Re: [RFC PATCH 0/9] dt: dependencies (for deterministic driver initialization order based on the DT)

From: Alexander Holler
Date: Tue Aug 26 2014 - 05:57:28 EST

Am 26.08.2014 10:51, schrieb Grant Likely:
On Mon, 25 Aug 2014 08:08:59 -0500, Jon Loeliger <jdl@xxxxxxx> wrote:

Anyway, instead of going back and forth between "deferred probe is good"
and "deferred probe is bad", how about we do something useful now and
concentrate on how to make use of the information we have in DT with the
goal to reduce the number of cases where deferred probing is required?

Good idea.

The proposal on the table is to allow the probe code
to make a topological sort of the devices based on
dependency information either implied, explicitly stated
or both. That is likely a fundamentally correct approach.

I believe some of the issues that need to be resolved are:

1) What constitutes a dependency?
2) How is that dependency expressed?
3) How do we add missing dependencies?
4) Backward compatability problems.

There are other questions, of course. Is it a topsort
per bus? Are there required "early devices"? Should
the inter-node dependencies be expressed at each node,
or in a separate hierarchy within the DTS? Others.

Topsort by bus wouldn't work. That would imply that nothing uses something involved with another bus. And early devices are handled fine by normal dependencies too (as long as they are complete), so there is no need to make an distinction.

Getting the dependency tree I think is only half the problem. The other
have is how to get the driver model to actually order probing using that
list. That problem is hard because the order drivers are probed is
currently determined by the interaction of driver link order, driver
initcall level, and device registration order. The first devices are
registered at an early initcall, before their drivers, and therefore
bind order is primarily determined by initcall level and driver link
order. However, later devices (ie. i2c clients) are registered by the
bus driver (ie. again, i2c) and probe order may be primarily link order
(if the driver is not yet registered) or registration order (if the
driver was registered before the parent bus).

That's why I've invented those "well-done"-initcalls. These are initcalls which just register a driver and don't probe it. Thats necessary to build a catalog of existing in-kernel-drivers (you have to know what you are sorting). Fortunately most drivers are already of that type. And those which aren't can be either just used like before (if it works) or they can be changed.

Changing them can be done per board (only enable dependency based order for a board if necessary drivers have changed).


Alexander Holler
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