drivers: i2c_nrfx_twim: Use concatenation buffer by default

Issue an error logging message when the i2c_nrfx_twim driver lacks
a concatenation buffer big enough to properly handle a call to
i2c_burst_write() function, to give the user a hint what is wrong.

Also use by default a 16-bytes long concatenation buffer for every
instance of the i2c_nrfx_twim driver. Such value should cover most
of the simple uses of the i2c_burst_write() function, like those
in the stmemsc sensor drivers, and when a longer buffer is needed,
the user will be provided with the above message pointing to the
property that should be adjusted.

Signed-off-by: Andrzej Głąbek <[email protected]>

Author: anangl

drivers/i2c/i2c_nrfx_twim.c

@@ -65,26 +65,31 @@ static int i2c_nrfx_twim_transfer(const struct device *dev,
 			break;
 		}
 
-		/* Merge this fragment with the next if we have a buffer, this
-		 * isn't the last fragment, it doesn't end a bus transaction,
-		 * the next one doesn't start a bus transaction, and the
-		 * direction of the next fragment is the same as this one.
+		/* This fragment needs to be merged with the next one if:
+		 * - it is not the last fragment
+		 * - it does not end a bus transaction
+		 * - the next fragment does not start a bus transaction
+		 * - the direction of the next fragment is the same as this one
 		 */
-		bool concat_next = (concat_buf_size > 0)
-			&& ((i + 1) < num_msgs)
-			&& !(msgs[i].flags & I2C_MSG_STOP)
-			&& !(msgs[i + 1].flags & I2C_MSG_RESTART)
-			&& ((msgs[i].flags & I2C_MSG_READ)
-			    == (msgs[i + 1].flags & I2C_MSG_READ));
+		bool concat_next = ((i + 1) < num_msgs)
+				&& !(msgs[i].flags & I2C_MSG_STOP)
+				&& !(msgs[i + 1].flags & I2C_MSG_RESTART)
+				&& ((msgs[i].flags & I2C_MSG_READ)
+				    == (msgs[i + 1].flags & I2C_MSG_READ));
 
 		/* If we need to concatenate the next message, or we've
 		 * already committed to concatenate this message, add it to
 		 * the buffer after verifying there's room.
 		 */
 		if (concat_next || (concat_len != 0)) {
 			if ((concat_len + msgs[i].len) > concat_buf_size) {
-				LOG_ERR("concat-buf overflow: %u + %u > %u",
-					concat_len, msgs[i].len, concat_buf_size);
+				LOG_ERR("Need to use concatenation buffer and "
+					"provided size is insufficient "
+					"(%u + %u > %u). "
+					"Adjust the zephyr,concat-buf-size "
+					"property in the \"%s\" node.",
+					concat_len, msgs[i].len,
+					concat_buf_size, dev->name);
 				ret = -ENOSPC;
 				break;
 			}

dts/bindings/i2c/nordic,nrf-twim.yaml

@@ -29,12 +29,14 @@ properties:
     zephyr,concat-buf-size:
         type: int
         required: false
-        description:
-            If concatenation buffer size is set, then multiple messages in the
-            same direction will be concatenated into single transfers as long
-            as there is space in buffer and no restart or stop flag is set.
+        default: 16
+        description: |
+            Size of a concatenation buffer that the driver is to use for merging
+            multiple same direction I2C messages that have no RESTART or STOP
+            flag between them (see e.g. the i2c_burst_write() function) into one
+            transfer on the bus.
 
             This property must be provided when interacting with devices like
             the SSD1306 display that cannot tolerate a repeated start and
-            address appearing on the bus between message fragments.  For many
+            address appearing on the bus between message fragments. For many
             devices a concatenation buffer is not necessary.