[Dev] Chromebook C201 wifi

Lee Strobel leestrobel at disroot.org
Fri Apr 5 00:27:28 GMT 2019

Hey, so I received that USB tester in the mail today. Got some
interesting results:

First, I tried the older Ralink wifi dongle (the one that works). When
connected, it seems to operate at around 110-120mA. Very rarely it went
up to around 160. So, that is quite a bit lower than the 450mA max
power that was reported by 'lsusb'.

I then unplugged that and plugged in my phone charger. That was able to
draw around 500mA (which is the spec for a USB 2.0 port).

I then tried the ThinkPenguin Atheros dongle. It was at about 70mA when
connected, but experienced the same freezing issue as when plugged
directly in to the Chromebook port. I tried it in my desktop and it
works fine on that, both direct and going through the USB tester. No
sign of the freezing issue. On the desktop, I think the max current I
saw was around 90mA.

So, clearly the issue is not a power limitation, as the Chromebook port
can deliver the full 500mA. My best guess is that it is some sort of
compatibilty bug between the dongle and the particular USB hub that the
Chromebook uses.



On Tue, 2019-04-02 at 15:08 +0200, Denis 'GNUtoo' Carikli wrote:
> On Mon, 1 Apr 2019 16:51:47 +0100
> Josh Branning <lovell.joshyyy at gmail.com> wrote:
> > Hi just had a look and there doesn't appear to be an easy option,
> > though there are a few things that could help in u-boot by the
> > looks
> > of things, I'm not too familiar with u-boot, and it would require
> > probably require someone who would know what they were doing to
> > really help.
> > 
> > But I guess probably you'd need to compile u-boot, changing some
> > stuff, like grepping for "uA", "uV", "max-current" , "max-current"
> > etc. have a look at arch/arm/dts and see what's going in there etc.
> > Probably there is a main voltage regulator and a usb voltage
> > regulator, someone who knows about electronics could tell you if it
> > could be a problem with either, or just one or the other.
> The best way to do that is to try to understand which chips are
> involved in the power management in that chromebook and try to find
> documentation on it.
> - Start by looking at the devicetree of the device, as it describes
> the
>   hardware that cannot be automatically detected.
> - Then try to find some documentation on the chips involved in the
>   voltage. Documentation can be found:
>   - in the datasheet/reference manual of the respective chips if they
>     are available online.
>   - In Linux/u-boot/coreboot/depthcharge/chrome-ec Documentation or
>     source code
> Also when grepping you could look for the following:
> - vbus: it's the USB 5v
> - buck boost converter: it's a kind of voltage regulator that raises
>   the voltage for instance from 3.3v to 5V.
> As I understand the C201 uses the following free software projects:
> - The EC source code is free software, so some information
>   might also be in chrome-ec[1].
> - It also uses Coreboot, but I don't know which payload u-boot chose
> to
>   use. It's probably depthcharge according to the kernel install
>   script, so it might also be interesting to look into that.
> References:
> -----------
> [1]https://review.coreboot.org/chrome-ec.git
> Denis.
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