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

[Doug Anderson]

Hi,

On Mon, Jul 13, 2020 at 7:11 AM Rob Herring <robh+dt at kernel.org> wrote:
>
> On Fri, Jul 10, 2020 at 5:02 PM Douglas Anderson <dianders at 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.


> >   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?
 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.


> > 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.


-Doug