drm/i915/skl: implement DP Aux Mutex framework

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

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.

Cc Rodrigo Vivi <[email protected]>

Signed-off-by Wayne Boyer <[email protected]>

---
 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(-)

drivers/gpu/drm/i915/i915_drv.h +1 -0

index b12594b..98e991d 100644
@@ -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))

drivers/gpu/drm/i915/i915_reg.h +5 -0

index 8942532..a033e70 100644
@@ -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
  *

drivers/gpu/drm/i915/intel_dp.c +51 -1

index 2fad873..e9e1239 100644
@@ -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;
 }

Quoted 3 lines · Wayne Boyer

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

Quoted 4 lines

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.

Please explain in the commit message what benefits this brings, and why it's a good idea. Just that something *can* be done in hardware does not mean that we should do it.

Quoted 13 lines · Jani Nikula

Quoted 11 lines · Wayne Boyer

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

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.

Quoted 5 lines

Please explain in the commit message what benefits this brings, and why
it's a good idea. Just that something *can* be done in hardware does not
mean that we should do it.

I also have trouble understanding a mutex that only some things use. Seems
like a mutex needs to be used everywhere to provide locking.

Dave.