Re: [PATCH 1/5] dt/bindings: Add binding for the DA8xx MUSB driver

Hello Rob,

On 16.02.2016 21:05, Rob Herring wrote:

On Fri, Feb 12, 2016 at 11:24 AM, Petr Kulhavy [off-list ref] wrote:

quoted

I understand your point that the description should be generic.
However the USB 2.0 specification does not define any relation between the
bMaxPower provided by the device and the actual power control.
As far as I understand the value just serves the purpose to raise a flag to
the user if the attached device would draw too much power, and not to enable
it at all.

That won't really work given devices lie. My bus powered USB disk
enclosure reports something like 10mA. That's pretty good considering
I have a 5W drive in it.

Yes, that's exactly what's happening. The USB 2.0 is in that matter 
naive and assumes that the devices don't lie :-)
But at the same time gives them the possibility to lie by not binding 
this value to the physical current.

quoted

For instance on my development board the real maximum current is limited
only by the mains power-supply used.
So it cannot even be modelled in the DT as a regulator because the
power-supply is not part of the HW and
everybody can take a different one.

Not part of which h/w? Different for everyone is exactly why Vbus
supply should be described in DT.

I mean the regulator is in the AC power supply, which is standalone and 
can be changed. Therefore not described in the DT.
The DT describes only the board.

quoted

So defining a regulator just to store this arbitrary USB 2.0 value is a bit
overkill for me.

If it is just arbitrary, then put it into the driver. I would do 500mA
and be done with it. I'd guess there is nothing real behind the
current default of 250mA.

Still there is some sense behind. The power_budget is used to possibly 
print a warning message and/or
disable a self powered device that announces too much power. So I would 
leave it configurable.

What would be your proposal then? Something like this?

     usb_reg: fixedregulator@0 {
         compatible = "regulator-fixed";
         regulator-name = "USB Vbus";
         regulator-always-on;
         regulator-max-microamp = <500000>;    /* 500mA */
     };

or a real regulator if there is one. And then reference it as:

     usb20: ... {
         ...
         vbus-supply = <&usb_reg>;
     }

As I said for me it's an overkill, knowing that this monster needs to be 
set-up and reside in memory just to write the power_budget variable for 
devices which anyway may lie.
But if you prefer that and Sergei and the guys from USB agree to it I 
don't really mind.--

Regards
Petr
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