[PATCH 4/4] drm/intel: Push get_scanout_position() timestamping into kms driver.

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

Move the ktime_get() clock readouts and potential preempt_disable()
calls from drm core into kms driver to make it compatible with the
api changes in the drm core.

The intel-kms driver needs to take the uncore.lock inside
i915_get_crtc_scanoutpos() and intel_pipe_in_vblank().
This is incompatible with the preempt_disable() on a
PREEMPT_RT patched kernel, as regular spin locks must not
be taken within a preempt_disable'd section. Lock contention
on the uncore.lock also introduced too much uncertainty in vblank
timestamps.

Push the ktime_get() timestamping for scanoutpos queries and
potential preempt_disable_rt() into i915_get_crtc_scanoutpos(),
so these problems can be avoided:

1. First lock the uncore.lock (might sleep on a PREEMPT_RT kernel).
2. preempt_disable_rt() (will be added by the rt-linux folks).
3. ktime_get() a timestamp before scanout pos query.
4. Do all mmio reads as fast as possible without grabbing any new locks!
5. ktime_get() a post-query timestamp.
6. preempt_enable_rt()
7. Unlock the uncore.lock.

This reduces timestamp uncertainty on a low-end HP Atom Mini netbook
with Intel GMA-950 nicely:

Before: 3-8 usecs with spikes > 20 usecs, triggering query retries.
After : Typically 1 usec (98% of all samples), occassionally 2 usecs
        (2% of all samples), with maximum of 3 usecs (a handful).

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
---
 drivers/gpu/drm/i915/i915_irq.c |   53 +++++++++++++++++++++++++++++++--------
 1 file changed, 42 insertions(+), 11 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
index 156a1a4..a3e41d3 100644
--- a/drivers/gpu/drm/i915/i915_irq.c
+++ b/drivers/gpu/drm/i915/i915_irq.c
@@ -599,35 +599,40 @@ static u32 gm45_get_vblank_counter(struct drm_device *dev, int pipe)
 	return I915_READ(reg);
 }
 
-static bool intel_pipe_in_vblank(struct drm_device *dev, enum pipe pipe)
+/* raw reads, only for fast reads of display block, no need for forcewake etc. */
+#define __raw_i915_read32(dev_priv__, reg__) readl((dev_priv__)->regs + (reg__))
+#define __raw_i915_read16(dev_priv__, reg__) readw((dev_priv__)->regs + (reg__))
+
+static bool intel_pipe_in_vblank_locked(struct drm_device *dev, enum pipe pipe)
 {
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	uint32_t status;
+	int reg;
 
 	if (IS_VALLEYVIEW(dev)) {
 		status = pipe == PIPE_A ?
 			I915_DISPLAY_PIPE_A_VBLANK_INTERRUPT :
 			I915_DISPLAY_PIPE_B_VBLANK_INTERRUPT;
 
-		return I915_READ(VLV_ISR) & status;
+		reg = VLV_ISR;
 	} else if (IS_GEN2(dev)) {
 		status = pipe == PIPE_A ?
 			I915_DISPLAY_PIPE_A_VBLANK_INTERRUPT :
 			I915_DISPLAY_PIPE_B_VBLANK_INTERRUPT;
 
-		return I915_READ16(ISR) & status;
+		reg = ISR;
 	} else if (INTEL_INFO(dev)->gen < 5) {
 		status = pipe == PIPE_A ?
 			I915_DISPLAY_PIPE_A_VBLANK_INTERRUPT :
 			I915_DISPLAY_PIPE_B_VBLANK_INTERRUPT;
 
-		return I915_READ(ISR) & status;
+		reg = ISR;
 	} else if (INTEL_INFO(dev)->gen < 7) {
 		status = pipe == PIPE_A ?
 			DE_PIPEA_VBLANK :
 			DE_PIPEB_VBLANK;
 
-		return I915_READ(DEISR) & status;
+		reg = DEISR;
 	} else {
 		switch (pipe) {
 		default:
@@ -642,12 +647,17 @@ static bool intel_pipe_in_vblank(struct drm_device *dev, enum pipe pipe)
 			break;
 		}
 
-		return I915_READ(DEISR) & status;
+		reg = DEISR;
 	}
+
+	if (IS_GEN2(dev))
+		return __raw_i915_read16(dev_priv, reg) & status;
+	else
+		return __raw_i915_read32(dev_priv, reg) & status;
 }
 
 static int i915_get_crtc_scanoutpos(struct drm_device *dev, int pipe,
-			     int *vpos, int *hpos)
+			     int *vpos, int *hpos, ktime_t *stime, ktime_t *etime)
 {
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	struct drm_crtc *crtc = dev_priv->pipe_to_crtc_mapping[pipe];
@@ -657,6 +667,7 @@ static int i915_get_crtc_scanoutpos(struct drm_device *dev, int pipe,
 	int vbl_start, vbl_end, htotal, vtotal;
 	bool in_vbl = true;
 	int ret = 0;
+	unsigned long irqflags;
 
 	if (!intel_crtc->active) {
 		DRM_DEBUG_DRIVER("trying to get scanoutpos for disabled "
@@ -671,14 +682,26 @@ static int i915_get_crtc_scanoutpos(struct drm_device *dev, int pipe,
 
 	ret |= DRM_SCANOUTPOS_VALID | DRM_SCANOUTPOS_ACCURATE;
 
+	/* Lock uncore.lock, as we will do multiple timing critical raw
+	 * register reads, potentially with preemption disabled, so the
+	 * following code must not block on uncore.lock.
+	 */
+	spin_lock_irqsave(&dev_priv->uncore.lock, irqflags);
+	
+	/* preempt_disable_rt() should go right here in PREEMPT_RT patchset. */
+
+	/* Get optional system timestamp before query. */
+	if (stime)
+		*stime = ktime_get();
+
 	if (IS_GEN2(dev) || IS_G4X(dev) || INTEL_INFO(dev)->gen >= 5) {
 		/* No obvious pixelcount register. Only query vertical
 		 * scanout position from Display scan line register.
 		 */
 		if (IS_GEN2(dev))
-			position = I915_READ(PIPEDSL(pipe)) & DSL_LINEMASK_GEN2;
+			position = __raw_i915_read32(dev_priv, PIPEDSL(pipe)) & DSL_LINEMASK_GEN2;
 		else
-			position = I915_READ(PIPEDSL(pipe)) & DSL_LINEMASK_GEN3;
+			position = __raw_i915_read32(dev_priv, PIPEDSL(pipe)) & DSL_LINEMASK_GEN3;
 
 		/*
 		 * The scanline counter increments at the leading edge
@@ -687,7 +710,7 @@ static int i915_get_crtc_scanoutpos(struct drm_device *dev, int pipe,
 		 * to get a more accurate picture whether we're in vblank
 		 * or not.
 		 */
-		in_vbl = intel_pipe_in_vblank(dev, pipe);
+		in_vbl = intel_pipe_in_vblank_locked(dev, pipe);
 		if ((in_vbl && position == vbl_start - 1) ||
 		    (!in_vbl && position == vbl_end - 1))
 			position = (position + 1) % vtotal;
@@ -696,7 +719,7 @@ static int i915_get_crtc_scanoutpos(struct drm_device *dev, int pipe,
 		 * We can split this into vertical and horizontal
 		 * scanout position.
 		 */
-		position = (I915_READ(PIPEFRAMEPIXEL(pipe)) & PIPE_PIXEL_MASK) >> PIPE_PIXEL_SHIFT;
+		position = (__raw_i915_read32(dev_priv, PIPEFRAMEPIXEL(pipe)) & PIPE_PIXEL_MASK) >> PIPE_PIXEL_SHIFT;
 
 		/* convert to pixel counts */
 		vbl_start *= htotal;
@@ -704,6 +727,14 @@ static int i915_get_crtc_scanoutpos(struct drm_device *dev, int pipe,
 		vtotal *= htotal;
 	}
 
+	/* Get optional system timestamp after query. */
+	if (etime)
+		*etime = ktime_get();
+
+	/* preempt_enable_rt() should go right here in PREEMPT_RT patchset. */
+
+	spin_unlock_irqrestore(&dev_priv->uncore.lock, irqflags);
+
 	in_vbl = position >= vbl_start && position < vbl_end;
 
 	/*
-- 
1.7.10.4