Instability is observed during display startup, for some, but not all, the displays. The symptom is: not all displays turn on and show picture. Some turn on only if uplugged and replugged while system is already running.

The root cause of the issue is that the drivers - both in linux drm and on r5 - always assume that eDP firmware will respond to the outgoing message with a reply with a defined length. However, tracing shows that sometimes the length of the response differs from expectation. This causes the current driver's operation to fail.

Here is an example trace, extracted from linux drm drivers, by adding printk's to cdns_mhdp_mailbox_validate_receive() routine:

[ 2.139075] GOOD REPLY opcode=17 module_id=1 size=1
[ 2.147970] BAD REPLY opcode=2 h[0]=2 module_id=1 h[1]=1 req_size=130 mbox_size=2
[ 2.164066] BAD REPLY opcode=2 h[0]=2 module_id=1 h[1]=1 req_size=130 mbox_size=2
[ 2.180162] BAD REPLY opcode=2 h[0]=2 module_id=1 h[1]=1 req_size=130 mbox_size=2
[ 2.196258] BAD REPLY opcode=2 h[0]=2 module_id=1 h[1]=1 req_size=130 mbox_size=2
[ 2.208339] cdns-mhdp a000000.dp-bridge: [drm:cdns_mhdp_get_edid_block] ERROR get block[0] edid failed: -22
[ 2.218233] cdns-mhdp a000000.dp-bridge: [drm:cdns_mhdp_get_modes] ERROR Failed to read EDID
[ 2.231842] GOOD REPLY opcode=7 module_id=10 size=8
[ 2.232873] BAD REPLY opcode=3 h[0]=3 module_id=1 h[1]=1 req_size=6 mbox_size=21
[ 2.232893] cdns-mhdp a000000.dp-bridge: [drm:mhdp_transfer] ERROR Failed to read DPCD
[ 2.234512] GOOD REPLY opcode=3 module_id=1 size=6
[ 2.235532] GOOD REPLY opcode=3 module_id=1 size=6
[ 2.236551] GOOD REPLY opcode=3 module_id=1 size=6
[ 2.239579] BAD REPLY opcode=3 h[0]=3 module_id=1 h[1]=1 req_size=20 mbox_size=5
[ 2.239586] cdns-mhdp a000000.dp-bridge: [drm:mhdp_transfer] ERROR Failed to read DPCD
[ 2.241205] GOOD REPLY opcode=3 module_id=1 size=20
[ 2.242234] GOOD REPLY opcode=3 module_id=1 size=6
[ 2.243254] GOOD REPLY opcode=3 module_id=1 size=6
[ 2.244274] GOOD REPLY opcode=4 module_id=1 size=5
[ 2.348270] GOOD REPLY opcode=7 module_id=10 size=8
[ 2.349299] GOOD REPLY opcode=4 module_id=1 size=5
[ 2.350318] GOOD REPLY opcode=4 module_id=1 size=5
[ 2.351337] GOOD REPLY opcode=4 module_id=1 size=5
[ 2.352378] GOOD REPLY opcode=4 module_id=1 size=5
[ 2.353397] GOOD REPLY opcode=4 module_id=1 size=5
[ 2.355473] GOOD REPLY opcode=4 module_id=1 size=5
[ 2.356492] GOOD REPLY opcode=3 module_id=1 size=6
[ 2.357511] GOOD REPLY opcode=3 module_id=1 size=11
[ 2.359556] GOOD REPLY opcode=3 module_id=1 size=11
[ 2.360588] GOOD REPLY opcode=4 module_id=1 size=5
[ 2.361606] GOOD REPLY opcode=3 module_id=1 size=6
[ 2.362625] GOOD REPLY opcode=3 module_id=1 size=11
[ 2.364668] GOOD REPLY opcode=3 module_id=1 size=11
[ 2.365693] GOOD REPLY opcode=4 module_id=1 size=5
[ 2.366711] GOOD REPLY opcode=7 module_id=10 size=8
[ 2.370911] GOOD REPLY opcode=7 module_id=10 size=8
[ 2.382942] Console: switching to colour frame buffer device 128x48

This requires firmwares of eDP to be updated.