[RFC] drm/dp: move hw_mutex up the call stack

[Rob Clark]

1) don't let other threads trying to bang on aux channel interrupt the
defer timeout/logic
2) don't let other threads interrupt the i2c over aux logic

---
This wasn't actually fixing things w/ problematic monitor, but seems
like generally a good idea.  At least AFAIU you shouldn't allow the
sequence of transfers for i2c over aux be interrupted but unrelated
chatter.

This applies on top of the DPCD logging patch.

 drivers/gpu/drm/drm_dp_helper.c | 27 +++++++++++++++++----------
 1 file changed, 17 insertions(+), 10 deletions(-)

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index ccb1f8a..e409b5f 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -163,8 +163,6 @@ static ssize_t aux_transfer(struct drm_dp_aux *aux, struct drm_dp_aux_msg *msg)
 {
 	ssize_t ret;
 
-	mutex_lock(&aux->hw_mutex);
-
 	DRM_DEBUG_DPCD("%s: req=0x%02x, address=0x%05x, size=%zu\n", aux->name,
 			msg->request, msg->address, msg->size);
 
@@ -196,8 +194,6 @@ static ssize_t aux_transfer(struct drm_dp_aux *aux, struct drm_dp_aux_msg *msg)
 		}
 	}
 
-	mutex_unlock(&aux->hw_mutex);
-
 	return ret;
 }
 
@@ -220,7 +216,7 @@ static int drm_dp_dpcd_access(struct drm_dp_aux *aux, u8 request,
 {
 	struct drm_dp_aux_msg msg;
 	unsigned int retry;
-	int err;
+	int err = 0;
 
 	memset(&msg, 0, sizeof(msg));
 	msg.address = offset;
@@ -228,6 +224,8 @@ static int drm_dp_dpcd_access(struct drm_dp_aux *aux, u8 request,
 	msg.buffer = buffer;
 	msg.size = size;
 
+	mutex_lock(&aux->hw_mutex);
+
 	/*
 	 * The specification doesn't give any recommendation on how often to
 	 * retry native transactions. We used to retry 7 times like for
@@ -241,18 +239,19 @@ static int drm_dp_dpcd_access(struct drm_dp_aux *aux, u8 request,
 			if (err == -EBUSY)
 				continue;
 
-			return err;
+			goto unlock;
 		}
 
 		switch (msg.reply & DP_AUX_NATIVE_REPLY_MASK) {
 		case DP_AUX_NATIVE_REPLY_ACK:
 			if (err < size)
-				return -EPROTO;
-			return err;
+				err = -EPROTO;
+			goto unlock;
 
 		case DP_AUX_NATIVE_REPLY_NACK:
 			DRM_DEBUG_DPCD("native nack (result=%d, size=%zu)\n", err, msg.size);
-			return -EIO;
+			err = -EIO;
+			goto unlock;
 
 		case DP_AUX_NATIVE_REPLY_DEFER:
 			DRM_DEBUG_DPCD("native defer\n");
@@ -262,7 +261,11 @@ static int drm_dp_dpcd_access(struct drm_dp_aux *aux, u8 request,
 	}
 
 	DRM_ERROR("DPCD: too many retries, giving up!\n");
-	return -EIO;
+	err = -EIO;
+
+unlock:
+	mutex_unlock(&aux->hw_mutex);
+	return err;
 }
 
 /**
@@ -698,6 +701,8 @@ static int drm_dp_i2c_xfer(struct i2c_adapter *adapter, struct i2c_msg *msgs,
 
 	memset(&msg, 0, sizeof(msg));
 
+	mutex_lock(&aux->hw_mutex);
+
 	for (i = 0; i < num; i++) {
 		msg.address = msgs[i].addr;
 		msg.request = (msgs[i].flags & I2C_M_RD) ?
@@ -741,6 +746,8 @@ static int drm_dp_i2c_xfer(struct i2c_adapter *adapter, struct i2c_msg *msgs,
 	msg.size = 0;
 	(void)drm_dp_i2c_do_msg(aux, &msg);
 
+	mutex_unlock(&aux->hw_mutex);
+
 	return err;
 }
 
-- 
2.5.0