Claude review: drm/msm/dpu: calculate data_width like downstream

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

Patch Review

**Positive aspects:**
- Uses the established `msm_dsc_get_bytes_per_line()` helper already used in `dsi_host.c`, maintaining consistency.
- `DIV_ROUND_UP` is the right choice to avoid off-by-one truncation in the data width.
- The `dce_bytes_per_line` field is properly zero-initialized (struct uses `= { 0 }`) and only read when `compression_en` is set.
- The expanded comment block is a welcome improvement over the previous terse comment.

**Concern — widebus video mode divisor:**

In `dsi_host.c:1028-1035`, the widebus divisor for DSC depends on the mode:
```c
if (wide_bus_enabled && !(msm_host->mode_flags & MIPI_DSI_MODE_VIDEO))
    bytes_per_pclk = 6;
else
    bytes_per_pclk = 3;
```

For **video mode** with widebus, DSI host uses `bytes_per_pclk = 3` (not 6). But this patch (which is in `dpu_encoder_phys_vid.c`, i.e., video mode only) divides by 6 when widebus is enabled:

```c
if (p->wide_bus_en)
    data_width = DIV_ROUND_UP(p->dce_bytes_per_line, 6);
```

These are admittedly computing different things (DSI packet timing vs. INTF data validity window), so this may be intentional and correct for the INTF hardware — but the reviewer should verify that the INTF timing engine truly expects the `data_width` to be halved for widebus even in video mode with DSC. It would be worth having a comment explaining why the INTF widebus behavior differs from the DSI host here, or citing the downstream source/hardware documentation that confirms the divide-by-6.

**Minor — commit message could be more specific:**

The commit message says "calculate data_width like downstream" but doesn't explain *what was wrong* with the existing calculation. A sentence like "The existing formula using `drm_dsc_get_bpp_int()` loses fractional BPP precision and doesn't account for actual slice geometry" would help reviewers understand the motivation.

**Nit — the "TODO: handle DSC+DP" comment is retained but could be updated:**

The comment at line 184 (`/* TODO: handle DSC+DP case... */`) still refers to the old behavior. Since this patch explicitly skips DP (`!dp_intf`) in the new compression branch, the TODO is still valid, but it might be worth noting in the commit message that DSC+DP remains unhandled.

**Summary:** The patch is a logical improvement, aligning the INTF data width calculation with the downstream approach using actual DSC byte counts. The main point to verify is whether the divide-by-6 for widebus video mode is correct for the INTF timing engine, given that the DSI host uses divide-by-3 for the same scenario.

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