mtd: spi-nor: micron-st: write 2 bytes when disabling Octal DTR mode

The Octal DTR configuration is stored in the CFR0V register. This
register is 1 byte wide. But 1 byte long transactions are not allowed in
8D-8D-8D mode. The next byte address contains the CFR1V register, which
contains the number of dummy cycles. This is very fortunate since the
enable path changes the value of this register. Reset the value to its
default when disabling Octal DTR mode. This way, both changes to the
flash state made when enabling can be reverted in one single
transaction.
Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20210531181757.19458-4-p.yadav@ti.com

drivers/mtd/spi-nor/micron-st.c

@@ -13,6 +13,7 @@
 #define SPINOR_OP_MT_WR_ANY_REG	0x81	/* Write volatile register */
 #define SPINOR_REG_MT_CFR0V	0x00	/* For setting octal DTR mode */
 #define SPINOR_REG_MT_CFR1V	0x01	/* For setting dummy cycles */
+#define SPINOR_REG_MT_CFR1V_DEF	0x1f	/* Default dummy cycles */
 #define SPINOR_MT_OCT_DTR	0xe7	/* Enable Octal DTR. */
 #define SPINOR_MT_EXSPI		0xff	/* Enable Extended SPI (default) */
 
@@ -48,17 +49,28 @@ static int spi_nor_micron_octal_dtr_enable(struct spi_nor *nor, bool enable)
 	if (ret)
 		return ret;
 
-	if (enable)
-		*buf = SPINOR_MT_OCT_DTR;
-	else
-		*buf = SPINOR_MT_EXSPI;
+	if (enable) {
+		buf[0] = SPINOR_MT_OCT_DTR;
+	} else {
+		/*
+		 * The register is 1-byte wide, but 1-byte transactions are not
+		 * allowed in 8D-8D-8D mode. The next register is the dummy
+		 * cycle configuration register. Since the transaction needs to
+		 * be at least 2 bytes wide, set the next register to its
+		 * default value. This also makes sense because the value was
+		 * changed when enabling 8D-8D-8D mode, it should be reset when
+		 * disabling.
+		 */
+		buf[0] = SPINOR_MT_EXSPI;
+		buf[1] = SPINOR_REG_MT_CFR1V_DEF;
+	}
 
 	op = (struct spi_mem_op)
 		SPI_MEM_OP(SPI_MEM_OP_CMD(SPINOR_OP_MT_WR_ANY_REG, 1),
 			   SPI_MEM_OP_ADDR(enable ? 3 : 4,
 					   SPINOR_REG_MT_CFR0V, 1),
 			   SPI_MEM_OP_NO_DUMMY,
-			   SPI_MEM_OP_DATA_OUT(1, buf, 1));
+			   SPI_MEM_OP_DATA_OUT(enable ? 1 : 2, buf, 1));
 
 	if (!enable)
 		spi_nor_spimem_setup_op(nor, &op, SNOR_PROTO_8_8_8_DTR);