[Bug 105406] No internal display on kernels > 4.4.x

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=105406

--- Comment #26 from Peteris <peteris.rudzusiks at gmail.com> ---
I have a POS computer that also uses CH7511. Just like in David's case, the
internal display of the POS is broken with Linux kernels 4.5+. The display
shows no output. I can confirm that commit 30d9aa4265fe is the first commit
since v4.4 that does not work.

For what it's worth, here is a patch for v4.19.35 that fixes the problem for
me. Not sure if this is a proper fix or hacky workaround for broken hardware,
but it works for me and maybe someone else finds it useful.

---
 drivers/gpu/drm/i915/intel_dp.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index f92079e19de8..ec908f605708 100644
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -3840,6 +3840,9 @@ intel_dp_get_dpcd(struct intel_dp *intel_dp)
         */
        intel_dp->sink_count = DP_GET_SINK_COUNT(sink_count);

+       if (!drm_dp_is_branch(intel_dp->dpcd))
+               return true; /* native DP sink */
+
        /*
         * SINK_COUNT == 0 and DOWNSTREAM_PORT_PRESENT == 1 implies that
         * a dongle is present but no display. Unless we require to know
@@ -3850,9 +3853,6 @@ intel_dp_get_dpcd(struct intel_dp *intel_dp)
        if (!intel_dp_is_edp(intel_dp) && !intel_dp->sink_count)
                return false;

-       if (!drm_dp_is_branch(intel_dp->dpcd))
-               return true; /* native DP sink */
-
        if (intel_dp->dpcd[DP_DPCD_REV] == 0x10)
                return true; /* no per-port downstream info */

-- 
2.17.1


The CH7511 is not a branch device according to DPCD, so intel_dp_get_dpcd()
function will return without actually using the incorrect SINK_COUNT variable.

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