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

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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@xxxxxxxxxxxxxxx wrote:
From: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>

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@xxxxxxxxxxxxxxxx>
Signed-off-by: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
---
   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@xxxxxxxxx


   	/* 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;



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