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

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Date:   Mon, 13 Jul 2020 14:32:59 -0700
From:   Rob Clark <robdclark@...il.com>
To:     Rob Herring <robh+dt@...nel.org>
Cc:     Doug Anderson <dianders@...omium.org>, Sean Paul <sean@...rly.run>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        freedreno <freedreno@...ts.freedesktop.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/9] drm/msm: Avoid possible infinite probe deferral and
 speed booting

On Mon, Jul 13, 2020 at 1:25 PM Rob Herring <robh+dt@...nel.org> wrote:
>
> On Mon, Jul 13, 2020 at 9:08 AM Doug Anderson <dianders@...omium.org> wrote:
> >
> > Hi,
> >
> > On Mon, Jul 13, 2020 at 7:11 AM Rob Herring <robh+dt@...nel.org> wrote:
> > >
> > > On Fri, Jul 10, 2020 at 5:02 PM Douglas Anderson <dianders@...omium.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?
>

yes, that is correct

BR,
-R