Re: [RFC PATCH 0/9] dt: dependencies (for deterministic driver initialization order based on the DT)
From: Alexander Holler
Date: Wed Aug 27 2014 - 13:00:12 EST
Am 27.08.2014 18:37, schrieb Stephen Warren:
On 08/27/2014 10:30 AM, Alexander Holler wrote:
Am 27.08.2014 18:22, schrieb Stephen Warren:
On 08/27/2014 08:44 AM, Catalin Marinas wrote:
It's not just optimisation but an important feature for new arm64 SoCs.
Given some Tegra discussions recently, in many cases the machine_desc
use on arm is primarily to initialise devices in the right order. If we
can solve this in a more deterministic way (other than deferred
probing), we avoid the need for a dedicated SoC platform driver (or
machine_desc) or workarounds like different initcall levels and
explicit
DT parsing.
A lot of the ordering is SW driver dependencies. I'm not sure how much
of that can accurately be claimed as HW dependencies. As such, I'm not
sure that putting dependencies into DT would be a good idea; it doesn't
feel like HW data, and might well change if we restructure SW. It'd need
some detailed research though.
Almost every phandle is a dependency, so the DT is already full with
them.
That's true, but not entirely relevant.
phandles in DT should only be present where there's an obvious HW
dependency. It's obvious that, for example, there's a real HW dependency
between an IRQ controller and a device that has an IRQ signal fed into
the IRQ controller. It makes perfect sense to represent that as a
phandle (+args).
However, most of the ordering imposed by the Tegra machine descriptor
callbacks is nothing to do with this. It's more that the SW driver for
component X needs some low level data (e.g. SKU/fuse information) before
it can run. However, there's no real HW dependency between the HW
component X and the fuse module. As such, it doesn't make sense to
represent such a dependency in DT, using a phandle or by any other means.
Of course, there are probably cases where we could/should add some more
phandles/... and likewise cases where we shouldn't. That's why detailed
research is needed.
Irrespective though, a new kernel needs to work against an old DT, so
always needs to work without any (of these new) dependencies being
represented in DT, since they aren't represented there today. So, I
think pushing the issue into DT is a non-starter either way, unless we
accept yet another ABI-breaking change, in which case we should just
give up on any claims of ABI and make everyone's lives simpler.
If I hear research, my response is usually "how many years"?
Fact is that there are already a lot of usable dependencies in the DT,
they just didn't find their way into the kernel and weren't used.
And I think it doesn't help much to make the picture more complicated
than it already is. Solve one step by another and not try to solve
everything at once.
So first enable the kernel to use dependencies at all. I've shown that
it doesn't need magic to do so. Afterwards you can extend or change the
existing solution. It's not always the best approach, but for
complicated things it often doesn't make sense trying to solve
everything at first.
Of course, my approach isn't perfect, but at least it is something
people can already use to play with.
Ok, the way how my patches do handle devices (not drivers) might be
completely wrong, but that's just because I've never got in contact with
the device-model before, it always just worked. So I haven't spend any
time to look into that before and I didn't spend much time to look into
that for my patches (I just discoverd that device-handling by drivers
looks sometimes awkward). I was happy with what I've achieved in the
short time I've spend, and therfor posted the patches to give other
people an easy possibility to try the stuff themself.
Regards,
Alexander Holler
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