[PATCH] component: Move host device to end of device lists on binding

[Rafael J. Wysocki]

On Sat, May 8, 2021 at 9:41 AM Stephen Boyd <swboyd at chromium.org> wrote:
>
> The device lists are poorly ordered when the component device code is
> used. This is because component_master_add_with_match() returns 0
> regardless of component devices calling component_add() first. It can
> really only fail if an allocation fails, in which case everything is
> going bad and we're out of memory. The host device (called master_dev in
> the code), can succeed at probe and be put on the device lists before
> any of the component devices are probed and put on the lists.
>
> 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.
>
> Normally, device links or probe defer would reorder the lists and put
> devices that depend on other devices in the lists at the correct
> location, but with component devices this doesn't happen because this
> information isn't expressed anywhere. Drivers simply succeed at
> registering their component or host with the component framework and
> wait for their bind() callback to be called once the other components
> are ready. We could make various device links between 'master_dev' and
> 'component->dev' but it's not necessary. Let's simply move the hosting
> device to the end of the device lists when the component device fully
> binds. This way we know that all components are present and have probed
> properly and now the host device has really probed so it's safe to
> assume the host driver ops can operate on any component device.

Moving a device to the end of dpm_list is generally risky in cases
when some dependency information may be missing.

For example, if there is a device depending on the hosting one, but
that dependency is not represented by a device link or a direct
ancestor-descendant relationship (or generally a path in the device
dependency graph leading from one of them to the other), then moving
it to the end of dpm_list would cause system-wide suspend to fail (the
hosting device would be suspended before the one depending on it).

That may not be a concern here, but at least it would be good to
document why it is not a concern.