Claude review: Re: [PATCH v2] drm: shmobile: Fix blank screen after resume when LCDC is stopped

[Claude Code Review Bot <claude-review@example.com>]

Overall Series Review

Subject: Re: [PATCH v2] drm: shmobile: Fix blank screen after resume when LCDC is stopped
Author: Geert Uytterhoeven <geert@linux-m68k.org>
Patches: 3
Reviewed: 2026-02-27T12:34:03.437511

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This mbox contains two versions of the same patch: v1 (`[PATCH]`) and v2 (`[PATCH v2]`), where v2 supersedes v1. The fix addresses a real issue with the SH Mobile LCDC driver where the display start address (SA) is written via the mirror register mechanism during resume, but the controller is stopped (DO=0) so no Frame End interrupt fires to transfer the mirrored value into the active register set.

The analysis is sound and well-supported by the hardware architecture:

- `lcdc_write_mirror()` writes to `reg + LCDC_MIRROR_OFFSET (0x2000)`, the standby/mirror register
- The MRS toggle requests the hardware to switch sets on the next Frame End
- When DO=0, no frames are processed, so the switch never occurs
- `lcdc_write()` writes directly to both Set A (`reg`) and Set B (`reg + LCDC_SIDE_B_OFFSET (0x1000)`) for banked registers (LDSA1R and LDSA2R are both banked per `lcdc_is_banked()`)

The commit ordering confirms the scenario: since shmobile uses the default `drm_atomic_helper_commit_tail()` (no custom `commit_tail` override), the sequence during a modeset/resume is:
1. `drm_atomic_helper_commit_planes()` → `shmob_drm_primary_plane_setup()` (DO=0 at this point)
2. `drm_atomic_helper_commit_modeset_enables()` → `shmob_drm_crtc_atomic_enable()` → `shmob_drm_crtc_start_stop(scrtc, true)` (sets DO=1)

So when the plane setup runs, the LCDC is indeed stopped and the mirror mechanism cannot work.

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