drm/intel: Push get_scanout_position() timestamping into kms driver.

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).

v2: Fix formatting of new multi-line code comments.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
index eb67bd98bc42dc9d7023110b3bbdec292d197fe9..a2274c713273f0a67e449fccecbf0e815319a636 100644 (file)
--- 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 +682,27 @@ 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 +711,7 @@ 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 +720,7 @@ 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 +728,14 @@ 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;
 
        /*