Re: [Intel-gfx] [PATCH 3/3] drm/i915: Support DDI lane reversal for DP

From: Sivakumar Thulasimani
Date: Wed Aug 26 2015 - 08:09:50 EST

On 8/18/2015 1:36 AM, Benjamin Tissoires wrote:
On Aug 14 2015 or thereabouts, StÃphane Marchesin wrote:
On Wed, Aug 5, 2015 at 12:34 PM, Benjamin Tissoires
<benjamin.tissoires@xxxxxxxxxx> wrote:
On Jul 30 2015 or thereabouts, Sivakumar Thulasimani wrote:

On 7/29/2015 8:52 PM, Benjamin Tissoires wrote:
On Jul 29 2015 or thereabouts, Sivakumar Thulasimani wrote:
why not detect reverse in intel_dp_detect/intel_hpd_pulse ? that way you can
identify both lane count and reversal state without touching anything in the
link training code. i am yet to upstream my changes for CHT that i can share
if required that does the same in intel_dp_detect without touching any line
in link training path.
With my current limited knowledge of the dp hotplug (and i915 driver) I
am not sure we could detect the reversed state without trying to train 1
lane only. I'd be glad to look at your changes and test them on my
system if you think that could help having a cleaner solution.

No, what i recommended was to do link training but in intel_dp_detect. Since
USB Type C cable
also has its own lane count restriction (it can have different lane count
than the one supported
by panel) you might have to figure that out as well. so both reversal and
lane count detection
can be done outside the modeset path and keep the code free of type C
changes outside
detection path.

Please find below the code to do the same. Do not waste time trying to apply
this directly on
nightly since this is based on a local tree and because this is pre- atomic
changes code, so you
might have to modify chv_upfront_link_train to work on top of the latest
nightly code. we
are supposed to upstream this and is in my todo list.

[original patch snipped...]

Hi Sivakumar,

So I managed to manually re-apply your patch on top of
drm-intel-nightly, and tried to port it to make Broadwell working too.
It works OK if the system is already boot without any external DP used.
In this case, the detection works and I can see my external monitor
working properly.

However, if the monitor is cold plugged, the cpu/GPU hangs and I can not
understand why. I think I enabled all that is mentioned in the PRM to be
able to train the DP link, but I am obviously missing something else.
Can you have a look?

Hi Benjamin,

I would recommend against this approach. Some adapters will claim that
they recovered a clock even when it isn't on the lanes you enabled,
which means that the reversal detection doesn't always work. The only
reliable way to do this is to go talk to the Chrome OS EC (you can
find these patches later in the Chrome OS tree). It's not as generic,
but we might be able to abstract that logic, maybe.

Hi StÃphane,

This is a very good news. I was afraid we would not have access to the
hardware controller because the Intel controller hub spec was not

I will try to have a look at it, but the latest chromeos branch (3.18)
seems to differ quite a lot from the upstream one. Anyway, fingers

Hi Benjamin/StÃphan,
All Intel platforms (at-least those i inquired about) handle lane reversal in HW.
your statement that link training will pass even on reversed lane seems to point
to the same fact. in such a scenario why should the encoder/connector care
if the lane is reversed or not ?


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