[PATCH v2] drm/core: Add drm_accurate_vblank_count_and_time, v2.

[Maarten Lankhorst]

Op 11-04-16 om 16:43 schreef Ville Syrjälä:
> On Mon, Apr 11, 2016 at 11:42:57AM +0200, Maarten Lankhorst wrote:
>> This function is useful for gen2 intel devices which have no frame
>> counter, but need a way to determine the current vblank count without
>> racing with the vblank interrupt handler.
>>
>> intel_pipe_update_start checks if no vblank interrupt will occur
>> during vblank evasion, but cannot check whether the vblank handler has
>> run to completion. This function uses the timestamps to determine
>> when the last vblank has happened, and interpolates from there.
>>
>> Changes since v1:
>> - Take vblank_time_lock and don't use drm_vblank_count_and_time.
>>
>> Cc: Mario Kleiner <mario.kleiner.de at gmail.com>
>> Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
>> Signed-off-by: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
>> ---
>>  drivers/gpu/drm/drm_irq.c | 31 +++++++++++++++++++++++++++++++
>>  include/drm/drmP.h        |  2 ++
>>  2 files changed, 33 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
>> index 3c1a6f18e71c..d3cecc1932e1 100644
>> --- a/drivers/gpu/drm/drm_irq.c
>> +++ b/drivers/gpu/drm/drm_irq.c
>> @@ -303,6 +303,37 @@ static void drm_update_vblank_count(struct drm_device *dev, unsigned int pipe,
>>  	store_vblank(dev, pipe, diff, &t_vblank, cur_vblank);
>>  }
>>  
>> +/**
>> + * drm_accurate_vblank_count_and_time - retrieve the master vblank counter
>> + * @crtc: which counter to retrieve
>> + * @tv_ret: last time counter was updated
>> + *
>> + * This function is similar to @drm_update_vblank_count_and_time but
>> + * this function interpolates to handle a race with vblank irq's, and
>> + * is only useful for crtc's that have no hw vblank counter.
>> + */
>> +
>> +u32 drm_accurate_vblank_count_and_time(struct drm_crtc *crtc,
>> +				       struct timeval *tv_ret)
>> +{
>> +	struct drm_device *dev = crtc->dev;
>> +	u32 vblank, pipe = drm_crtc_index(crtc);
>> +	unsigned long flags;
>> +
>> +	WARN(dev->max_vblank_count, "This function is only useful when a hw counter is unavailable.");
> It's useful on everything if you actually want to trust the
> software counter to not be stale.
>
>> +
>> +	spin_lock_irqsave(&dev->vblank_time_lock, flags);
>> +	drm_update_vblank_count(dev, pipe, 0);
>> +
>> +	vblank = dev->vblank[pipe].count;
>> +	*tv_ret = vblanktimestamp(dev, pipe, vblank);
> Do we have a user in mind that would care about the timestamp?
>
No, I guess allowing a NULL pointer for tv_ret might help.

~Maarten