[PATCH v6 1/8] drm/msm/dp: Add eDP support via aux_bus

[Dmitry Baryshkov]

On Fri, 8 Apr 2022 at 16:56, Doug Anderson <dianders at chromium.org> wrote:
>
> Hi,
>
> On Fri, Apr 8, 2022 at 5:13 AM Dmitry Baryshkov
> <dmitry.baryshkov at linaro.org> wrote:
> >
> > On Fri, 8 Apr 2022 at 03:28, Doug Anderson <dianders at chromium.org> wrote:
> > >
> > > Hi,
> > >
> > > On Thu, Apr 7, 2022 at 4:36 PM Dmitry Baryshkov
> > > <dmitry.baryshkov at linaro.org> wrote:
> > > >
> > > > The ps8640 driver looks 'working by coincidence'. It calls
> > > > dp_aux_populate, then immediately after the function returns it checks
> > > > for the panel. If panel-edp is built as a module, the probe might fail
> > > > easily.
> > > > The anx7625 driver has the same kind of issue. The DP AUX bus is
> > > > populated from the probe() and after some additional work the panel is
> > > > being checked.
> > > > This design is fragile and from my quick glance it can break (or be
> > > > broken) too easy. It reminds me of our drm msm 'probe' loops
> > > > preventing the device to boot completely if the dsi bridge/panel could
> > > > not be probed in time.
> > >
> > > I did spend some time thinking about this, at least for ps8640. I
> > > believe that as long as the panel's probe isn't asynchronous.
> >
> > By panel probe (as a probe of any device) is potentially asynchronous.
> > As in your example, the PWM might not be present, the regulator probe
> > might have been delayed, the panel-edp might be built as a module,
> > which is not present for some reason.
>
> Well, in those cases it's not exactly asynchronous, or at least not in
> the way I was thinking about. Either it will work now, or we should
> try again later when more drivers have probed. The case I was worried
> about was:
>
> 1. We call devm_of_dp_aux_populate_ep_devices()
>
> 2. devm_of_dp_aux_populate_ep_devices() kicks off a probe to the panel
> in the background
>
> 3. devm_of_dp_aux_populate_ep_devices() returns to us without waiting
> for the panel's probe to finish.
>
> I think that's possible if the panel driver sets PROBE_PREFER_ASYNCHRONOUS.

That would be a separate story, yes. However the general case is still
valid: one can not guarantee that the panel device is available
immediately after aux bus population.
So ps8640 works at this moment and in the particular configuration.

>
> I was less worried about the resources missing since I thought that
> would be handled by the normal probe deferral case. IIRC the "infinite
> loop" that we used to end up with MSM's probe was because something
> about the way the MSM DRM driver worked would cause the deferral
> mechanisms to retry instantly each time we deferred. I don't remember
> exactly what triggered it, but I don't _think_ that's true for ps8640?

I'm not sure either. If you have a system with that bridge, it can be
easily tried by returning -EPROBE_DEFER from the panel driver's
probe().

For the msm driver it was the following sequence of events:
- mdss probes
- mdss creates subdevices including dsi host
- subdevices are probed
- mdss drivers tries to perform component binding
- dsi host determines that the next item is not available
- it returns -EPROBE_DEFER to the component bind
- mdss reverts registration of subdevices, returning probe defer.

However as there were devices added to the device list, the deferral
list was retried immediately. Thus we faced the probe loop.

I think this looks close to the ps8640, but I wouldn't bet on that.

> > > Basically if the panel isn't ready then ps8640 should return and we'll
> > > retry later. I do remember the probe loops that we used to have with
> > > msm and I don't _think_ this would trigger it.
> >
> > I don't have proof here. But as I wrote, this thing might break at some point.
> >
> > > That being said, if we need to separate out the AUX bus into a
> > > sub-device like we did in sn65dsi86 we certainly could.
> >
> > Ideally we should separate the "bridge" subdevice, like it was done in
> > sn65dsi86.
> > So that the aux host probes, registers the EP device, then the bridge
> > device can not be probed (and thus the drm_bridge is not created)
> > until the next  bridge becomes available.
>
> You're definitely right, that's best. I was hesitant to force the
> issue during review of the ps8640 because it adds a bunch of
> complexity and didn't _seem_ to be needed. Maybe it makes sense to
> just code it up, though...

As I wrote, the test is easy. If the system boots fine, there is no
need to fix that immediately.
However I think in general we should stop depending on the panel being
available right after populating the aux bus.

--
With best wishes
Dmitry