[PATCH] drm/i915/display: Use drm_crtc_vblank_count in intel_crtc_get_vblank_counter()

[Jani Nikula]

On Mon, 04 Aug 2025, Maarten Lankhorst <dev at lankhorst.se> wrote:
> Hey,
>
> Den 2025-08-04 kl. 16:46, skrev Jani Nikula:
>> On Thu, 24 Jul 2025, Maarten Lankhorst <dev at lankhorst.se> wrote:
>>> drm_crtc_accurate_vblank_count takes a spinlock, which we should avoid
>>> in tracepoints and debug functions.
>>>
>>> This also prevents taking the spinlock 2x during the critical
>>> section of pipe updates.
>> 
>> Don't we have vblank->max_vblank_count != 0 in most cases, and don't we
>> want accuracy in the rest of the cases?
>> 
>> The commit message should explain why it's okay to make this change.
>
> The whole point of tracepoints and debugging infra is that the functions should
> observe, not alter. Even for the vblank_count == 0 we should not be taking the
> locking that we do.
>
> If you look at drm_update_vblank_count, that's way more than the simple
> atomic64_read(&vblank->count) from drm_vblank_count.
>
> It takes the vblank_time_lock, loops potentially DRM_TIMESTAMP_MAXRETRIES
> times, prints a few of drm_dbg() messages (which also has issues), and then
> returns after updating. Being approximately correct is a win here.

It's really hard for me to say okay here when the one person I
absolutely know will be using vblank tracing is Ville, and his comment
was, "The non-accurate version is completely useless here." [1]

BR,
Jani.


[1] https://lore.kernel.org/r/aHaV7vtp2Y1RbIAt@intel.com


-- 
Jani Nikula, Intel