Generic communication of boot loader state to the OS

[Russell King - ARM Linux]

On Tue, Aug 26, 2014 at 02:39:45PM -0400, jonsmirl at gmail.com wrote:
> On Tue, Aug 26, 2014 at 2:28 PM, Russell King - ARM Linux
> <linux at arm.linux.org.uk> wrote:
> > On Tue, Aug 26, 2014 at 12:10:00PM -0400, jonsmirl at gmail.com wrote:
> >> As a side effect this will eliminate the need for kernel command line
> >> parameters describing boot state. Like console="". Over time it might
> >> even be able to pass a DHCP IP address from uboot into the kernel.
> >
> > Err no.  Don't even think about that.  DHCP may be wonderful and all,
> > but there's a fundamental issue with it: entries time out unless they
> > are renewed.
> >
> > Why is that a problem?  Well, take a DHCP server which hands out
> > dynamic addresses, and updates the DNS.  When the lease expires, it
> > tears down the DNS entry.
> >
> > Now take a target booting using DHCP in uboot, which then mounts its
> > root NFS.  If it tries to startup a DHCP client, the first thing the
> > DHCP client does is to clean up the interface... resulting in it
> > killing the root NFS connection.  If that doesn't happen, then you
> > end up with a problem at shutdown, because DHCP clients always
> > deconfigure the interface when they're killed off - resulting in
> > "reboot" not being functional.
> 
> I run the same configuration  -  uboot in flash, tftp to load kernel,
> nfs mount the root. And I do reboot fifty times a day too.  I just put
> everything on static IPs to avoid problems.
> 
> Would this work if the DHCP client was notified that there already was
> an IP lease in place?  It could remember that lease and then be
> responsible for setting it into the interface and renewing it. Right
> now there is no way to pass this information.
> 
> Doesn't the DHCP client do the wrong thing simply because it lacks the
> info to do something better?

It can get the already configured IP address if the kernel (or initramfs)
has already configured the interface - it's already available from the
interface itself.  So it's not that there's no way to pass this to DHCP,
it's that DHCP always starts out assuming that it's the new kid on the
block.

Solving that problem doesn't get away from the shutdown problem though.
You can partially avoid it by not having interfaces brought down, but
one of the things that distros commonly do is kill all processes during
shutdown... which kills the DHCP client, and then causes the interface
to be deconfigured... which then stalls the reboot operation because
suddenly the root-NFS server is no longer available... Gosh, why did
that happen I wonder...

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