Re: [PATCH v2 1/3] drm/i915: Skip register reads in i915_get_crtc_scanoutpos()

[Mario Kleiner <mario.kleiner.de@gmail.com>]

On 25.09.13 10:14, Ville Syrjälä wrote:
> On Wed, Sep 25, 2013 at 04:35:56AM +0200, Mario Kleiner wrote:
>> On 23.09.13 13:48, ville.syrjala@linux.intel.com wrote:
>>> From: Ville Syrjälä <ville.syrjala@linux.intel.com>
>>>
>>> We have all the information we need in the mode structure, so going and
>>> reading it from the hardware is pointless, and slower.
>>>
>>> We never populated ->get_vblank_timestamp() in the UMS case, and as that
>>> is the only way we'd ever call ->get_scanout_position(), we can
>>> completely ignore UMS in i915_get_crtc_scanoutpos().
>>>
>>> Also reorganize intel_irq_init() a bit to clarify the KMS vs. UMS
>>> situation.
>>>
>>> v2: Drop UMS code
>>>
>>> Cc: Mario Kleiner <mario.kleiner@tuebingen.mpg.de>
>>> Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
>>> ---
>>>    drivers/gpu/drm/i915/i915_irq.c | 43 ++++++++++++++++-------------------------
>>>    1 file changed, 17 insertions(+), 26 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
>>> index b356dc1..058f099 100644
>>> --- a/drivers/gpu/drm/i915/i915_irq.c
>>> +++ b/drivers/gpu/drm/i915/i915_irq.c
>>> @@ -570,24 +570,29 @@ static u32 gm45_get_vblank_counter(struct drm_device *dev, int pipe)
>>>    static int i915_get_crtc_scanoutpos(struct drm_device *dev, int pipe,
>>>    			     int *vpos, int *hpos)
>>>    {
>>> -	drm_i915_private_t *dev_priv = (drm_i915_private_t *) dev->dev_private;
>>> -	u32 vbl = 0, position = 0;
>>> +	struct drm_i915_private *dev_priv = dev->dev_private;
>>> +	struct drm_crtc *crtc = dev_priv->pipe_to_crtc_mapping[pipe];
>>> +	struct intel_crtc *intel_crtc = to_intel_crtc(crtc);
>>> +	const struct drm_display_mode *mode = &intel_crtc->config.adjusted_mode;
>>> +	u32 position;
>>>    	int vbl_start, vbl_end, htotal, vtotal;
>>>    	bool in_vbl = true;
>>>    	int ret = 0;
>>> -	enum transcoder cpu_transcoder = intel_pipe_to_cpu_transcoder(dev_priv,
>>> -								      pipe);
>>>
>>> -	if (!i915_pipe_enabled(dev, pipe)) {
>>> +	if (!intel_crtc->active) {
>>>    		DRM_DEBUG_DRIVER("trying to get scanoutpos for disabled "
>>>    				 "pipe %c\n", pipe_name(pipe));
>>>    		return 0;
>>>    	}
>>>
>>> -	/* Get vtotal. */
>>> -	vtotal = 1 + ((I915_READ(VTOTAL(cpu_transcoder)) >> 16) & 0x1fff);
>>> +	htotal = mode->crtc_htotal;
>>> +	vtotal = mode->crtc_vtotal;
>>> +	vbl_start = mode->crtc_vblank_start;
>>> +	vbl_end = mode->crtc_vblank_end;
>>>
>>> -	if (INTEL_INFO(dev)->gen >= 4) {
>>> +	ret |= DRM_SCANOUTPOS_VALID | DRM_SCANOUTPOS_ACCURATE;
>>> +
>>> +	if (IS_G4X(dev) || INTEL_INFO(dev)->gen >= 5) {
>>>    		/* No obvious pixelcount register. Only query vertical
>>>    		 * scanout position from Display scan line register.
>>>    		 */
>>> @@ -605,29 +610,16 @@ static int i915_get_crtc_scanoutpos(struct drm_device *dev, int pipe,
>>>    		 */
>>>    		position = (I915_READ(PIPEFRAMEPIXEL(pipe)) & PIPE_PIXEL_MASK) >> PIPE_PIXEL_SHIFT;
>>>
>>> -		htotal = 1 + ((I915_READ(HTOTAL(cpu_transcoder)) >> 16) & 0x1fff);
>>>    		*vpos = position / htotal;
>>>    		*hpos = position - (*vpos * htotal);
>>>    	}
>>>
>>> -	/* Query vblank area. */
>>> -	vbl = I915_READ(VBLANK(cpu_transcoder));
>>> -
>>> -	/* Test position against vblank region. */
>>> -	vbl_start = vbl & 0x1fff;
>>> -	vbl_end = (vbl >> 16) & 0x1fff;
>>> -
>>> -	if ((*vpos < vbl_start) || (*vpos > vbl_end))
>>> -		in_vbl = false;
>>> +	in_vbl = *vpos >= vbl_start && *vpos < vbl_end;
>>
>> I think this should be a <= instead of < in *vpos < vbl_end, if it is
>> meant to be equal to the line it replaces (not >  is <=), unless the
>> original comparison was off-by-one?
>
> Yeah, I think the original was wrong, in more ways than one. It forgot
> to add +1 to vbl_start/end, and then it did the comparison wrong as
> well.
>

Ah ok, that's possible. Then you have my blessing :).

On the Intel side i only had and have sporadic access to an old Intel 
GMA-950 (Gen-3?) when writing that function, so i could only really test 
one half of the code-path in that function. Also that card only has a 
VGA output, which limits my actual measurements to use of a photo-diode 
attached to a CRT monitor. That means i can only verify the accuracy of 
timestamping down to about 0.2 msecs variability and 0.5 msecs bias due 
to the limitations/noise of the measurement setup (depending how close i 
get the photo-diode to the corner of the monitor, how dark it is, etc.). 
So i know that the jitter in the timestamps is very low, less than 1 
usec standard deviation iirc, and that the absolute error wrt. reality 
is lower than 0.2 msecs, but i wouldn't be able to detect absolute 
errors of a few scanlines.

-mario

>>
>>   > +	in_vbl = *vpos >= vbl_start && *vpos <= vbl_end;
>>
>> Other than that, it looks good.
>>
>> Reviewed-by: mario.kleiner.de@gmail.com
>>
>>>
>>>    	/* Inside "upper part" of vblank area? Apply corrective offset: */
>>>    	if (in_vbl && (*vpos >= vbl_start))
>>>    		*vpos = *vpos - vtotal;
>>>
>>> -	/* Readouts valid? */
>>> -	if (vbl > 0)
>>> -		ret |= DRM_SCANOUTPOS_VALID | DRM_SCANOUTPOS_ACCURATE;
>>> -
>>>    	/* In vblank? */
>>>    	if (in_vbl)
>>>    		ret |= DRM_SCANOUTPOS_INVBL;
>>> @@ -3148,11 +3140,10 @@ void intel_irq_init(struct drm_device *dev)
>>>    		dev->driver->get_vblank_counter = gm45_get_vblank_counter;
>>>    	}
>>>
>>> -	if (drm_core_check_feature(dev, DRIVER_MODESET))
>>> +	if (drm_core_check_feature(dev, DRIVER_MODESET)) {
>>>    		dev->driver->get_vblank_timestamp = i915_get_vblank_timestamp;
>>> -	else
>>> -		dev->driver->get_vblank_timestamp = NULL;
>>> -	dev->driver->get_scanout_position = i915_get_crtc_scanoutpos;
>>> +		dev->driver->get_scanout_position = i915_get_crtc_scanoutpos;
>>> +	}
>>>
>>>    	if (IS_VALLEYVIEW(dev)) {
>>>    		dev->driver->irq_handler = valleyview_irq_handler;
>>>
>