[Intel-gfx] [PATCH] drm/i915/skl: implement DP Aux Mutex framework

[Wayne Boyer]

Beginning with SKL the DP Aux channel communication can be protected
using a built in HW mutex.

When PSR is enablabled the HW takes control on AUX and uses it to
control panel exit/entry states.

When validating PSR with automated tests, grabbing CRC from sink
revealed strange aux communication issues.  Aux reads were returning
a message read size equal to 0 and 0 is a forbidden message.

By using the HW mutex the HW is blocked from using aux when running
the automated PSR tests.

This patch provides an initial implementation for using that mutex.
The use is currently limited to protecting the sink crc request based
on feedback from the H/W designers indicating that using the mutex
for all aux channel communication is not recommended.

v2: Improved commit message to explain the case where the HW mutex is
helpful.  Also added bug reference.

Signed-off-by: Wayne Boyer <wayne.boyer at intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91437
Reviewed-by: Rodrigo Vivi <rodrigo.vivi at intel.com>
Tested-by: Rodrigo Vivi <rodrigo.vivi at intel.com>
---
 drivers/gpu/drm/i915/i915_drv.h |  1 +
 drivers/gpu/drm/i915/i915_reg.h |  5 ++++
 drivers/gpu/drm/i915/intel_dp.c | 52 ++++++++++++++++++++++++++++++++++++++++-
 3 files changed, 57 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index b12594b..98e991d 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -2604,6 +2604,7 @@ struct drm_i915_cmd_table {
 
 #define HAS_DDI(dev)		(INTEL_INFO(dev)->has_ddi)
 #define HAS_FPGA_DBG_UNCLAIMED(dev)	(INTEL_INFO(dev)->has_fpga_dbg)
+#define HAS_AUX_MUTEX(dev)	(INTEL_INFO(dev)->gen >= 9)
 #define HAS_PSR(dev)		(IS_HASWELL(dev) || IS_BROADWELL(dev) || \
 				 IS_VALLEYVIEW(dev) || IS_CHERRYVIEW(dev) || \
 				 IS_SKYLAKE(dev) || IS_KABYLAKE(dev))
diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
index 8942532..a033e70 100644
--- a/drivers/gpu/drm/i915/i915_reg.h
+++ b/drivers/gpu/drm/i915/i915_reg.h
@@ -4288,6 +4288,11 @@ enum skl_disp_power_wells {
 #define   DP_AUX_CH_CTL_FW_SYNC_PULSE_SKL(c) (((c) - 1) << 5)
 #define   DP_AUX_CH_CTL_SYNC_PULSE_SKL(c)   ((c) - 1)
 
+#define DP_AUX_MUTEX_A			0x6402C
+#define DP_AUX_MUTEX_B			0x6412C
+#define   DP_AUX_MUTEX_ENABLE		    (1 << 31)
+#define   DP_AUX_MUTEX_STATUS		    (1 << 30)
+
 /*
  * Computing GMCH M and N values for the Display Port link
  *
diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index 2fad873..e9e1239 100644
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -781,6 +781,47 @@ static uint32_t skl_get_aux_send_ctl(struct intel_dp *intel_dp,
 	       DP_AUX_CH_CTL_SYNC_PULSE_SKL(32);
 }
 
+static bool skl_aux_mutex(struct intel_dp *intel_dp, bool get)
+{
+	struct intel_digital_port *intel_dig_port = dp_to_dig_port(intel_dp);
+	struct drm_device *dev = intel_dig_port->base.base.dev;
+	struct drm_i915_private *dev_priv = dev->dev_private;
+	uint32_t aux_ch_mutex, status;
+	int count = 0;
+
+	if (!HAS_AUX_MUTEX(dev))
+		return false;
+
+	/*
+	 * FIXME: determine actual aux channel
+	 * Hard coded to channel A for now to protect sink crc requests on eDP.
+	 */
+	aux_ch_mutex = DP_AUX_MUTEX_A;
+
+	if (!get) {
+		I915_WRITE(aux_ch_mutex, DP_AUX_MUTEX_ENABLE | DP_AUX_MUTEX_STATUS);
+		return false;
+	}
+
+	/*
+	 * The Bspec specifies waiting 500us between attempts to acquire the
+	 * mutex.  Ten retries should be adequate to balance successfully
+	 * acquirng the mutex and spending too much time trying.
+	 */
+	while (count++ < 10) {
+		I915_WRITE(aux_ch_mutex, DP_AUX_MUTEX_ENABLE);
+		status = I915_READ(aux_ch_mutex);
+		if (!(status & DP_AUX_MUTEX_STATUS))
+			return true;
+		udelay(500);
+	}
+
+	return false;
+}
+
+#define skl_aux_mutex_get(dev) skl_aux_mutex(dev, true)
+#define skl_aux_mutex_put(dev) skl_aux_mutex(dev, false)
+
 static int
 intel_dp_aux_ch(struct intel_dp *intel_dp,
 		const uint8_t *send, int send_bytes,
@@ -4188,10 +4229,14 @@ int intel_dp_sink_crc(struct intel_dp *intel_dp, u8 *crc)
 	u8 buf;
 	int count, ret;
 	int attempts = 6;
+	bool aux_mutex_acquired = false;
+
+	aux_mutex_acquired = skl_aux_mutex_get(intel_dp);
 
 	ret = intel_dp_sink_crc_start(intel_dp);
+
 	if (ret)
-		return ret;
+		goto release;
 
 	do {
 		intel_wait_for_vblank(dev, intel_crtc->pipe);
@@ -4218,6 +4263,11 @@ int intel_dp_sink_crc(struct intel_dp *intel_dp, u8 *crc)
 
 stop:
 	intel_dp_sink_crc_stop(intel_dp);
+
+release:
+	if (aux_mutex_acquired)
+		aux_mutex_acquired = skl_aux_mutex_put(intel_dp);
+
 	return ret;
 }
 
-- 
2.5.0