kernel 6.18: generic panel-dsi binding regression — parse_panel_timing_node rejects override before panel_dsi_dt_probe runs

[issic]

Describe the bug

After upgrading from linux-image-6.12.75+rpt-rpi-v8 to linux-image-6.18.29+rpt-rpi-v8, DSI panels using the generic compatible = "panel-dsi" binding fail to initialise. The display shows the DSI test pattern (rainbow bars) instead of the framebuffer. dmesg shows:

panel-simple-dsi fe700000.dsi.0: Specify missing connector_type
panel-simple-dsi fe700000.dsi.0: supply power not found, using dummy regulator
WARNING: CPU: 1 PID: 216 at drivers/gpu/drm/panel/panel-simple.c:487 panel_simple_probe+0x758/0x7d8 [panel_simple]
panel-simple-dsi fe700000.dsi.0: Reject override mode: no timings specified

The same overlay and panel work correctly on 6.12.75 and 6.12.62.

Hardware

Raspberry Pi CM4 on BTT Manta M4P V2.1 carrier
Waveshare 3.5" DSI LCD (E) panel, 640×480, RGB888
Overlay: Waveshare_35DSI with the 35E parameter preset

Root cause analysis
In drivers/gpu/drm/panel/panel-simple.c, panel_simple_dsi_probe() was refactored between 6.12 and 6.18:
6.12 builds the DT descriptor before calling panel_simple_probe:

cdesc = of_device_get_match_data(...);
if (desc == &panel_dsi) {
    panel_dsi_dt_probe(dev, dt_desc);
    desc = dt_desc;
}
panel_simple_probe(dev, &desc->desc);

6.18 calls panel_simple_probe first, then tries to build the DT descriptor:

cpanel = panel_simple_probe(dev);   // hits parse_panel_timing_node, dies
desc = container_of(panel->desc, ...);
if (desc == &panel_dsi) {
    panel_dsi_dt_probe(dev, dt_desc);
    desc = dt_desc;
}

Inside panel_simple_probe, this code runs for all non-DPI panels and rejects the generic DSI case before the DT descriptor is built:

cif (!of_device_is_compatible(dev->of_node, "panel-dpi") &&
    !of_get_display_timing(dev->of_node, "panel-timing", &dt))
    panel_simple_parse_panel_timing_node(dev, panel, &dt);

panel_simple_parse_panel_timing_node() requires desc->num_timings > 0, which is never true for the generic &panel_dsi descriptor — that's the whole point of the generic binding.

Steps to reproduce the behaviour

Use any overlay with compatible = "panel-dsi" + panel-timing subnode (e.g. in-tree vc4-kms-dsi-generic or Waveshare_35DSI)
Boot 6.18.29 — display shows DSI test bars
Boot 6.12.75 — display works

Device (s)

Raspberry Pi CM4

System

Raspberry Pi reference 2022-09-22
Generated using pi-gen, https://github.com/RPi-Distro/pi-gen, 005a8c73b05a2cab394073150208bf4f069e861a, stage2

May 8 2026 18:13:05
Copyright (c) 2012 Broadcom
version ae9a8ea9f3ca745de6f357bd7fc8307721ad38b7 (clean) (release) (start)

Linux reddwarf 6.12.75+rpt-rpi-v8 #1 SMP PREEMPT Debian 1:6.12.75-1+rpt1 (2026-03-11) aarch64 GNU/Linux
Linux reddwarf 6.18.29+rpt-rpi-v8 #1 SMP PREEMPT Debian 1:6.18.29-1+rpt1 aarch64 GNU/Linux

Logs

panel-simple-dsi fe700000.dsi.0: Specify missing connector_type
panel-simple-dsi fe700000.dsi.0: supply power not found, using dummy regulator
WARNING: CPU: 1 PID: 216 at drivers/gpu/drm/panel/panel-simple.c:487 panel_simple_probe+0x758/0x7d8 [panel_simple]
panel-simple-dsi fe700000.dsi.0: Reject override mode: no timings specified

Additional context

Suggested fix
Mirror the existing DPI fallback for the generic DSI case: either teach panel_simple_probe to call panel_dsi_dt_probe() early when the matched desc is &panel_dsi, or have panel_simple_dsi_probe build dt_desc before invoking panel_simple_probe (the 6.12 ordering).

[6by9]

@timonsku panel_dsi was your contribution via #5640. Do you wish to fix it up on 6.18?

Otherwise @issic if you fancied creating a PR it would get reviewed.

[6by9]

Sheer chance means I've just had a requirement to use panel-dsi, so I'm looking at fixing this up cleanly. It's not quite as simple to do without major code changes as it at first looks.

[timonsku]

I would only find time in mid June to take a look as I don't have access to a hardware setup to try things out on handy. If you want to take the fix on I'd appreciate it, happy to assist or review.

[6by9]

Potential fix in #7382.

It works for my test case, but could do with extra testing. sudo rpi-update pulls/7382 should get the CI build once complete (about 25mins from now).

[timonsku]

no chance to test it yet but reading over it looks good

[issic]

Confirmed fixed on CM4 + BTT Manta M4P + Waveshare 3.5" DSI (E). rpi-update pulls/7382 → 6.18.32-v8+, panel binds, no 'Reject override mode', display + touch work."

[    0.023529] /soc/dsi@7e700000/panel-dsi-generic@0: Fixed dependency cycle(s) with /soc/dsi@7e700000
[    0.023840] /soc/cprman@7e101000: Fixed dependency cycle(s) with /soc/dsi@7e700000
[    0.025049] /soc/cprman@7e101000: Fixed dependency cycle(s) with /soc/dsi@7e700000
[    0.025091] /soc/dsi@7e700000: Fixed dependency cycle(s) with /soc/dsi@7e700000/panel-dsi-generic@0
[    0.025111] /soc/dsi@7e700000: Fixed dependency cycle(s) with /soc/cprman@7e101000
[    0.025154] /soc/dsi@7e700000/panel-dsi-generic@0: Fixed dependency cycle(s) with /soc/dsi@7e700000
[    4.321885] /soc/dsi@7e700000: Fixed dependency cycle(s) with /soc/dsi@7e700000/panel-dsi-generic@0
[    4.322082] /soc/dsi@7e700000/panel-dsi-generic@0: Fixed dependency cycle(s) with /soc/dsi@7e700000
[    5.257043] panel-simple-dsi fe700000.dsi.0: supply power not found, using dummy regulator
[    5.575416] vc4-drm gpu: bound fe700000.dsi (ops vc4_dsi_ops [vc4])
 uname -r 6.18.32-v8+

Thank You