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

From: Lee Jones
Date: Wed Feb 18 2015 - 11:26:57 EST

On Wed, 18 Feb 2015, Sebastian Hesselbarth wrote:

> 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
> really.
> >>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,
> right?
> >* 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?

FWIW, my person opinion would be to keep the drivers separate (not
least to keep drivers/soc from becoming the next dumping ground after
drivers/mfd and drivers/misc had had enough) and place them inside
their own subsystems, keep the device tree node hierarchy and place the
parent node under syscon. After all, that is what this device

Then, as you say, create a nice framework which elegantly probes each
of the platform device drivers in turn. That way each of the drivers
get to keep their own compatible string which you can use to match on
using current framework helpers.

simple-bus already does what you want. The only issue here is
semantics (naming). As I've alluded to already, people are currently
using (abusing?) this stuff, therefore it must be suitable, at least
on a functional (rather than religious) level.

Lee Jones
Linaro STMicroelectronics Landing Team Lead â Open source software for ARM SoCs
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