Change in coreboot[master]: mb/google/corsola: Revise power-on sequence of PS8640

[Rex-BC Chen (Code Review)]

Attention is currently required from: Hung-Te Lin, Rex-BC Chen.
Hello Hung-Te Lin, build bot (Jenkins), Paul Menzel, Yu-Ping Wu,

I'd like you to reexamine a change. Please visit

https://review.coreboot.org/c/coreboot/+/62893

to look at the new patch set (#8).

Change subject: mb/google/corsola: Revise power-on sequence of PS8640
......................................................................

mb/google/corsola: Revise power-on sequence of PS8640

Although the panel initializes fine and the fw recovery screen is
displayed without issues, the current power-on sequence of the
PS8640 violates the spec of the PS8640, which can be confirmed by
measuring it with an oscilloscope.

The sequence is:
- set VDD12 to be 1.2V
- set VDD33 to be 3.3V
- pull hign PD#
- pull down RST#
- delay 2ms
- pull high RST#
- delay more than 50ms (55ms for margin)
- pull down RST#
- delay more than 50ms (55ms for margin)
- pull high RST#

This flow will increase 110ms if firmware display is enabled in
krabby. For normal booting flow, the firmware will not be enabled,
so it will meet boot time requirements of Chrome OS. (Less than 1s.)

Datasheet name: PS8640_DS_V1.4_20200210.docx.
Chapter: 14.

BUG=b:222650141
TEST=show fw display normally in krabby.
TEST=result of waveform meets the spec.

Signed-off-by: Rex-BC Chen 
Change-Id: I7706c56dc7fc13ac84c0d52a6e534bc0988e8fd3
---
M src/mainboard/google/corsola/display.c
1 file changed, 18 insertions(+), 0 deletions(-)

git pull ssh://review.coreboot.org:29418/coreboot refs/changes/93/62893/8