[RFC PATCH 0/9] dt: dependencies (for deterministic driver initialization order based on the DT)
swarren at wwwdotorg.org
Wed Aug 27 09:37:58 PDT 2014
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
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.
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