On 9 February 2015 at 04:54, Siarhei Siamashka siarhei.siamashka@gmail.com wrote:
On Sat, 31 Jan 2015 04:54:47 +0200 Siarhei Siamashka siarhei.siamashka@gmail.com wrote:
On Sun, 11 Jan 2015 20:34:55 +0100 Hans de Goede hdegoede@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