edp backtrace spam on MacBookAir4,1

[Daniel Vetter]

On Mon, May 28, 2012 at 1:09 AM, Linus Torvalds
<torvalds at linux-foundation.org> wrote:
> A new worry about excessively verbose i915 driver "errors" that don't
> actually seem to be errors.
>
> I got myself a micro-DP to VGA adapter so that I can use my Macbook
> Air for presentations.
>
> And testing it, hotplugging seems to work very nicely, and gone are
> the days when you had to do xrandr and crap. Things "JustWork(tm)".
>
> Goodie.
>
> Until you start looking at the dmesg log. Then it gets ugly. It's full
> of scary-looking stuff like
>
>  ------------[ cut here ]------------
>  WARNING: at drivers/gpu/drm/i915/intel_dp.c:350 intel_dp_check_edp+0x5d/0xb0()
>  Hardware name: MacBookAir4,1
>  eDP powered off while attempting aux channel communication.
>  Modules linked in: fuse rfcomm bnep nf_conntrack_netbios_ns
> nf_conntrack_broadcast ..
>  Pid: 2126, comm: kworker/0:0 Tainted: G        W    3.4.0-08209-gae32adc #9
>  Call Trace:
>    warn_slowpath_common+0x7a/0xb0
>    warn_slowpath_fmt+0x41/0x50
>    intel_dp_check_edp+0x5d/0xb0
>    intel_dp_aux_ch+0x3f/0x330
>    intel_dp_aux_native_read_retry+0xad/0x130
>    intel_dp_detect+0x240/0x2c0
>    output_poll_execute+0xba/0x1a0
>    process_one_work+0x11b/0x3c0
>    worker_thread+0x12e/0x2d0
>    kthread+0x8e/0xa0
>    kernel_thread_helper+0x4/0x10
>  ---[ end trace 5756a4d08d9e2a83 ]---
>
> which seems a bit extreme. I just unplugged the connector, I don't
> think it should necessarily make quite this big a deal over it. What
> does the WARN_ON() with the full call trace really buy us?

Well, all these edp checks (I presume all the other WARN backtraces
yell around about edp, too) ensure that we have edp vdd or the panel
power on while we try to do dp aux channel communication. And because
we do need to talk to the monitor at tons of places, the backtrace is
actually really useful to debug such edp vdd confusions.

The real problem here is that we think that the DP connector on your
MBA is connected to an edp panel, which is pretty bogus. Cc'ing Chris
and Keith who have the hw. Btw, you don't see all that dmesg spam
until you connect something real because we check the hotplug pin
status before we try to do use the dp aux channel (to get at the
edid).

Yours, Daniel

PS: Add cc's and changed the subject, I've figured you wanted to bash
me in public ;-)
-- 
Daniel Vetter
daniel.vetter at ffwll.ch - +41 (0) 79 365 57 48 - http://blog.ffwll.ch