Re: [PATCH 00/21] On-demand device registration

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Am 12.06.2015 um 13:36 schrieb Alexander Holler:
Am 12.06.2015 um 13:19 schrieb Alexander Holler:
Am 12.06.2015 um 09:25 schrieb Linus Walleij:
On Thu, Jun 11, 2015 at 6:40 PM, Alexander Holler
<holler@xxxxxxxxxxxxx> wrote:
Am 11.06.2015 um 14:30 schrieb Linus Walleij:

Certainly it is possible to create deadlocks in this scenario, but the
scope is not to create an ubreakable system.

IAnd what happens if you run into a deadlock? Do you print "you've
lost, try
changing your kernel config" in some output hidden by a
splash-screen? ;)

Sorry it sounds like a blanket argument, the fact that there are
mutexes in the kernel makes it possible to deadlock, it doesn't
mean we don't use mutexes. Some programming problems are
just like such.

I'm not talking about specific deadlocks through mutexes. I'm talking
about what happens when driver A needs driver B which needs driver A.
How do you recognise and handle that with your instrumented on-demand
device initialization? Such a circular dependency might happen by just
adding a new fucntion call or by changing the kernel configuration. And
with the on-demand stuff, the possibility that the developer introducing
this new (maybe optional) call will never hit such a circular dependency
is high. So you will end up with a never ending stream of problem
reports whenever someone introduced such a circular dependecy without
having noticed it.

And to come back to specific deadlocks, if you are extending function
calls from something former simple to something which might initialize a
whole bunch of drivers, needing maybe seconds, I wouldn't say this is a
blanket argument, but a real thread.

Keep in mind, that the possibility that a function call ends up with
initializing a whole bunch of other drivers, is not determined
statically, but depends on the configuration and runtime behaviour of
the actual system the on-demand stuff actually happens.

E.g. if driver A is faster one system that driver B, the whole bunch of
drivers might become initialized by a call in driver A. But if driver B
was faster on the developers system (or the system is configured to
first init driver B), than the whole bunch of drivers might have become
initialized by driver B on the developers system. Thus he never might
have hit a possible problem when the whole bunch of drivers got
initialized in driver A.

That means it isn't always a good idea to create dynamic systems (like
on-demand device initialization), because it's very hard to foresee and
correctly handle their runtime behaviour.

And because you've said that "problem space is a bit convoluted" and I disagree, here's a summary from my point of view:

1. All the necessary information (dependencies between drivers) already exists at compile time. The set of dependencies between drivers might become smaller by configuration, but will not become larger. So there should be NO need to collect them at runtime, e.g. by instrumenting function calls. I've described the problems I see with that above. I've choosen DT as source of dependencies because it offers an easy accessible and almost complete set of dependencies. I just had to add some type information to the dtb in order to identify the dependencies (phandles). But other ways to collect the dependencies would work too. Even the most simple way to add a static list of dependencies to each driver (which later on might be automated by some more clever stuff than adding them manually) would do the trick.

2. The problem to sort a set of nodes (drivers) with dependencies is solved since a long time and almost any developers uses it regularly in form of make. And everyone who used make -jN knows that the possible parallel initialization of drivers I've talked about, is already solved too.

3. In order to initialize the drivers in some specific order, their initcalls must be identified. I've offered a possible solution to that without much changes, but many other, even better ways, are possible too. It just depends on how much you want to change and on how much of these changes you will be able to feed into mainline kernel (which depends on your connections/relations inside the core kernel crew). E.g. instead of still just relying on one-dimensional arrays with (anonymous) pointers to initcalls, a multidimensional array of initcalls and drivername (and maybe more information) might be thinkable.

4. x86/amd64/ACPI-people, so most longtime and core kernel maintainers obviously don't have much interest until you've solved 1. in a way they can use too. So the necessary changes for 2. or 3. will have a big hurdle to take if 1. isn't solved usable for them too.

Alexander Holler

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