[U-Boot] [PATCH 17/17] sunxi: Hookup OTG USB controller support

[Michal Suchanek]

On 9 February 2015 at 04:54, Siarhei Siamashka
<siarhei.siamashka at gmail.com> wrote:
> On Sat, 31 Jan 2015 04:54:47 +0200
> Siarhei Siamashka <siarhei.siamashka at gmail.com> wrote:
>
>> On Sun, 11 Jan 2015 20:34:55 +0100
>> Hans de Goede <hdegoede at redhat.com> wrote:
>>
>> > Hookup OTG USB controller support and enable the otg controller + USB-keyb
>> > on various tablets.
>> >
>> > This allows tablet owners to interact with u-boot without needing to solder
>> > a serial console onto their tablet PCB.
>>
>> Thanks. Works great on my Primo73 and Primo81 tablets. However I have
>> just one naive question before happily taking it into use by default.
>>
>> Is the hardcoded otg host mode (without checking the id pin) always
>> safe? For example, what happens if somebody connects a charger instead
>> of a usb keyboard to the tablet?
>
> So, does anyone have any idea if it is theoretically safe to have two
> USB hosts connected to each other and both driving VBUS at +5V?
>
> A somewhat realistic scenario is booting an Allwinner tablet over
> USB using the FEL mode. Is it a good idea for such tablet to have
> CONFIG_USB_MUSB_SUNXI=y and CONFIG_USB0_VBUS_PIN="axp_drivebus"
> in defconfig?
>

>From my past experiments with some non-standard-compliant cables which
allow connecting a tablet with a PC without telling the tablet to go
OTG mode it seems the tablet battery is drained really quick in this
case.

There is a protocol for telling the tablet not to power the bus (or
telling the PC not to power it for that matter) but both sides have to
support matching part of the protocol. Either way, ideal case is that
tablet gets power form PC or powered hub even in host mode which is
not standards-compliant and obviously does not work with stuff like
keyboards.

However, it should be possible to detect that there is bus power at
the moment a device is connected and not power the device in that case
regardless of usb mode negotiated after the connection.

Thanks

Michal