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

[Daniel Vetter]

On Tue, May 11, 2021 at 10:19:09AM -0700, Stephen Boyd wrote:
> Quoting Daniel Vetter (2021-05-11 06:39:36)
> > On Tue, May 11, 2021 at 12:52 PM Rafael J. Wysocki <rafael at kernel.org> wrote:
> > >
> > > On Mon, May 10, 2021 at 9:08 PM Stephen Boyd <swboyd at chromium.org> wrote:
> > >
> > > [cut]
> > >
> > > >
> > > > >
> > > > > > I will try it, but then I wonder about things like system wide
> > > > > > suspend/resume too. The drm encoder chain would need to reimplement the
> > > > > > logic for system wide suspend/resume so that any PM ops attached to the
> > > > > > msm device run in the correct order. Right now the bridge PM ops will
> > > > > > run, the i2c bus PM ops will run, and then the msm PM ops will run.
> > > > > > After this change, the msm PM ops will run, the bridge PM ops will run,
> > > > > > and then the i2c bus PM ops will run. It feels like that could be a
> > > > > > problem if we're suspending the DSI encoder while the bridge is still
> > > > > > active.
> > > > >
> > > > > Yup suspend/resume has the exact same problem as shutdown.
> > > >
> > > > I think suspend/resume has the exact opposite problem. At least I think
> > > > the correct order is to suspend the bridge, then the encoder, i.e. DSI,
> > > > like is happening today. It looks like drm_atomic_helper_shutdown()
> > > > operates from the top down when we want bottom up? I admit I have no
> > > > idea what is supposed to happen here.
> > >
> > > Why would the system-wide suspend ordering be different from the
> > > shutdown ordering?
> >
> > At least my point was that both shutdown and suspend/resume have the
> > same problem, and the righ fix is (I think at least) to add these
> > hooks to the component.c aggregate ops structure. Hence just adding
> > new callbacks for shutdown will be an incomplete solution.
> 
> To add proper hooks to component.c we'll need to make the aggregate
> device into a 'struct device' and make a bus for them that essentially
> adds the aggregate device to the bus once all the components are
> registered. The bind/unbind can be ported to probe/remove, and then the
> aggregate driver can get PM ops that run before the component devices
> run their PM ops.
> 
> Let me go try it out and see if I can make it minimally invasive so that
> the migration path is simple.

Thanks for volunteeering. Please cc Greg KH so we make sure we're not
doing this wrongly wrt the device model.
-Daniel

> > I don't feel like changing the global device order is the right
> > approach, since essentially that's what component was meant to fix.
> > Except it's incomplete since it only provides a solution for
> > bind/unbind and not for shutdown or suspend/resume as other global
> > state changes. I think some drivers "fixed" this by putting stuff like
> > drm_atomic_helper_shutdown/suspend/resume into early/late hooks, to
> > make sure that everything is ready with that trick. But that doesn't
> > compose very well :-/
> 
> Yeah it looks like msm is using prepare/complete for this so that it can
> jump in early and suspend the display pipeline before the components
> suspend themselves. The shutdown path only has one callback so we can't
> play the same games.

Yeah there's tons of hacks. i915 component usage with audio has similar
tricks to make suspend/resume work.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch