Re: Question about regulators

[Miquel Raynal <miquel.raynal@bootlin.com>]

Hello,

Adding Greg & Hans just in case they have an idea, or someone else to
ping in mind.

Miquel Raynal <miquel.raynal@bootlin.com> wrote on Wed, 3 Apr 2019
10:13:43 +0200:

> Hello,
> 
> I am reading the document named "xHCI interoperability testing v0.94"
> available on usb.org. There is this paragraph:
> 
>         Devices operating at Enhanced SuperSpeed GenX are allowed to
>         draw a maximum configured current of 900mA and unconfigured
>         current of 150mA. Devices operating at High-Speed or below may
>         draw a maximum configured current of 500mA and unconfigured
>         current of 100mA*. Devices must report their maximum configured
>         current draw and their power configuration as self or bus
>         powered to the host and must operate within the regions
>         reported. Additionally any device that is in the suspended
>         state may draw no more than 2.5 mA.
> 
> My understanding is that the current limitation on Vbus should be tuned
> dynamically between 100, 150, 500 and 900mA depending on the type of
> device and if has been configured or not.
> 
> Despite some researches in the USB core I can't find any mechanism
> handling this in Linux implementation.
> 
> Behind this remark, I actually have boards (Armada 7040/8040 DB) with a
> regulator on Vbus to limit the current to either 500 or 900mA. I
> checked the xhci-platform driver which is used with these SoCs and I
> don't find any mean to tweak this value depending on the inserted
> device.
> 
> Should I actually care? Is it enough to limit to 900mA whenever the
> xHCI driver is probed?
> 
> Thanks for your guidance,
> Miquèl


Thanks,
Miquèl