[PATCH v2 4/4] drm/dp: Allow registering AUX channels as I2C busses

[Thierry Reding <thierry.reding@gmail.com>]

Implements an I2C-over-AUX I2C adapter on top of the generic drm_dp_aux
infrastructure. It extracts the retry logic from existing drivers, which
should help in porting those drivers to this new helper.

Signed-off-by: Thierry Reding <treding@nvidia.com>
---
 drivers/gpu/drm/drm_dp_helper.c | 178 ++++++++++++++++++++++++++++++++++++++++
 include/drm/drm_dp_helper.h     |   4 +
 2 files changed, 182 insertions(+)

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index 01a8173c6e83..8a64cf8ac8cc 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -555,3 +555,181 @@ int drm_dp_link_power_up(struct drm_dp_aux *aux, struct drm_dp_link *link)
 
 	return 0;
 }
+
+/*
+ * I2C-over-AUX implementation
+ */
+
+struct drm_dp_i2c_adapter {
+	struct i2c_adapter adapter;
+	struct drm_dp_aux *aux;
+};
+
+static u32 drm_dp_i2c_functionality(struct i2c_adapter *adapter)
+{
+	return I2C_FUNC_I2C | I2C_FUNC_SMBUS_EMUL |
+	       I2C_FUNC_SMBUS_READ_BLOCK_DATA |
+	       I2C_FUNC_SMBUS_BLOCK_PROC_CALL |
+	       I2C_FUNC_10BIT_ADDR;
+}
+
+/*
+ * Transfer a single I2C-over-AUX message and handle various error conditions,
+ * retrying the transaction as appropriate.
+ */
+static int drm_dp_i2c_do_msg(struct drm_dp_aux *aux, struct drm_dp_aux_msg *msg)
+{
+	unsigned int retry;
+	int err;
+
+	/*
+	 * 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.
+	 */
+	for (retry = 0; retry < 7; retry++) {
+		err = aux->transfer(aux, msg);
+		if (err < 0)
+			return err;
+
+		switch (msg->reply & DP_AUX_NATIVE_REPLY_MASK) {
+		case DP_AUX_NATIVE_REPLY_ACK:
+			/*
+			 * For I2C-over-AUX transactions this isn't enough, we
+			 * need to check for the I2C ACK reply.
+			 */
+			break;
+
+		case DP_AUX_NATIVE_REPLY_NACK:
+			return -EREMOTEIO;
+
+		case DP_AUX_NATIVE_REPLY_DEFER:
+			/*
+			 * We could check for I2C bitrate capabilities and if
+			 * available adjust this interval. We could also be
+			 * more careful with DP-to-legacy adapters where a
+			 * long legacy cable may forc very low I2C bit rates.
+			 *
+			 * For now just defer for long enough to hopefully be
+			 * safe for all use-cases.
+			 */
+			usleep_range(500, 600);
+			continue;
+
+		default:
+			DRM_ERROR("invalid native reply %#04x\n", msg->reply);
+			return -EREMOTEIO;
+		}
+
+		switch (msg->reply & DP_AUX_I2C_REPLY_MASK) {
+		case DP_AUX_I2C_REPLY_ACK:
+			/*
+			 * Both native ACK and I2C ACK replies received. We
+			 * can assume the transfer was successful.
+			 */
+			return 0;
+
+		case DP_AUX_I2C_REPLY_NACK:
+			return -EREMOTEIO;
+
+		case DP_AUX_I2C_REPLY_DEFER:
+			usleep_range(400, 500);
+			continue;
+
+		default:
+			DRM_ERROR("invalid I2C reply %#04x\n", msg->reply);
+			return -EREMOTEIO;
+		}
+	}
+
+	DRM_ERROR("too many retries, giving up\n");
+	return -EREMOTEIO;
+}
+
+static int drm_dp_i2c_xfer(struct i2c_adapter *adapter, struct i2c_msg *msgs,
+			   int num)
+{
+	struct drm_dp_i2c_adapter *dp = adapter->algo_data;
+	unsigned int i, j;
+
+	for (i = 0; i < num; i++) {
+		struct drm_dp_aux_msg msg;
+		int err;
+
+		/*
+		 * Many hardware implementations support FIFOs larger than a
+		 * single byte, but it has been empirically determined that
+		 * transferring data in larger chunks can actually lead to
+		 * decreased performance. Therefore each message is simply
+		 * transferred byte-by-byte.
+		 */
+		for (j = 0; j < msgs[i].len; j++) {
+			memset(&msg, 0, sizeof(msg));
+			msg.address = msgs[i].addr;
+
+			msg.request = (msgs[i].flags & I2C_M_RD) ?
+					DP_AUX_I2C_READ :
+					DP_AUX_I2C_WRITE;
+
+			/*
+			 * All messages except the last one are middle-of-
+			 * transfer messages.
+			 */
+			if (j < msgs[i].len - 1)
+				msg.request |= DP_AUX_I2C_MOT;
+
+			msg.buffer = msgs[i].buf + j;
+			msg.size = 1;
+
+			err = drm_dp_i2c_do_msg(dp->aux, &msg);
+			if (err < 0)
+				return err;
+		}
+	}
+
+	return num;
+}
+
+static const struct i2c_algorithm drm_dp_i2c_algo = {
+	.functionality = drm_dp_i2c_functionality,
+	.master_xfer = drm_dp_i2c_xfer,
+};
+
+/**
+ * drm_dp_add_i2c_bus() - register an I2C adapter for I2C-over-AUX access
+ * @aux: DisplayPort AUX channel
+ *
+ * Returns a pointer to an I2C adapter on success or an ERR_PTR()-encoded
+ * error code on failure.
+ */
+struct i2c_adapter *drm_dp_add_i2c_bus(struct drm_dp_aux *aux)
+{
+	struct drm_dp_i2c_adapter *adapter;
+	int err;
+
+	adapter = kzalloc(sizeof(*adapter), GFP_KERNEL);
+	if (!adapter)
+		return ERR_PTR(-ENOMEM);
+
+	adapter->adapter.algo = &drm_dp_i2c_algo;
+	adapter->adapter.algo_data = adapter;
+	adapter->adapter.retries = 3;
+	adapter->aux = aux;
+
+	adapter->adapter.class = I2C_CLASS_DDC;
+	adapter->adapter.owner = THIS_MODULE;
+	adapter->adapter.dev.parent = aux->dev;
+	adapter->adapter.dev.of_node = aux->dev->of_node;
+
+	strncpy(adapter->adapter.name, dev_name(aux->dev),
+		sizeof(adapter->adapter.name));
+
+	err = i2c_add_adapter(&adapter->adapter);
+	if (err < 0) {
+		kfree(adapter);
+		return ERR_PTR(err);
+	}
+
+	return &adapter->adapter;
+}
+EXPORT_SYMBOL(drm_dp_add_i2c_bus);
diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
index bad9ee11a2e2..c4acdbc2a172 100644
--- a/include/drm/drm_dp_helper.h
+++ b/include/drm/drm_dp_helper.h
@@ -423,6 +423,8 @@ struct drm_dp_aux_msg {
  * @transfer: transfers a message representing a single AUX transaction
  */
 struct drm_dp_aux {
+	struct device *dev;
+
 	ssize_t (*transfer)(struct drm_dp_aux *aux,
 			    struct drm_dp_aux_msg *msg);
 };
@@ -494,4 +496,6 @@ int drm_dp_link_probe(struct drm_dp_aux *aux, struct drm_dp_link *link);
  */
 int drm_dp_link_power_up(struct drm_dp_aux *aux, struct drm_dp_link *link);
 
+struct i2c_adapter *drm_dp_add_i2c_bus(struct drm_dp_aux *aux);
+
 #endif /* _DRM_DP_HELPER_H_ */
-- 
1.8.4.2