[Bug 97666] Kernel "NULL pointer dereference" with MST monitor on HSW

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=97666

--- Comment #34 from Carlos Santa <carlos.santa at intel.com> ---
(In reply to Ville Syrjala from comment #33)
> (In reply to Carlos Santa from comment #32)
> > Hi Ville,
> > 
> > I tested the following 3 patches on today's drm-nightly:
> > 
> > 1. drm/i915: Refresh that status of MST capable connectors in ->detect()
> > 2. drm/fb-helper: Fix connector ref leak on error
> > 3. drm/fb-helper: Keep references for the current set of used connectors
> > 
> > You're right, after applying #1, the null pointer issue comes back but after
> > applying #2, and doing the display power on/off to repro the bug, the system
> > freezes and never comes back again. Same behavior if then applying patch #3
> > on top, which is supposed to be the fix.
> 
> Freezes hard enough to not leave a trace in netconsole/serial etc. ? Logs
> would be nice.

Ok, I got the log using dmesg -w. It's actually oopsing the same way as before,
see the attachment 10-21-2016.

More on the traces,

the "adapter" parameter being passed to i2c_transfer() from
drm_do_probe_ddc_edid()  although is not NULL it appears to arrive with invalid
data, the pointer that gets de-referenced is this one
"adapter->algo->master_xfer".

Adding more traces I was able to track the sequence to:

i2c_transfer()
drm_do_probe_ddc_edid()
drm_get_edid()
port->connector = (*mstb->mgr->cbs->add_connector)(mstb->mgr, port, proppath);
drm_dp_add_port(port->connector, &port->aux.ddc)
drm_dp_send_link_address()

The port->connector is what gets passed to drm_get_edid() as a reference, I am
guessing that this "port" somehow is becoming invalid from within
intel_dp_add_mst_connector().

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