spi: cadence-qspi: minimise register accesses on each op if !DTR

cqspi_enable_dtr() is called for each operation, commands or not, reads
or writes. It writes CQSPI_REG_CONFIG then waits for idle (three
successful reads). Skip that in the no-DTR case if DTR is already
disabled.

It cannot be skipped in the DTR case as cqspi_setup_opcode_ext() writes
to a register and we must wait for idle state.

According to ftrace, the average cqspi_exec_mem_op() call goes from
85.4µs to 83.6µs when reading 235M over UBIFS on an octal flash.
Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
Link: https://msgid.link/r/20240405-cdns-qspi-mbly-v2-6-956679866d6d@bootlin.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-cadence-quadspi.c

@@ -491,8 +491,11 @@ static int cqspi_enable_dtr(struct cqspi_flash_pdata *f_pdata,
 		if (ret)
 			return ret;
 	} else {
-		reg &= ~CQSPI_REG_CONFIG_DTR_PROTO;
-		reg &= ~CQSPI_REG_CONFIG_DUAL_OPCODE;
+		unsigned int mask = CQSPI_REG_CONFIG_DTR_PROTO | CQSPI_REG_CONFIG_DUAL_OPCODE;
+		/* Shortcut if DTR is already disabled. */
+		if ((reg & mask) == 0)
+			return 0;
+		reg &= ~mask;
 	}
 
 	writel(reg, reg_base + CQSPI_REG_CONFIG);