can: mcp251xfd: regmap: optimizing transfer size for CRC transfers size 1

For CRC transfers with size 1 it is more efficient to use the
write_safe command instead of the write_crc command. This saves the
length byte on the SPI transfer.

changes since v1: https://lore.kernel.org/all/20230127124258.2764-1-thomas.kopp@microchip.com
- change logic to remove 1 level of indention

Link: https://lore.kernel.org/all/20230202141811.2581795-1-mkl@pengutronix.deSigned-off-by: Thomas Kopp <thomas.kopp@microchip.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/spi/mcp251xfd/mcp251xfd-ring.c

@@ -30,11 +30,23 @@ mcp251xfd_cmd_prepare_write_reg(const struct mcp251xfd_priv *priv,
 	last_byte = mcp251xfd_last_byte_set(mask);
 	len = last_byte - first_byte + 1;
 
-	data = mcp251xfd_spi_cmd_write(priv, write_reg_buf, reg + first_byte);
+	data = mcp251xfd_spi_cmd_write(priv, write_reg_buf, reg + first_byte, len);
 	val_le32 = cpu_to_le32(val >> BITS_PER_BYTE * first_byte);
 	memcpy(data, &val_le32, len);
 
-	if (priv->devtype_data.quirks & MCP251XFD_QUIRK_CRC_REG) {
+	if (!(priv->devtype_data.quirks & MCP251XFD_QUIRK_CRC_REG)) {
+		len += sizeof(write_reg_buf->nocrc.cmd);
+	} else if (len == 1) {
+		u16 crc;
+
+		/* CRC */
+		len += sizeof(write_reg_buf->safe.cmd);
+		crc = mcp251xfd_crc16_compute(&write_reg_buf->safe, len);
+		put_unaligned_be16(crc, (void *)write_reg_buf + len);
+
+		/* Total length */
+		len += sizeof(write_reg_buf->safe.crc);
+	} else {
 		u16 crc;
 
 		mcp251xfd_spi_cmd_crc_set_len_in_reg(&write_reg_buf->crc.cmd,
@@ -46,8 +58,6 @@ mcp251xfd_cmd_prepare_write_reg(const struct mcp251xfd_priv *priv,
 
 		/* Total length */
 		len += sizeof(write_reg_buf->crc.crc);
-	} else {
-		len += sizeof(write_reg_buf->nocrc.cmd);
 	}
 
 	return len;

drivers/net/can/spi/mcp251xfd/mcp251xfd.h

@@ -504,6 +504,11 @@ union mcp251xfd_write_reg_buf {
 		u8 data[4];
 		__be16 crc;
 	} crc;
+	struct __packed {
+		struct mcp251xfd_buf_cmd cmd;
+		u8 data[1];
+		__be16 crc;
+	} safe;
 } ____cacheline_aligned;
 
 struct mcp251xfd_tx_obj {
@@ -758,6 +763,13 @@ mcp251xfd_spi_cmd_write_crc_set_addr(struct mcp251xfd_buf_cmd_crc *cmd,
 	cmd->cmd = cpu_to_be16(MCP251XFD_SPI_INSTRUCTION_WRITE_CRC | addr);
 }
 
+static inline void
+mcp251xfd_spi_cmd_write_safe_set_addr(struct mcp251xfd_buf_cmd *cmd,
+				     u16 addr)
+{
+	cmd->cmd = cpu_to_be16(MCP251XFD_SPI_INSTRUCTION_WRITE_CRC_SAFE | addr);
+}
+
 static inline void
 mcp251xfd_spi_cmd_write_crc(struct mcp251xfd_buf_cmd_crc *cmd,
 			    u16 addr, u16 len)
@@ -769,14 +781,20 @@ mcp251xfd_spi_cmd_write_crc(struct mcp251xfd_buf_cmd_crc *cmd,
 static inline u8 *
 mcp251xfd_spi_cmd_write(const struct mcp251xfd_priv *priv,
 			union mcp251xfd_write_reg_buf *write_reg_buf,
-			u16 addr)
+			u16 addr, u8 len)
 {
 	u8 *data;
 
 	if (priv->devtype_data.quirks & MCP251XFD_QUIRK_CRC_REG) {
-		mcp251xfd_spi_cmd_write_crc_set_addr(&write_reg_buf->crc.cmd,
-						     addr);
-		data = write_reg_buf->crc.data;
+		if (len == 1) {
+			mcp251xfd_spi_cmd_write_safe_set_addr(&write_reg_buf->safe.cmd,
+							     addr);
+			data = write_reg_buf->safe.data;
+		} else {
+			mcp251xfd_spi_cmd_write_crc_set_addr(&write_reg_buf->crc.cmd,
+							     addr);
+			data = write_reg_buf->crc.data;
+		}
 	} else {
 		mcp251xfd_spi_cmd_write_nocrc(&write_reg_buf->nocrc.cmd,
 					      addr);