Re: Question about regulators

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

Hi Hans,

Hans de Goede <hdegoede@redhat.com> wrote on Tue, 9 Apr 2019 09:38:01
+0200:

> Hi,
> 
> On 09-04-19 09:26, Miquel Raynal wrote:
> > 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?  
> 
> To be clear you are talking about a regulator producing Vbus for device
> plugged into the system, right ?
> 
> In that case just configuring th current-limit at 900mA is fine, it really
> is up to the device to not consume more then it may and even if it does since
> the port is capable of delivering 900mA anyways that is not a problem.
> 
> There is code in various places in the kernel for Linux devices to not
> consume too much energy (mostly in charger ic drivers) when the Linux
> device itself is an USB device drawing power.
> 
> Regards,
> 
> Hans
> 

Thank you very much for answering my questions. I'll just stick to a
900mA limitation in all cases then.

Thanks,
Miquèl