linux-kernel - Re: [RFC PATCH v3] drivercore: Add driver probe deferral mechanism

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Date:	Thu, 22 Sep 2011 15:19:01 -0600
From:	Grant Likely <grant.likely@...retlab.ca>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Arnd Bergmann <arnd@...db.de>, Greg Kroah-Hartman <greg@...ah.com>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Manjunath GKondaiah <manjunath.gkondaiah@...aro.org>,
	linux-kernel@...r.kernel.org, Dilan Lee <dilee@...dia.com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC PATCH v3] drivercore: Add driver probe deferral mechanism

On Thu, Sep 22, 2011 at 2:29 PM, Alan Cox <alan@...rguk.ukuu.org.uk> wrote:
> Definitely what is needed for some of the x86 SoC stuff and would let us
> rip out some of the special case magic for the SCU discovery.
>
> First thing that strikes me is driver_bound kicks the processing queue
> again. That seems odd - surely this isn't needed because any driver that
> does initialise this time and may allow something else to get going will
> queue the kick itself. Thus this seems to just add overhead.
>
> It all looks a bit O(N²) if we don't expect the drivers that might
> trigger something else binding to just say 'hey I'm one of the
> troublemakers'

The way I read it, absolute worst case is when every device but one
depends on another device.  In that case I believe it will be
O(Nlog(N)).  (Every device gets probed on the first pass, but only the
last one gets probed.  Then it goes through N-1 devices to the result
of only 1 more device getting probed, then N-2, etc.).  Actual usage
won't be anywhere near that complexity because there tends to be a
small number of devices on these SoCs that a lot of other devices
depend on, so I expect that there will be somewhere on the order of
3-4 passes for the typical embedded SoCs that I've seen.

driver_bound is used to kick off the process because anytime a new
device becomes active (bound to a driver), there is a high likelyhood
that one of the devices in the pending list can now be probed.
However, the re-probing is performed in a workqueue to make sure that
it is somewhat serialized to prevent the *entire* pending list to be
processed for every driver_bound() call.  Instead, by kicking a
workqueue, it guarantees that all the pending items are retried, but
kicking the workqueue 6 times doesn't mean that the list is going to
be processed 6 times.

In other words, driver_bound will kick off the process, but it doesn't
actually do the re-probing work.

g.
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