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Re: xhci power to external device (Michael van Elst) writes:

> USB2 ports are limited to 100mA and devices may communicate that they
> want more (up to 500mA) and then are signaled to draw more power.
> USB3 is similar but allows other limits, 150mA default and 900mA max.
> There are also USB3 options that allow even more power, e.g. for
> USB chargers.

Thanks for straigtening me out!
> That information is mostly used to calculate the power budget, If
> it would be exceeded, the device isn't allowed to draw more power.

How common is it for USB controllers to actually do this calculation and
selectively enable, vs being built to supply 500/900 per port?
Looking at NetBSD drivers quickly, I don't see this.

> Many USB controllers cannot limit the power output, so even when
> a device is denied the extra power (or didn't even dare to communicate
> and is only allowed the minimum) it could just draw higher currents.

That seems cheaper to build.

>>  I have never heard of a port that can throttle what it supplies.
> Throttling would cause the voltage to drop, which of course happens
> when a device tries to draw more current than the port can provide.
> A USB controller that limits power output on a port does this to
> protect the port.

Certainly current limiting on power supplies is normal.   How common is
it for a USB controller to not only have current limiting for the
500/900 level (to protect the port hardware as you say) to also have
configured lower current limiting so that devices that should be under
100/150 will be prevented from going much over that?

Overall, it sounds to me like Rhialto's problem is more likely some kind
of bug, dodgy wiring, marginal power supply, etc.

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