On Wed, Oct 07, 2020 at 12:42:29PM -0700, Matthias Kaehlcke wrote:
> On Wed, Oct 07, 2020 at 03:25:42PM -0400, Alan Stern wrote:
> > On Wed, Oct 07, 2020 at 10:28:47AM -0700, Matthias Kaehlcke wrote:
> > > On Wed, Oct 07, 2020 at 12:38:38PM -0400, Alan Stern wrote:
> > > > On Wed, Oct 07, 2020 at 09:03:36AM -0700, Matthias Kaehlcke wrote:
> > > > > Ok, I wasn't sure if the hubs suspend asynchronously from each other. If they
> > > > > do it should indeed not be a problem to have the "master" wait for its peers.
> > > >
> > > > Well, order of suspending is selectable by the user. It can be either
> > > > asynchronous or reverse order of device registration, which might pose a
> > > > problem. We don't know in advance which of two peer hubs will be
> > > > registered first. It might be necessary to introduce some additional
> > > > explicit synchronization.
> > >
> > > I'm not sure we are understanding each other completely. I agree that
> > > synchronization is needed to have the primary hub wait for its peers, that
> > > was one of my initial concerns.
> > >
> > > Lets use an example to clarify my secondary concern: a hub chip provides a
> > > USB 3 and a USB 2 hub, lets say the USB 3 hub is the primary.
> > >
> > > Here is some pseudo-code for the suspend function:
> > >
> > > hub_suspend(hub)
> > > ...
> > >
> > > if (hub->primary) {
> > > device_pm_wait_for_dev(hub->peer)
> > >
> > > // check for connected devices and turn regulator off
> > > }
> > >
> > > ...
> > > }
> > >
> > > What I meant with 'asynchronous suspend' in this context:
> > >
> > > Can hub_suspend() of the peer hub be executed (asynchronously) while the
> > > primary is blocked on device_pm_wait_for_dev(),
> >
> > Yes, that's exactly what would happen with async suspend.
> >
> > > or would the primary wait
> > > forever if the peer hub isn't suspended yet?
> >
> > That wouldn't happen. device_pm_wait_for_dev is smart; it will return
> > immediately if neither device uses async suspend. But in that case you
> > could end up removing power from the peer hub before it had suspended.
> >
> > That's why I said you might need to add additional synchronization. The
> > suspend routines for the two hubs could each check to see whether the
> > other device had suspended yet, and the last one would handle the power
> > regulator. The additional synchronization is for the case where the two
> > checks end up being concurrent.
>
> That was exactly my initial concern and one of the reasons I favor(ed) a
> platform instead of a USB driver:
Clearly there's a tradeoff.
> > otherwise all hubs need to know their peers and check in suspend if they
> > are the last hub standing, only then the power can be switched off.
>
> To which you replied:
>
> > you just need to make the "master" hub wait for its peer to suspend, which
> > is easy to do.
>
> However that apparently only works if async suspend is enabled, and we
> can't rely on that.
Yes, I had forgotten about the possibility of synchronous suspend. My
mistake.
> With the peers checking on each other you lose effectively the notion
> of a primary.
Well, you can still want to put the sysfs power-control attribute file
into just one of the hubs' directories, and that one would be considered
the primary. But I agree, it's a weak notion.
> Going back to the binding:
>
> &usb_1_dwc3 {
> hub_2_0: hub@1 {
> compatible = "usbbda,5411";
> reg = <1>;
> };
>
> hub_3_0: hub@2 {
> compatible = "usbbda,411";
> reg = <2>;
> vdd-supply = <&pp3300_hub>;
> companion-hubs = <&hub_2_0>;
> };
> };
>
> How does 'hub_2_0' know that its peer is hub_3_0 and that it has a regulator
> (and potentially other resources)?
The peering relation goes both ways, so it should be included in the
hub_2_0 description too. Given that, the driver could check hub_2_0's
peer's DT description for the appropriate resources.
> All this mess can be avoided by having a single instance in control of the
> resources which is guaranteed to suspend after the USB devices.
Yes. At the cost of registering, adding a driver for, and making users
aware of a fictitious platform device.
Alan Stern
