linux-kernel - Re: [PATCH] component: Move host device to end of device lists on binding

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Date:   Tue, 11 May 2021 10:22:21 -0700
From:   Stephen Boyd <swboyd@...omium.org>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Rob Clark <robdclark@...il.com>,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH] component: Move host device to end of device lists on binding

Quoting Russell King - ARM Linux admin (2021-05-11 07:42:37)
> On Sat, May 08, 2021 at 12:41:18AM -0700, Stephen Boyd wrote:
> > Within the component device framework this usually isn't that bad
> > because the real driver work is done at bind time via
> > component{,master}_ops::bind(). It becomes a problem when the driver
> > core, or host driver, wants to operate on the component device outside
> > of the bind/unbind functions, e.g. via 'remove' or 'shutdown'. The
> > driver core doesn't understand the relationship between the host device
> > and the component devices and could possibly try to operate on component
> > devices when they're already removed from the system or shut down.
>
> You really are not supposed to be doing anything with component devices
> once they have been unbound. You can do stuff with them only between the
> bind() and the unbind() callbacks for the host device.

Got it. The device is not unbound in this case so this isn't the
problem.

>
> Access to the host devices outside of that is totally undefined and
> should not be done.
>
> The shutdown callback should be fine as long as the other devices are
> still bound, but there will be implications if the shutdown order
> matters.
>
> However, randomly pulling devices around in the DPM list sounds to me
> like a very bad idea. What happens if such re-orderings result in a
> child device being shutdown after a parent device has been shut down?
>

Fair enough. I'll cook up a 'component' bus and see if that can fix this
properly. It will add a new device for the aggregate driver that does
the bind/unbind so the host/parent device will still be ordered on the
DPM list at the same place. The new aggregate device will be after the
components and we'll attach the PM ops and shutdown hooks to that.