linux-kernel - Re: [RFC][PATCH 2/3] usb: roles: Add usb role switch notifier.

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Message-ID: <fa44a7ab-14bc-24ec-a19b-7bf15e100ce1@redhat.com>
Date:   Sun, 6 Oct 2019 17:22:28 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     John Stultz <john.stultz@...aro.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Yu Chen <chenyu56@...wei.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Chunfeng Yun <chunfeng.yun@...iatek.com>,
        Felipe Balbi <balbi@...nel.org>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Jun Li <lijun.kernel@...il.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Linux USB List <linux-usb@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>
Subject: Re: [RFC][PATCH 2/3] usb: roles: Add usb role switch notifier.

Hi,

On 10/3/19 11:33 PM, John Stultz wrote:
> On Thu, Oct 3, 2019 at 1:56 PM Hans de Goede <hdegoede@...hat.com> wrote:
>> On 03-10-2019 22:45, John Stultz wrote:
>>> The HiKey960 has only one USB controller, but in order to support both
>>> USB-C gadget/OTG and USB-A (host only) ports. When the USB-C
>>> connection is attached, it powers down and disconnects the hub. When
>>> the USB-C connection is detached, it powers the hub on and connects
>>> the controller to the hub.
>>
>> When you say one controller, do you mean 1 host and 1 gadget controller,
>> or is this one of these lovely devices where a gadget controller gets
>> abused as / confused with a proper host controller?
> 
> I'm not totally sure myself, but I believe it's the latter, as the
> host ports have to be disabled in order for the gadet/otg port to
> function.
> 
> There was a similar situation w/ the original HiKey board (dwc2
> controller) as well, though the switching was done fully in hardware
> and we only needed some minor tweaks to the driver to keep the state
> transitions straight.
> 
>> And since you are doing a usb-role-switch driver, I guess that the
>> role-switch is integrated inside the SoC, so you only get one pair
>> of USB datalines to the outside ?
> 
> I believe so, but again, I don't have a ton of knowledge about the SoC
> details, Chen Yu would probably be the right person to answer, but I
> don't know if he's doing upstreaming anymore.
> 
>> This does seem rather special, it might help if you can provide a diagram
>> with both the relevant bits inside the SoC as well as what lives outside
>> the Soc. even if it is in ASCII art...
> 
> There is a schematic pdf here:
> https://github.com/96boards/documentation/raw/master/consumer/hikey/hikey960/hardware-docs/HiKey960_Schematics.pdf
> 
> The larger block diagram on page 3 might be helpful, but you can find
> more details on the usb hub bits on page 17 and 18.

Ok, so I took a quick look at the schematic and it is really funky.

The USB3 superspeed data pairs are only going to the USB-3 hub and
only the USB-2 lines are muxed between the TypeC and the HUB, so
in theory superspeed devices could keep working while the TypeC is
in device mode, since their data lines will still be connected,
but I guess the controller in the SoC is switched to device mode
then so this does not work. Likewise Vbus is an all or
nothing thing, either both the TypeC connector + the 2 Type-A
reeptacles get Vusb or none of them get Vusb. Also it is seems to use
the TypeC connector in host-mode together with the A receptacles.
I must say this is a weird design...

Anyways back the code to add a usb role switch notifier. I do
not think that this is a good idea, this is making "core" changes
to deal with a special case. If you are going to use a notfier for
this then IMHO the notifier should be part of the hikey960 usb role
swtich driver and not be in the usb-role-switch class code, since
this is very much a device specific hack.

Regards,

Hans