Re: Nokia N900: musb is in wrong state after boot

From: joerg Reisenweber
Date: Thu Jan 21 2016 - 13:44:49 EST

On Thu 21 January 2016 09:41:46 Tony Lindgren wrote:
> Then for supporting the USB host mode.. We should add regulator support
> to the USB PHY driver so if the ID pin is grounded, the PHY driver enables
> the VBUS regulator. That too seems to need some coordination between the
> drivers/phy/phy-twl4030-usb.c and 1707 driver if the ID pin interrupt is
> only detected in drivers/phy/phy-twl4030-usb.c.

Note that, while this is probably a good thing to do, it needs to be
sufficiently loose coupling to allow user to 'intercept' this VBOOS regulator
enabling and instead allow device charging while in externally powered
hostmode. There's even a spec for this in USB-docs-foo iirc, something along a
certain resistor value on ID to GND - alas I guess the twl4030 is not capable
to detect such sophisticated signaling, and anyway it's always desirable to
allow user to manually override the VBOOST and enable VBUS-charging while in

On N900 the situation is even more complex since the 1707 doesn't support
genuine ID detection, neither does it support emulated ID grounding. And
there's no other method than a ID=GND message from PHY to musb core to make
the musb core state engine transfer into proper hostmode. Thus my H-E-N
hostmode botch abuses debug flags to force the musb core into a "emulated"
hostmode and this mode doesn't support USB speed detection. Thus speed
settings are forced onto musb core and PHY by software, and the musb core
speed bits are only effective before session enabled.
Bottom line: you need VBUS to try and negotiate speed with the attached device
in hostmode, but to actually set this speed you detected by software means,
you need to disable and discharge VBUS again, or musb core won't care about
the speed you set. To be utterly clear: unconditional enabling of VBUS in
ID=GND won't work.

This is quite complex and it's questionable if it could get handled reasonably
in kernel space. *Very* N900 specific niche solution, I'd not think it's suited
for upstreaming.

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