Re: xhci power to external device

[Greg Troxel <gdt%lexort.com@localhost>]

Rhialto <rhialto%falu.nl@localhost> writes:

> I have an external harddisk, like so: (output from usbdevs -v)
>
> Controller /dev/usb0:
> addr 0: super speed, self powered, config 1, xHCI Root Hub(0x0000), vendor 8086(0x8086), rev 1.00(0x0100)
>  port 1 addr 9: super speed, power 224 mA, config 1, Elements 25A1(0x25a1), Western Digital(0x1058), rev 10.14(0x1014), serial xxxxxxxxxxxxxxxxxxxxxxxx
>
> I have some reason to believe it does nog get enough power from the
> port. Is the "power 224 mA" how the current is actually limited? Or can
> the device draw more without telling us?

My impression is:

  USB ports and devices are limited to 500 mA, per the spec.  (But,
  there are various schemes for USB chargers to communciate that they
  support more, so devices can be willing to draw more)

  what you are seeing is the device's declaration of how much power it
  will draw

  there is, very maybe, some notion that a computer will only enable a
  port if some power conditions are met, such as the total declared
  power of all enabled devices remains below some level believed to be
  what can in aggregate is supplied.  But I'm not sure this exists.

  I have never heard of a port that can throttle what it supplies.

  I have definitely seen problems from trying to pull too much power in
  various ways.

  I don't have a good reason to believe all ports in any one computer
  will have exactly the same behavior.

  if you have an external disk that is USB powered, and it's flaky, you
  should get a powered hub