1 files changed, 82 insertions, 2 deletions

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index 7069e54e5b43..214a4c649d56 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -424,6 +424,81 @@ static u32 drm_dp_i2c_functionality(struct i2c_adapter *adapter)
                I2C_FUNC_10BIT_ADDR;
 }
 
+#define AUX_PRECHARGE_LEN 10 /* 10 to 16 */
+#define AUX_SYNC_LEN (16 + 4) /* preamble + AUX_SYNC_END */
+#define AUX_STOP_LEN 4
+#define AUX_CMD_LEN 4
+#define AUX_ADDRESS_LEN 20
+#define AUX_REPLY_PAD_LEN 4
+#define AUX_LENGTH_LEN 8
+
+/*
+ * Calculate the duration of the AUX request/reply in usec. Gives the
+ * "best" case estimate, ie. successful while as short as possible.
+ */
+static int drm_dp_aux_req_duration(const struct drm_dp_aux_msg *msg)
+{
+        int len = AUX_PRECHARGE_LEN + AUX_SYNC_LEN + AUX_STOP_LEN +
+                AUX_CMD_LEN + AUX_ADDRESS_LEN + AUX_LENGTH_LEN;
+
+        if ((msg->request & DP_AUX_I2C_READ) == 0)
+                len += msg->size * 8;
+
+        return len;
+}
+
+static int drm_dp_aux_reply_duration(const struct drm_dp_aux_msg *msg)
+{
+        int len = AUX_PRECHARGE_LEN + AUX_SYNC_LEN + AUX_STOP_LEN +
+                AUX_CMD_LEN + AUX_REPLY_PAD_LEN;
+
+        /*
+         * For read we expect what was asked. For writes there will
+         * be 0 or 1 data bytes. Assume 0 for the "best" case.
+         */
+        if (msg->request & DP_AUX_I2C_READ)
+                len += msg->size * 8;
+
+        return len;
+}
+
+#define I2C_START_LEN 1
+#define I2C_STOP_LEN 1
+#define I2C_ADDR_LEN 9 /* ADDRESS + R/W + ACK/NACK */
+#define I2C_DATA_LEN 9 /* DATA + ACK/NACK */
+
+/*
+ * Calculate the length of the i2c transfer in usec, assuming
+ * the i2c bus speed is as specified. Gives the the "worst"
+ * case estimate, ie. successful while as long as possible.
+ * Doesn't account the the "MOT" bit, and instead assumes each
+ * message includes a START, ADDRESS and STOP. Neither does it
+ * account for additional random variables such as clock stretching.
+ */
+static int drm_dp_i2c_msg_duration(const struct drm_dp_aux_msg *msg,
+                                   int i2c_speed_khz)
+{
+        /* AUX bitrate is 1MHz, i2c bitrate as specified */
+        return DIV_ROUND_UP((I2C_START_LEN + I2C_ADDR_LEN +
+                             msg->size * I2C_DATA_LEN +
+                             I2C_STOP_LEN) * 1000, i2c_speed_khz);
+}
+
+/*
+ * Deterine how many retries should be attempted to successfully transfer
+ * the specified message, based on the estimated durations of the
+ * i2c and AUX transfers.
+ */
+static int drm_dp_i2c_retry_count(const struct drm_dp_aux_msg *msg,
+                              int i2c_speed_khz)
+{
+        int aux_time_us = drm_dp_aux_req_duration(msg) +
+                drm_dp_aux_reply_duration(msg);
+        int i2c_time_us = drm_dp_i2c_msg_duration(msg, i2c_speed_khz);
+
+        return DIV_ROUND_UP(i2c_time_us, aux_time_us + AUX_RETRY_INTERVAL);
+}
+
 /*
  * Transfer a single I2C-over-AUX message and handle various error conditions,
  * retrying the transaction as appropriate.  It is assumed that the
@@ -436,13 +511,18 @@ static int drm_dp_i2c_do_msg(struct drm_dp_aux *aux, struct drm_dp_aux_msg *msg)
 {
         unsigned int retry, defer_i2c;
         int ret;
-
         /*
          * DP1.2 sections 2.7.7.1.5.6.1 and 2.7.7.1.6.6.1: A DP Source device
          * is required to retry at least seven times upon receiving AUX_DEFER
          * before giving up the AUX transaction.
+         *
+         * We also try to account for the i2c bus speed.
+         * FIXME currently assumes 10 kHz as some real world devices seem
+         * to require it. We should query/set the speed via DPCD if supported.
          */
-        for (retry = 0, defer_i2c = 0; retry < (7 + defer_i2c); retry++) {
+        int max_retries = max(7, drm_dp_i2c_retry_count(msg, 10));
+
+        for (retry = 0, defer_i2c = 0; retry < (max_retries + defer_i2c); retry++) {
                 mutex_lock(&aux->hw_mutex);
                 ret = aux->transfer(aux, msg);
                 mutex_unlock(&aux->hw_mutex);