linux-kernel - Re: [PATCH v4 1/2] dt-bindings: usb: Add binding for discrete onboard USB hubs

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Date:   Wed, 7 Oct 2020 10:28:47 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Doug Anderson <dianders@...omium.org>,
        Rob Herring <robh@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Frank Rowand <frowand.list@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux USB List <linux-usb@...r.kernel.org>,
        Bastien Nocera <hadess@...ess.net>,
        Stephen Boyd <swboyd@...omium.org>,
        Ravi Chandra Sadineni <ravisadineni@...omium.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, Peter Chen <peter.chen@....com>
Subject: Re: [PATCH v4 1/2] dt-bindings: usb: Add binding for discrete
 onboard USB hubs

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:
> > On Tue, Oct 06, 2020 at 09:00:23PM -0400, Alan Stern wrote:
> > > On Tue, Oct 06, 2020 at 12:25:36PM -0700, Matthias Kaehlcke wrote:
> > > > On Tue, Oct 06, 2020 at 01:15:24PM -0400, Alan Stern wrote:
> > > > > You don't need a platform device or a new driver to do this.  The code 
> > > > > can go in the existing hub driver.
> > > > 
> > > > Maybe. IIUC currently USB drivers don't support/use suspend_late. Could that
> > > > be added? It would simplify matters, 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. It would be simpler if a single instance (e.g. the
> > > > hub with the DT entries) is in control.
> > > 
> > > Adding suspend_late would be a little painful.  But you don't really 
> > > need it; you just need to make the "master" hub wait for its peer to 
> > > suspend, which is easy to do.
> > 
> > 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(), or would the primary wait
forever if the peer hub isn't suspended yet?

> > > And hubs would need to know their peers in any case, because you have to
> > > check if any devices attached to the peer have wakeup enabled.
> > 
> > My concern was about all hubs (including 'secondaries') having to know their
> > peers and check on each other, in the scenario we are now talking about only
> > the "master" hub needs to know and check on its peers, which is fine.
> 
> Not all hubs would need this.  Only ones marked in DT as having a power 
> regulator.

Sure, as long as the primary (with a power regulator) can wait for its peers
to suspend without the risk of blocking forever (my doubt above).