net: ethernet: oa_tc6: implement register read operation

Implement register read operation according to the control communication
specified in the OPEN Alliance 10BASE-T1x MACPHY Serial Interface
document. Control read commands are used by the SPI host to read
registers within the MAC-PHY. Each control read commands are composed of
a 32 bits control command header.

The MAC-PHY ignores all data from the SPI host following the control
header for the remainder of the control read command. Control read
commands can read either a single register or multiple consecutive
registers. When multiple consecutive registers are read, the address is
automatically post-incremented by the MAC-PHY. Reading any unimplemented
or undefined registers shall return zero.
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Parthiban Veerasooran <Parthiban.Veerasooran@microchip.com>
Link: https://patch.msgid.link/20240909082514.262942-4-Parthiban.Veerasooran@microchip.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/oa_tc6.c

@@ -38,6 +38,7 @@ enum oa_tc6_header_type {
 };
 
 enum oa_tc6_register_op {
+	OA_TC6_CTRL_REG_READ = 0,
 	OA_TC6_CTRL_REG_WRITE = 1,
 };
 
@@ -113,7 +114,8 @@ static void oa_tc6_prepare_ctrl_spi_buf(struct oa_tc6 *tc6, u32 address,
 
 	*tx_buf = oa_tc6_prepare_ctrl_header(address, length, reg_op);
 
-	oa_tc6_update_ctrl_write_data(tc6, value, length);
+	if (reg_op == OA_TC6_CTRL_REG_WRITE)
+		oa_tc6_update_ctrl_write_data(tc6, value, length);
 }
 
 static int oa_tc6_check_ctrl_write_reply(struct oa_tc6 *tc6, u8 size)
@@ -132,6 +134,30 @@ static int oa_tc6_check_ctrl_write_reply(struct oa_tc6 *tc6, u8 size)
 	return 0;
 }
 
+static int oa_tc6_check_ctrl_read_reply(struct oa_tc6 *tc6, u8 size)
+{
+	u32 *rx_buf = tc6->spi_ctrl_rx_buf + OA_TC6_CTRL_IGNORED_SIZE;
+	u32 *tx_buf = tc6->spi_ctrl_tx_buf;
+
+	/* The echoed control read header must match with the one that was
+	 * transmitted.
+	 */
+	if (*tx_buf != *rx_buf)
+		return -EPROTO;
+
+	return 0;
+}
+
+static void oa_tc6_copy_ctrl_read_data(struct oa_tc6 *tc6, u32 value[],
+				       u8 length)
+{
+	__be32 *rx_buf = tc6->spi_ctrl_rx_buf + OA_TC6_CTRL_IGNORED_SIZE +
+			 OA_TC6_CTRL_HEADER_SIZE;
+
+	for (int i = 0; i < length; i++)
+		value[i] = be32_to_cpu(*rx_buf++);
+}
+
 static int oa_tc6_perform_ctrl(struct oa_tc6 *tc6, u32 address, u32 value[],
 			       u8 length, enum oa_tc6_register_op reg_op)
 {
@@ -152,8 +178,62 @@ static int oa_tc6_perform_ctrl(struct oa_tc6 *tc6, u32 address, u32 value[],
 	}
 
 	/* Check echoed/received control write command reply for errors */
-	return oa_tc6_check_ctrl_write_reply(tc6, size);
+	if (reg_op == OA_TC6_CTRL_REG_WRITE)
+		return oa_tc6_check_ctrl_write_reply(tc6, size);
+
+	/* Check echoed/received control read command reply for errors */
+	ret = oa_tc6_check_ctrl_read_reply(tc6, size);
+	if (ret)
+		return ret;
+
+	oa_tc6_copy_ctrl_read_data(tc6, value, length);
+
+	return 0;
+}
+
+/**
+ * oa_tc6_read_registers - function for reading multiple consecutive registers.
+ * @tc6: oa_tc6 struct.
+ * @address: address of the first register to be read in the MAC-PHY.
+ * @value: values to be read from the starting register address @address.
+ * @length: number of consecutive registers to be read from @address.
+ *
+ * Maximum of 128 consecutive registers can be read starting at @address.
+ *
+ * Return: 0 on success otherwise failed.
+ */
+int oa_tc6_read_registers(struct oa_tc6 *tc6, u32 address, u32 value[],
+			  u8 length)
+{
+	int ret;
+
+	if (!length || length > OA_TC6_CTRL_MAX_REGISTERS) {
+		dev_err(&tc6->spi->dev, "Invalid register length parameter\n");
+		return -EINVAL;
+	}
+
+	mutex_lock(&tc6->spi_ctrl_lock);
+	ret = oa_tc6_perform_ctrl(tc6, address, value, length,
+				  OA_TC6_CTRL_REG_READ);
+	mutex_unlock(&tc6->spi_ctrl_lock);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(oa_tc6_read_registers);
+
+/**
+ * oa_tc6_read_register - function for reading a MAC-PHY register.
+ * @tc6: oa_tc6 struct.
+ * @address: register address of the MAC-PHY to be read.
+ * @value: value read from the @address register address of the MAC-PHY.
+ *
+ * Return: 0 on success otherwise failed.
+ */
+int oa_tc6_read_register(struct oa_tc6 *tc6, u32 address, u32 *value)
+{
+	return oa_tc6_read_registers(tc6, address, value, 1);
 }
+EXPORT_SYMBOL_GPL(oa_tc6_read_register);
 
 /**
  * oa_tc6_write_registers - function for writing multiple consecutive registers.

include/linux/oa_tc6.h

@@ -15,3 +15,6 @@ struct oa_tc6 *oa_tc6_init(struct spi_device *spi);
 int oa_tc6_write_register(struct oa_tc6 *tc6, u32 address, u32 value);
 int oa_tc6_write_registers(struct oa_tc6 *tc6, u32 address, u32 value[],
 			   u8 length);
+int oa_tc6_read_register(struct oa_tc6 *tc6, u32 address, u32 *value);
+int oa_tc6_read_registers(struct oa_tc6 *tc6, u32 address, u32 value[],
+			  u8 length);