Claude review: drm/vblank: timer: Use absolute timer since boot

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

Patch Review

**Critical issue: Clock domain mismatch.** The timer is changed from `CLOCK_MONOTONIC` to `CLOCK_BOOTTIME`, but the initial start time uses `ktime_get_ns()`, which returns `CLOCK_MONOTONIC` time:

```c
hrtimer_setup(&vtimer->timer, drm_vblank_timer_function,
              CLOCK_BOOTTIME, HRTIMER_MODE_ABS);
...
vblank_time_ns = roundup(ktime_get_ns(), vblank->framedur_ns);
hrtimer_start(&vtimer->timer, ns_to_ktime(vblank_time_ns), HRTIMER_MODE_ABS);
```

This should be `ktime_get_boottime_ns()`. After any system suspend, `CLOCK_BOOTTIME > CLOCK_MONOTONIC`, so a monotonic timestamp used as a boottime absolute value would be in the past, causing the timer to fire immediately and then forward rapidly.

This mismatch also breaks the comparison in the existing `drm_crtc_vblank_get_vblank_timeout()`: `vtimer->timer.node.expires` is now in the boottime domain, while `cur_time` from `drm_crtc_vblank_count_and_time()` is in the monotonic domain (vblank timestamps are stored via `ktime_get()` in the vblank core). The `!ktime_compare(*vblank_time, cur_time)` check compares values from different clock domains.

**Division by zero risk:** If `vblank->framedur_ns` is 0 (e.g., dotclock is 0), `roundup(ktime_get_ns(), 0)` will trigger a division by zero. The `drm_calc_timestamping_constants` called just before can leave `framedur_ns` at 0 if `dotclock <= 0`. A guard is needed.

**The underrun detection** (`!ret_overrun` case) is a nice addition for debugging.

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Generated by Claude Code Patch Reviewer