Re: [PATCH 01/11] mfd: add the Berlin controller driver

From: Sebastian Hesselbarth
Date: Wed Feb 18 2015 - 10:59:52 EST

On 02/18/2015 04:06 PM, Arnd Bergmann wrote:
On Wednesday 18 February 2015 14:09:04 Sebastian Hesselbarth wrote:
I fundamentally disagree that either this registers or syscon in general
should in any way be seen as a bus. The chip control registers is an
highly unsorted bunch of bits that we try to match with cleanly
separated subsystems. This makes it a resource but no bus of any sort.

It really depends on what you mean by 'bus'. It's certainly not a bus_type
in Linux, but if you have a node in DT with multiple children, we
sometimes think of that as a bus. I believe it makes sense to have
the child devices under the syscon node here, at least the ones that
have no other registers.

Arnd, Lee,

First of all, I think I get the points of both of you.

With 'bus' I was referring to anything that implies a fixed number to
address any of the sub-units. As the register is more or less unsorted
and unordered, I'd see it as a resource - but that isn't important

The problem that we try to solve here is not a DT problem but solely
driven by the fact that we need something to register platform_devices
for pinctrl and reset. The unit we describe in DT is a pinctrl-clock-
power-reset-unit - or short chip control.

There are two problems here that we need to look at separately,
even though there is some interaction:

* For the DT representation, we need to make it something that
corresponds to the hardware. We could either have a single device
node for the set of registers, or we could have one node for each
child. With syscon, we could also put the functional device nodes
somewhere else, which we have to do if any device uses multiple
syscon parents.

Well, during the discussion, I think we can also get along with a
single node for the whole chip-registers - even in DT. Clock and
reset just require corresponding #foo-cells and pinctrl will have
its pinmux sub-nodes right in that single node. If we have separate
sub-nodes for each of the subsystems is mainly a matter of taste,

* For the driver code, we need a way to fit into the kernel model
while at the same time using the information that is in DT.
I agree with Lee that your current driver is not a good solution
here: you create a driver for the parent device that knows what
child devices there are, which is not a good abstraction. Instead
we should have a way for the child devices to get probed automatically,
just like we do for simple-bus, whether we use simple-bus or not.

With no sub-nodes, we'd have to have a driver that knows the linux
subsystem platform_devices. With sub-nodes, you are proposing a
"syscon-bus"-like compatible hook to populate sub-devices. Ok, I
get this.

If you argue that mfd is not the right place for this "driver" we'll
have to find a different place for it. I remember Mike has no problem
with extending early probed clock drivers to register additional
platform_devices - so I guess we end up putting it in there ignoring
mfd's ability to do it for us.

If you have a driver that is responsible for the entire register
area, I think it would be best to make that driver just register
to all the subsystems itself rather than creating child devices.

Hmm. That would create a beast of a driver wouldn't it? We could
get along with a library-like structure we each of foo-related
code resides in drivers/foo but still we'd need some common include
to reference to the sub-driver.

The alternative is to come up with a way to probe all the child
devices automatically, but then we should make that parent device
have a generic driver that does not need to know about the children
and that can work on any platform with similar requirements.

Ok, this is most likely the part that Lee doesn't like on the current
driver: a platform_device for registering platform_devices *only* and
only for Berlin.

So, out of the two options:

(a) Go for syscon_of_populate_devices() with a new compatible (I guess)
and having sub-nodes for each Linux subsystem that we want to have
a platform_device for. I fear that this will clash with early
registration of clk and we still have to find a way, i.e. device
naming policy, to match the drivers with their devices.

(b) Join clk, pinctrl, reset into a single chip/soc-control node and
rewrite the sub-drivers to not directly rely on DT compatible.
With this, joining all sub-drivers into drivers/soc/berlin would
be a sane approach, right? Also, I have the strong feeling, that
we will encounter situations later that will require the clk driver
to pull a reset before changing a specific clk rate, e.g. for GPU.

Anyway, I appreciate your discussion but still I am a little lost
between the two options. I thought that Antoine's mfd approach is
good, but I understand your concerns.

Any direction we should go for?

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