[PATCH 2/3] Documentation: dt-bindings: Add snps, need-phy-for-wake for dwc2 USB

[Doug Anderson]

Rob,

On Tue, Jun 23, 2015 at 7:17 AM, Rob Herring <robherring2 at gmail.com> wrote:
> On Mon, Jun 22, 2015 at 6:52 PM, Douglas Anderson <dianders at chromium.org> wrote:
>> Some SoCs with a dwc2 USB controller may need to keep the PHY on to
>> support remote wakeup.  Allow specifying this as a device tree
>> property.
>
> I find it hard to believe that any host can support wake-up without
> the PHY.

I am told by Andrew Bresticker that on tegra there is complicated
logic in the PMU that allows USB wakeup while powering off the PHY.
If I hadn't been aware of such a feature I probably would have called
the property "wakeup-supported" or something like that.

> Does this really need to be conditional? Perhaps other cases
> are just always-on or remote wake-up has not been tested.

When I worked on exynos products I was told by Samsung that USB wakeup
was simply not possible.  True that they have a different USB
controller, but I can certainly believe that someone could design a
system with dwc2 where USB wakeup was not possible.

Specifically, in order to get USB wakeup we had to switch on the 24MHz
clock at suspend time.  Had that not been possible (or had the 24MHz
clock not been able to be a clock source for the USB controller) then
USB wakeup would not be possible on rk3288 even if we left the phy on.

So to me the three states are:
1. USB wake not possible
2. USB wake possible, don't need PHY
3. USB wake possible, do need PHY

I don't know of any dwc2 users that are in #2 (so I didn't add a
property now), but I figure that when they exist someone should add a
property then.


-Doug