On Wed, Sep 30, 2020 at 1:00 PM Doug Anderson <dianders@xxxxxxxxxxxx> wrote: > > Hi, > > > On Wed, Sep 30, 2020 at 7:44 AM Rob Herring <robh@xxxxxxxxxx> wrote: > > > > > > We already have hubs in DT. See [1][2][3][4]. What's new here? > > After I sent my response I kept thinking about this and I realized > that I have prior art I can point out too! :-) Check out > "smsc,usb3503a". That is describing a USB hub too and, at least on > "exynos5250-spring.dts" is is a top level node. Since "smsc,usb3503a" > can be optionally connected to an i2c bus too, it could be listed > under an i2c controller as well (I believe it wasn't hooked up to i2c > on spring). > > Interestingly enough, the USB Hub that Matthias is trying to add > support for can _also_ be hooked up to i2c. We don't actually have > i2c hooked up on our board, but conceivably it could be. Presumably, > if i2c was hooked up, we would have no other choice but to represent > this chip as several device tree nodes: at least one under the i2c > controller and one (or two) under the USB controller. Just because > (on this board) i2c isn't hooked up doesn't change the fact that there > is some extra control logic that could be represented in its own > device tree node. To me, this seems to give extra evidence that the > correct way to model this device in device tree is with several nodes. > > I'll point out that on "exynos5250-spring.dts" we didn't have to solve > the problem that Matthias is trying to solve here because we never > actually supported waking up from USB devices there. Thus the > regulator for the hub on spring can be unconditionally powered off in > suspend. On newer boards we'd like to support waking up from USB > devices but also to save power if no wakeup devices are plugged into > USB. In order to achieve this we need some type of link from the > top-level hub device to the actual USB devices that were enumerated. Yes, in a prior version I mentioned we already had 2 ways to describe hubs. I view this as a 3rd way. There's prior art in how we reference an i2c bus for a slave device that's already on another bus. That's 'i2c-bus' and 'ddc-i2c-bus'. But that's not really this case. Rob
