Re: [PATCH 0/9] drm/msm: Avoid possible infinite probe deferral and speed booting

[Rob Herring <robh+dt@kernel.org>]

On Mon, Jul 13, 2020 at 9:08 AM Doug Anderson <dianders@chromium.org> wrote:
>
> Hi,
>
> On Mon, Jul 13, 2020 at 7:11 AM Rob Herring <robh+dt@kernel.org> wrote:
> >
> > On Fri, Jul 10, 2020 at 5:02 PM Douglas Anderson <dianders@chromium.org> wrote:
> > >
> > > I found that if I ever had a little mistake in my kernel config,
> > > or device tree, or graphics driver that my system would sit in a loop
> > > at bootup trying again and again and again.  An example log was:
> >
> > Why do we care about optimizing the error case?
>
> It actually results in a _fully_ infinite loop.  That is: if anything
> small causes a component of DRM to fail to probe then the whole system
> doesn't boot because it just loops trying to probe over and over
> again.  The messages I put in the commit message are printed over and
> over and over again.

Sounds like a bug as that's not what should happen.

If you defer during boot (initcalls), then you'll be on the deferred
list until late_initcall and everything is retried. After
late_initcall, only devices getting added should trigger probing. But
maybe the adding and then removing a device is causing a re-trigger.

> > >   msm ae00000.mdss: bound ae01000.mdp (ops 0xffffffe596e951f8)
> > >   msm_dsi ae94000.dsi: ae94000.dsi supply gdsc not found, using dummy regulator
> > >   msm_dsi_manager_register: failed to register mipi dsi host for DSI 0
> > >   [drm:ti_sn_bridge_probe] *ERROR* could not find any panel node
> > >   ...
> > >
> > > I finally tracked it down where this was happening:
> > >   - msm_pdev_probe() is called.
> > >   - msm_pdev_probe() registers drivers.  Registering drivers kicks
> > >     off processing of probe deferrals.
> > >   - component_master_add_with_match() could return -EPROBE_DEFER.
> > >     making msm_pdev_probe() return -EPROBE_DEFER.
> > >   - When msm_pdev_probe() returned the processing of probe deferrals
> > >     happens.
> > >   - Loop back to the start.
> > >
> > > It looks like we can fix this by marking "mdss" as a "simple-bus".
> > > I have no idea if people consider this the right thing to do or a
> > > hack.  Hopefully it's the right thing to do.  :-)
> >
> > It's a simple test. Do the child devices have any dependency on the
> > parent to probe and/or function? If so, not a simple-bus.
>
> Great!  You can see in the earlier patch in the series that the very
> first thing that happens when the parent device probes is that it
> calls devm_of_platform_populate().  That means no dependencies, right?

It should. But then I reviewed the MDSS binding today and it looks
like the MDSS is the interrupt parent for at least some child devices?

>  So that means it's fine/correct to add "simple-bus" here?
>
>
> > > Once I do this I notice that my boot gets marginally faster (you
> > > don't need to probe the sub devices over and over) and also if I
> >
> > Can you quantify that?
>
> I'd say < 100 us.  I can try to quantify more if needed, but it wasn't
> the point of this patch.
>
>
> > Have you run with devlinks enabled. You need a command line option to
> > enable. That too should reduce deferred probes.
>
> Ah, good idea!  I will try it.  However, even with devlinks, if there
> is any chance of deferred probes then we need a fix like this.  The
> point of the patch isn't about speeding things up but about avoiding
> an infinite loop at bootup due to a small bug.

I think a deferred probe would only happen if there's a dependency we
don't track (but we're tracking about everything that's common). But
if there's some error, I'm not sure what would happen. Seems like a
good test case. :)

> > > have a problem it doesn't loop forever (on my system it still
> > > gets upset about some stuck clocks in that case, but at least I
> > > can boot up).
> >
> > Deferred probe only runs when a device is added, so it's not like it
> > is continually running.
>
> If you don't mind looking at the code patch, see:
>
> https://lore.kernel.org/r/20200710160131.4.I358ea82de218ea5f4406572ade23f5e121297555@changeid/
>
> Specifically you can see that each time we try to probe we were
> calling of_platform_populate().  That appeared to be enough to trigger
> things.

Like I said, sounds like a bug. Even if 'simple-bus' is the
appropriate thing to do here, it should be fixed or at least
understood.

Rob