Re: [PATCH v2 10/11] staging: typec: fusb302: Hook up mux support using tcpc_gen_mux support

[Peter Rosin]

On 2017-09-25 13:35, Hans de Goede wrote:
> Hi,
> 
> On 25-09-17 12:34, Peter Rosin wrote:
>> On 2017-09-13 17:48, Hans de Goede wrote:
>>> Hi,
>>>
>>> On 13-09-17 17:07, Rob Herring wrote:
>>>> On Wed, Sep 13, 2017 at 9:06 AM, Hans de Goede <hdegoede@xxxxxxxxxx> wrote:
>>>>> Hi,
>>>>>
>>>>>
>>>>> On 13-09-17 15:38, Rob Herring wrote:
>>>>>>
>>>>>> On Wed, Sep 13, 2017 at 3:56 AM, Hans de Goede <hdegoede@xxxxxxxxxx>
>>>>>> wrote:
>>>>>>>
>>>>>>> Hi,
>>>>>>>
>>>>>>>
>>>>>>> On 13-09-17 00:20, Rob Herring wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>> On Tue, Sep 05, 2017 at 06:42:20PM +0200, Hans de Goede wrote:
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Add mux support to the fusb302 driver, call devm_tcpc_gen_mux_create()
>>>>>>>>> to let the generic tcpc_mux_dev code create a tcpc_mux_dev for us.
>>>>>>>>>
>>>>>>>>> Also document the mux-names used by the generic tcpc_mux_dev code in
>>>>>>>>> our devicetree bindings.
>>>>>>>>>
>>>>>>>>> Cc: Rob Herring <robh+dt@xxxxxxxxxx>
>>>>>>>>> Cc: Mark Rutland <mark.rutland@xxxxxxx>
>>>>>>>>> Cc: devicetree@xxxxxxxxxxxxxxx
>>>>>>>>> Signed-off-by: Hans de Goede <hdegoede@xxxxxxxxxx>
>>>>>>>>> ---
>>>>>>>>>      Documentation/devicetree/bindings/usb/fcs,fusb302.txt |  3 +++
>>>>>>>>>      drivers/staging/typec/fusb302/fusb302.c               | 11
>>>>>>>>> ++++++++++-
>>>>>>>>>      2 files changed, 13 insertions(+), 1 deletion(-)
>>>>>>>>>
>>>>>>>>> diff --git a/Documentation/devicetree/bindings/usb/fcs,fusb302.txt
>>>>>>>>> b/Documentation/devicetree/bindings/usb/fcs,fusb302.txt
>>>>>>>>> index 472facfa5a71..63d639eadacd 100644
>>>>>>>>> --- a/Documentation/devicetree/bindings/usb/fcs,fusb302.txt
>>>>>>>>> +++ b/Documentation/devicetree/bindings/usb/fcs,fusb302.txt
>>>>>>>>> @@ -6,6 +6,9 @@ Required properties :
>>>>>>>>>      - interrupts             : Interrupt specifier
>>>>>>>>>        Optional properties :
>>>>>>>>> +- mux-controls           : List of mux-ctrl-specifiers containing 1 or
>>>>>>>>> 2
>>>>>>>>> muxes
>>>>>>>>> +- mux-names              : "type-c-mode-mux" when using 1 mux, or
>>>>>>>>> +                           "type-c-mode-mux", "usb-role-mux" when
>>>>>>>>> using
>>>>>>>>> 2 muxes
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> I'm not sure this is the right place for this. The mux is outside the
>>>>>>>> FUSB302. In a type-C connector node or USB phy node would make more
>>>>>>>> sense to me.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> The mux certainly does not belong in the USB phy node, it sits between
>>>>>>> the
>>>>>>> USB PHY
>>>>>>> and the Type-C connector and can for example also mux the Type-C pins to
>>>>>>> a
>>>>>>> Display
>>>>>>> Port PHY.
>>>>>>
>>>>>>
>>>>>> Thinking about this some more, the mux(es) should be its own node(s).
>>>>>> Then the question is where to put the nodes.
>>>>>
>>>>>
>>>>> Right, the mux will be its own node per
>>>>> Documentation/devicetree/bindings/mux/mux-controller.txt
>>>>> the bindings bit this patch is adding is only adding phandles pointing
>>>>> to that mux-node as the fusb302 "consumes" the mux functionality.
>>>>>
>>>>> So as such (the fusb302 is the component which should control the mux)
>>>>> I do believe that the bindings this patch adds are correct.
>>>>
>>>> Humm, that's not how the mux binding works. The mux controller is what
>>>> drives the mux select lines and is the provider. The consumer is the
>>>> mux device itself. What decides the mux state is determined by what
>>>> you are muxing, not which node has mux-controls property.
>>>>
>>>> By putting mux-controls in fusb302 node, you are saying fusb302 is a
>>>> mux (or contains a mux).
>>>>
>>>>
>>>>>>> As for putting it in a type-C connector node, currently we do not have
>>>>>>> such
>>>>>>> a node,
>>>>>>
>>>>>>
>>>>>> Well, you should. Type-C connectors are certainly complicated enough
>>>>>> that we'll need one. Plus we already require connector nodes for
>>>>>> display outputs, so what do we do once you add display muxing?
>>>>>
>>>>>
>>>>> An interesting question, I'm working on this for x86 + ACPI boards actually,
>>>>> not a board using DT I've been adding DT bindings docs for device-properties
>>>>> I use because that seems like the right thing to do where the binding is
>>>>> obvious
>>>>> (which I believe it is in this case as the fusb302 is the mux consumer) and
>>>>> because the device-property code should work the same on x86 + ACPI
>>>>> (where some platform-specific drivers attach the device properties) and
>>>>> on e.g. ARM + DT.
>>>>>
>>>>> The rest should probably be left to be figured out when an actual DT
>>>>> using device using the fusb302 or another Type-C controller shows up.
>>>>
>>>> Well this is a new one (maybe, I suppose others have sneaked by). If
>>>> ACPI folks want to use DT bindings, then what do I care. But I have no
>>>> interest in reviewing ACPI properties. The whole notion of sharing
>>>> bindings between DT and ACPI beyond anything trivial is flawed IMO.
>>>> The ptifalls have been discussed multiple times before, so I'm not
>>>> going to repeat them here.
>>>
>>> Ok, so shall I just drop the Documentation/devicetree/bindings/usb/fcs,fusb302.txt
>>> part of this patch then ?
>>
>> I totally agree with the concern that Rob expressed about handling USB C
>> muxes in a non-adhoc way. And this makes this series scary. I don't know
>> enough details about USB C muxes and PD (I just have a very superficial
>> mental model) to tell if this series is going down the right path. Or
>> how terrible it will be to fix things up if not?
>>
>> The series extends the mux subsystem with muxes that pins semantics
>> to specific states. That is new, and shows up exactly here when Rob is
>> not happy about the binding. And if Rob does not want this in the
>> DT bindings then I'm not so sure it is wise to do it at all? This
>> problem doesn't go away just because you remove the binding. I think
>> I would feel much better if there was a path forward on how to
>> represent USB C muxes in DT and how that would fit with the driver
>> structure.
>>
>> If you compare to the i2c-muxes, there is a relatively new i2c-mux-gpmux
>> driver that uses some general purpose mux from the mux subsystem to
>> implement an i2c-mux. If USB C muxes where to be done similarly, I'd
>> imagine there should be a general abstraction of what USB C muxes provide
>> somewhere outside of the mux subsystem, and a bunch of implementations
>> of that abstraction. One of those implementations could be to use "raw"
>> muxes from the mux subsystem. Of course, this is not what this series is
>> doing.
>>
>> Also, muxes that are not general purpose such as the ones added to the
>> mux subsystem by this series could perhaps be repurposed for some other
>> application, but since the interface implemented does not really obey
>> the rules (the provided mux controller interacts with different sets of
>> signals depending on the state) this will not be possible.
>>
>> These issues are what has caused me to do a lot of thinking and to sit
>> silent, sorry about that, but I would like input from someone more
>> experienced. If possible. But I'm not sure where to turn?
>>
>> As a crazy example, why is it not possible to hook up one signal pair
>> from the USB C mux, not to DP, but instead to some I2C controller? Then,
>> if done right, i2c-mux-gpmux could be hooked up with the relevant mux
>> controller and use the signal pair for I2C, with the mux controller
>> acting as a gate. So, maybe a bit crazy, but something like that is how
>> I think it should work from the mux subsystem point of view. And while
>> maybe crazy and while it might not be technically possible to do I2C
>> over a USB C connector for some reason, I do think that whatever
>> abstraction you come up with for USB C muxes, it has to deal with and
>> broker requests from both the USB subsystem and whatever other
>> subsystems deals with the alt pairs. Be it graphics for DP signals, or
>> whatever. IIUC, the alt signals need not be graphics, and it would be
>> sad to implement the USB C mux it in a way that makes it hard to use
>> the alt pairs for something else.
>>
>> [maybe my understanding of USB C is just wrong]
> 
> So 2 things:
> 
> 1) The Type-C subsys does actually abstract the mux outside of the
> Type-C port-manager (TCPM) core in the form of a tcpc_mux_dev, so for
> DT based platforms we could instantiate a tcpc_mux_dev from a node
> which then represents the mux as usual.
> 
> 2) What is very different from how the mux subsys currently is used,
> is who is in control of the mux. Currently a subsys which wants to
> route data through the mux selects the mux in the right mode, and
> then deselects it when done. This assumes that the mux can mux
> the data paths to the requested destination at all times, just not
> to multiple destinations at once. With Type-C and any moment in
> time, there really is only one correct setting as the mux is
> connect to a Type-C device on the other side which typically
> only has one config. So what happens with Type-C is that the TCPM
> negotiates with the externally connected Type-C device, and then
> sets the mux to the one and only correct setting for that device
> to be fully functional.
> 
> So your i2c example, for example, will not work with the normal
> way where the i2c subsys asks the mux to mux a data-pair to the i2c
> controller, as that only makes sense when your theoretical Type-C
> device which talks i2c over a pair is connected externally, where
> as when something else is connected then trying to talk i2c might
> even damage the connected device. So with Type-C it is the TCPM
> and only the TCPM which controls the mux.

If I get this correctly, some code (the TCPM?) negotiates with the
other side over PD, and after that the way to set up the USB C mux
is known and will not change until the cable is unplugged (unless
we go wild and renegotiate, but let's assume that will not happen). 
What I don't get is why do you want to squeeze what the TCPM(?) is
doing with its specialized mux into the existing mux subsystem
interface?

I mean, the mux interface isn't really a perfect fit with its
locking and support for multiple consumers etc that really just gets
in the way of setting a register in some chip to some value. Which
is all that is really needed when you know that the TCPM is the only
one accessing the chip.

And from the point of view of the mux subsystem, the USB C muxes
like the Pericom driver will be in a class of their own. Muxes of
that class can really only be used by one thing -- the TCPM code.

So, I don't see the benefit of doing this through the mux subsystem.
My hope is that the TCPM code is generic, and that by putting the
USB C mux inside the mux subsystem, you can keep the TCPM code
generic? That might be good for the TCPM code, but it does create a
new class of devices in the mux subsystem, and opens the door for
other classes later on.

Why not just add a function pointer that the generic TCPM code calls
if it is set, and then add some code somewhere more local to the TCPM
code to back that call, and completely avoid the mux subsystem?

Cheers,
Peter