Re: [v6 2/4] dt-bindings: hwmon: Add ASPEED TACH Control documentation

[Thierry Reding]

On Mon, Jul 17, 2023 at 11:54:26PM -0700, Guenter Roeck wrote:
> On 7/17/23 23:39, Thierry Reding wrote:
> > On Tue, Jul 18, 2023 at 08:04:24AM +0200, Krzysztof Kozlowski wrote:
> > > On 18/07/2023 06:01, 蔡承達 wrote:
> > > > > 
> > > > > On 17/07/2023 11:01, 蔡承達 wrote:
> > > > > > Guenter Roeck <linux@xxxxxxxxxxxx> 於 2023年7月17日 週一 上午1:00寫道：
> > > > > > > 
> > > > > > > On 7/16/23 09:08, Krzysztof Kozlowski wrote:
> > > > > > > 
> > > > > > > [ ... ]
> > > > > > > 
> > > > > > > > > 
> > > > > > > > > This patch serial doesn't use to binding the fan control h/w. It is
> > > > > > > > > used to binding the two independent h/w blocks.
> > > > > > > > > One is used to provide pwm output and another is used to monitor the
> > > > > > > > > speed of the input.
> > > > > > > > > My patch is used to point out that the pwm and the tach is the
> > > > > > > > > different function and don't need to
> > > > > > > > > bind together. You can not only combine them as the fan usage but also
> > > > > > > > > treat them as the individual module for
> > > > > > > > > use. For example: the pwm can use to be the beeper (pwm-beeper.c), the
> > > > > > > > > tach can be used to monitor the heart beat signal.
> > > > > > > > 
> > > > > > > > Isn't this exactly the same as in every other SoC? PWMs can be used in
> > > > > > > > different ways?
> > > > > > > > 
> > > > > > > 
> > > > > > > ... and in every fan controller. Not that it really makes sense because
> > > > > > > normally the pwm controller part of such chips is tied to the fan input,
> > > > > > > to enable automatic fan control, but it is technically possible.
> > > > > > > In many cases this is also the case in SoCs, for example, in ast2500.
> > > > > > > Apparently this was redesigned in ast2600 where they two blocks are
> > > > > > > only lightly coupled (there are two pwm status bits in the fan status
> > > > > > > register, but I have no idea what those mean). If the blocks are tightly
> > > > > > > coupled, separate drivers don't really make sense.
> > > > > > > 
> > > > > > > There are multiple ways to separate the pwm controller part from the
> > > > > > > fan inputs if that is really necessary. One would be to provide a
> > > > > > > sequence of address mappings, the other would be to pass the memory
> > > > > > > region from an mfd driver. It is not necessary to have N instances
> > > > > > > of the fan controller, even if the address space is not continuous.
> > > > > > > 
> > > > > > 
> > > > > > Hi Guenter,
> > > > > > 
> > > > > > May I ask about the meaning of the sequence of address mappings? It appears
> > > > > > to consist of multiple tuples within the 'reg' property, indicating
> > > > > > the usage of PWM/Tach
> > > > > > registers within a single instance. After that I can use the dts like following:
> > > > > > 
> > > > > > pwm: pwm@1e610000 {
> > > > > > ...
> > > > > > reg = <0x1e610000 0x8
> > > > > > 0x1e610010 0x8
> > > > > > 0x1e610020 0x8
> > > > > > 0x1e610030 0x8
> > > > > > 0x1e610040 0x8
> > > > > > 0x1e610050 0x8
> > > > > > 0x1e610060 0x8
> > > > > > 0x1e610070 0x8
> > > > > > 0x1e610080 0x8
> > > > > > 0x1e610090 0x8
> > > > > > 0x1e6100A0 0x8
> > > > > > 0x1e6100B0 0x8
> > > > > > 0x1e6100C0 0x8
> > > > > > 0x1e6100D0 0x8
> > > > > > 0x1e6100E0 0x8
> > > > > > 0x1e6100F0 0x8>;
> > > > > 
> > > > > 
> > > > > Uh, no... I mean, why? We keep pointing out that this should not be done
> > > > > differently than any other SoC. Open any other SoC PWM controller and
> > > > > tell me why this is different? Why this cannot be one address space?
> > > > 
> > > > Hi Krzysztof,
> > > > 
> > > > This is because the register layout for PWM and Tach is not continuous.
> > > > Each PWM/Tach instance has its own set of controller registers, and they
> > > > are independent of each other.
> > > 
> > > Register layout is not continuous in many other devices, so again - why
> > > this must be different?
> > > 
> > > > 
> > > > For example:
> > > > PWM0 uses registers 0x0 and 0x4, while Tach0 uses registers 0x8 and 0xc.
> > > > PWM1 uses registers 0x10 and 0x14, while Tach1 uses registers 0x18 and 0x1c.
> > > > ...
> > > > 
> > > > To separate the PWM controller part from the fan inputs, Guenter has
> > > > provided two methods.
> > > > The first method involves passing the memory region from an MFD
> > > > driver, which was the
> > > 
> > > I have no clue how can you pass memory region
> > > (Documentation/devicetree/bindings/reserved-memory/) from MFD and why
> > > does it make sense here.
> > > 
> > > > initial method I intended to use. However, it seems that this method
> > > > does not make sense to you.
> > > > 
> > > > Therefore, I would like to explore the second method suggested by
> > > > Guenter, which involves providing
> > > > a sequence of address mappings.
> > 
> > At the risk of saying what others have said: given that there's a single
> > reset line and a single clock line controlling all of these channels and
> > given what I recall of how address demuxers work in chips, everything
> > indicates that this is a single hardware block/device.
> > 
> > So the way that this should be described in DT is:
> > 
> > 	pwm@1e610000 {
> > 		reg = <0x1e610000 0x100>;
> > 		clocks = ...;
> > 		resets = ...
> > 	};
> > 
> > That'd be the most accurate representation of this hardware in DT. It is
> > then up to the driver to expose this in any way you see fit. For Linux
> > it may make sense to expose this as 16 PWM channels and 16 hardware
> > monitoring devices. Other operating systems using the same DT may choose
> 
> It is single chip. It should be a single hardware monitoring device with
> 16 channels. I don't even want to think about the mess we'd get if people
> start modeling a single chip as N hardware monitoring devices, one for
> each monitoring channel supported by that chip. It would be even more messy
> if the driver supporting those N devices would be marked for asynchronous
> probe, which would result in random hwmon device assignments.

Sorry, I badly worded it. What I meant to say was: one hardware
monitoring device with 16 channels.

> > to expose this differently, depending on their frameworks, etc. A simple
> > operating system may not expose this as separate resources at all but
> > instead directly program individual registers from this block.
> > 
> > I'd also like to add that I think trying to split this up into multiple
> > drivers in Linux is a bit overkill. In my opinion, though I know not
> > everyone shares this view, it's perfectly fine for one driver to expose
> > multiple types of resources. There's plenty of use-cases across the
> > kernel where tightly coupled devices like this have a single driver that
> > registers with multiple subsystems. Going through MFD only because this
> > particular hardware doesn't split registers nicely along Linux subsystem
> > boundaries.
> > 
> > So FWIW, I'm fine carrying hwmon code in a PWM driver and I'm equally
> > fine if PWM code ends up in a hwmon driver (or any other subsystem
> > really) if that makes sense for a given hardware.
> > 
> 
> I am fine either way as well, as long as we are talking about a single
> hwmon device and not 16 of them.

Excellent. Should make it pretty clear in which direction this should
go.

Thierry

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