[PATCH v2 0/7] musb: sunxi: Add support for run-time changing dr-mode through sysfs

[Hans de Goede]

Hi,

On 19-08-16 23:25, Bin Liu wrote:
> Hi,
>
> On Mon, Aug 15, 2016 at 09:21:25PM +0200, Hans de Goede wrote:
>> Hi All,
>>
>> Here is a patch series which implements run-time changing the dr-mode
>> of sunxi musb controllers through the (already existing) musb "mode"
>> sysfs attribute.
>>
>> This is useful on boards where there is no id pin, e.g. some tv-boxes
>> use the musb controller to get an extra usb A port without needing
>> a hub chip. Except for the missing id pin when using a usb A<->A cable
>> these ports can do peripheral mode just fine. This series makes it
>> possible to do e.g. this by doing echo "peripheral" > mode before
>> plugging in the usb A<->A cable.
>
> Well, this is an illegal usecase. A-A cable is invalid by USB Spec.

Yet they exist and there are USB devices (e.g. harddisk enclosures)
which use a USB-A connector even though they are a device not
a host. I own several such devices myself. I agree that this is
wrong but these devices exist, typical case of reality trumping
the spec.

> With type-A receptacle the controller should be in host-only mode,
> switching to peripheral mode should not be allowed.

Peripheral mode can still be quite useful, e.g. to do ethernet
over USB (some of these devices lack wired ethernet).

Also the manual of these devices typically instructs users to
use an A<->A cable for firmware updates, since the bootROM in
the SoC does not care anmd will happily put the device in
Allwinner's custom DFU mode using the USB-A connector.

Although I agree with this being not ideal, the fact is that
an USB-A connector combined with a A<->A cable is electrically
100% identical to a USB-B connector with its id-pin not connected
(on the PCB side), which also happens see below.

>> This series has both sun4i-usb-phy driver and sunxi-musb-glue changes,
>> both are necessary for the run-time changing to work, but they can be
>> merged independently without breaking anything.
>>
>> Changed in v2:
>>
>> After sleeping on it a night I realized that always passing port_mode =
>> DUAL_ROLE to the musb-core was wrong. There is a distintion between
>> the id-pin not working properly which we can work around with software
>> mode switching (and we want to register both the host and udc driver
>> in this case) vs cases where we really only want to register a host
>> (wifi module connected to musb soldered onto the PCB).
>>
>> So v2 of this series drops the
>> "musb: sunxi: Always register both host and udc when build with dual-role support"
>> patch.
>>
>> Instead systems which are dual-role capable, but lack an id-pin should use
>> dr_mode = "otg" in the dts file. There is one problem with this, some
>> systems are dual-role capable but use a female USB-A connector connected
>> to the musb controller. These should come up in host mode by default,
>
> This is not a problem. With a type-A connector, the dual-role controller
> should work in host-only mode.
 > Role switching should only be allowed if an AB connector is used.

Yet people may want to use it in peripheral mode, I strongly believe
that we should not be telling people what they can and cannot do
with their hardware. That is policy and the kernel does not set
such policy, that is up to userspace. OTOH the port should clearly
default to host mode hence the new property.

> Using the sysfs entry to switch roles for generic purpose is really a
> bad idea, it opens up ton of problems.

Yet the sysfs entry exists already, and the problems depend on how
you hook it up. I'm merely using the sysfs entry as an id-pin
for cases where there is no id pin hooked up. If you look at the
patches you will see that all that is changed by writing the
sysfs entry is the phy reporting a different id-pin value to
the musb ip. Everything else is handled the same as for any normal
otg mode port.

> For systems which lack of id-pin should use a discrete circuit (for
> example GPIO) to detect the id pin in the AB receptacle, then the USB
> driver will handle the role switching transparently.

You're again talking theory here in reality there is hardware where
for whatever reasons the PCB uses a mini or micro usb receptacle,
but they did not bother to _physically_ hook up the id pin.

As said before reality trumps the Spec / theory.

>> rather then peripheral mode which is the default for systems which lack an
>> id-pin. This patch set introduces:
>>
>> "phy-sun4i-usb: Add "allwinner,usb0-usb-a-connector" dt property"
>>
>> Which allows specifying the use of a female USB-A connector for the
>> musb controller in the phy dt node, the presence of this dt property
>> changes the default to host mode.
>
> This is unnecessary, if using a type-A connector, dr_mode should be
> "host" in DT.

Which means we're telling users what they can and cannot do with
their hardware, which we should not be doing.

Regards,

Hans