Re: [PATCH v10 2/7] usb: mux: add generic code for dual role port mux

[Lu Baolu <baolu.lu@linux.intel.com>]

Hi Peter,

On 06/04/2016 10:28 AM, Peter Chen wrote:
> On Sat, Jun 04, 2016 at 12:06:06AM +0800, Lu Baolu wrote:
>>> from my point,it is a dual-role switch
>>> driver too,
>> No, it's not a dual-role switch driver, but a driver for USB port multiplexing.
>>
>> One example of port multiplexing can be found in several Intel SOC and PCH
>> chips, inside of which, there are two independent USB controllers: host and
>> device. They might share a single port and this port could be configured to
>> route the line to one of these two controllers. This patch introduced a generic
>> framework for port mux drivers. It aids the drivers to handle port mux by
>> providing interfaces to 1) register/unregister a mux device; 2) lookup the
>> mux device; and 3) switch the port.
>>
> For this case, I can't see it is different with dual-role switch.

Port mux is part of dual role switch, but not the whole thing.

Dual role switch includes at least below things:
 - ID or type-C event detection
 - port mux
 - VBUS management
 - start/stop host/device controllers

An OTG/Dual-role framework can be used to keep all these
things run together with an internal state machine. But it's
not duplicated with a generic framework for port mux and
the port mux drivers.

> Your
> case is just like Renesas case, which uses two different drivers between
> peripheral and host[1].

In my case, the port mux devices are physical devices and they
can be controlled through GPIO pins or device registers. They
are independent of both peripheral and host controllers.


>> Port multiplexing isn't equal to USB dual role. There are other cases in today's
>> systems. In several Intel PCH chips, there equips two USB host controllers: ehci
>> and xhci. The xhci USB2 ports are multiplexed with ehci. This guarantees all
>> USB ports work even running an old version of OS which lacks of USB3 support.
>> In theory, we can create a driver for the port mux and switch the ports between
>> xhci and ehci, but that's silly, isn't it? Why not always USB3?:-)
>>
>> Another case is xHCI debug capability. The xHCI host controller might equip
>> a unit for system debugging (refer to 7.6 of xHCI spec). The debugging unit is
>> independent of xhci host controller. But it shares its port with xhci. Software
>> could switch the port between xhci and the debugging unit through the registers
>> defined in xHCI spec.
>>
> Yes, above two are different with dual role switch. But in your code and
> Kconfig, it seems this framework is dedicated for dual-role. Eg:
>
> +menuconfig USB_PORTMUX
> +       bool "USB dual role port MUX support"
> +       help
> +         Generic USB dual role port mux support.

Above two cases are examples for port multiplexing, but I don't think we
need drivers for them. At this moment, this framework is only for dual
role port mux devices.

>
> I think a general dual role port mux is necessary, it can be used to
> manage different dual-role switch method, eg

Yes, I agree.

> - ID pin
> - External connector through GPIO
> - SoC register
> - sysfs
> - type-C events

ID pin and type-C events are the *reasons* which trigger the port
mux switch. Currently, on our platforms, gpio, registers and sysfs
are methods to control the mux.

>
> But this code is better co-work with OTG/Dual-role framework, we'd
> better have only interface that the user can know which role for the
> current port.

OTG/Dual-role framework and portmux framework are not overlapped.
The sysfs interface shouldn't be overlapped as well. Say, I have a port
mux device and I have a driver for it. I am able to read the status of my
port mux device through sysfs. This is not part of OTG/Dual-role as far
as I can see.

Best regards,
Lu Baolu