When using a device based on DCN32/321,
we have an issue where a second
4k@60Hz display does not light up,
and the system becomes unresponsive
for a few minutes. In the debug process,
it was possible to see a hang
in the function dcn20_post_unlock_program_front_end
in this part:

for (j = 0; j < TIMEOUT_FOR_PIPE_ENABLE_MS*1000
	&& hubp->funcs->hubp_is_flip_pending(hubp); j++)
	mdelay(1);
}

The hubp_is_flip_pending always returns positive
for waiting pending flips which is a symptom of
pipe hang. Additionally, the dmesg log shows
this message after a few minutes:

  BUG: soft lockup - CPU#4 stuck for 26s!
  ...
  [  +0.000003]  dcn20_post_unlock_program_front_end+0x112/0x340 [amdgpu]
  [  +0.000171]  dc_commit_state_no_check+0x63d/0xbf0 [amdgpu]
  [  +0.000155]  ? dc_validate_global_state+0x358/0x3d0 [amdgpu]
  [  +0.000154]  dc_commit_state+0xe2/0xf0 [amdgpu]

This confirmed the hypothesis that we had a pipe
hanging somewhere. Next, after checking the
ftrace entries, we have the below weird
sequence:

 [..]
  2)               |        dcn10_lock_all_pipes [amdgpu]() {
  2)   0.120 us    |          optc1_is_tg_enabled [amdgpu]();
  2)               |          dcn20_pipe_control_lock [amdgpu]() {
  2)               |            dc_dmub_srv_clear_inbox0_ack [amdgpu]() {
  2)   0.121 us    |              amdgpu_dm_dmub_reg_write [amdgpu]();
  2)   0.551 us    |            }
  2)               |            dc_dmub_srv_send_inbox0_cmd [amdgpu]() {
  2)   0.110 us    |              amdgpu_dm_dmub_reg_write [amdgpu]();
  2)   0.511 us    |            }
  2)               |            dc_dmub_srv_wait_for_inbox0_ack [amdgpu]() {
  2)   0.110 us    |              amdgpu_dm_dmub_reg_read [amdgpu]();
  2)   0.110 us    |              amdgpu_dm_dmub_reg_read [amdgpu]();
  2)   0.110 us    |              amdgpu_dm_dmub_reg_read [amdgpu]();
  2)   0.110 us    |              amdgpu_dm_dmub_reg_read [amdgpu]();
  2)   0.110 us    |              amdgpu_dm_dmub_reg_read [amdgpu]();
  2)   0.110 us    |              amdgpu_dm_dmub_reg_read [amdgpu]();
  2)   0.110 us    |              amdgpu_dm_dmub_reg_read [amdgpu]();
 [..]

We are not expected to read from dmub register
so many times and for so long. From the trace log,
it was possible to identify that the function
dcn20_pipe_control_lock was triggering the dmub
operation when it was unnecessary and causing
the hang issue. This commit drops the unnecessary
dmub code and, consequently, fixes the second display not
lighting up the issue.
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>