> > What I'm getting at is that I don't see the benefit of this in the
> > light of Arjan's async boot system, which can also tell us when all
> > discovery is complete ... what added benefit am I missing here?
> 
> How does Arjan's async boot system tell use when all discovery is
> complete?  AFAICS, it only tells you when all its async tasks are
> finished.  But device discovery and registration sometimes use other
> asynchronous techniques which Arjan's code is unaware of.
> Examples: the USB khubd thread, the USB mass-storage scanning thread,
> and the SCSI async-scanning thread.

for normal device probing we already have infrastructure though...
wait_for_device_probe, driver_probe_done and friends...
(the scsi scanning thread is being converted to the async
infrastructure btw)

do we need to invent more ?

-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org
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