ARM: dts: at91-sama5d27_som1: Set sst26vf064b SPI NOR flash at its ma…

…ximum frequency

sama5d27-som1 populates an sst26vf064b SPI NOR flash. Its maximum
operating frequency for 2.7-3.6V is 104 MHz. As the flash is operated
at 3.3V, increase its maximum supported frequency to 104MHz. The
increasing of the spi-max-frequency value requires the setting of the
"CE# Not Active Hold Time", thus set the spi-cs-setup-ns to a value of 7.

The sst26vf064b datasheet specifies just a minimum value for the
"CE# Not Active Hold Time" and it advertises it to 5 ns. There's no
maximum time specified. I determined experimentally that 5 ns for the
spi-cs-setup-ns is not enough when the flash is operated close to its
maximum frequency and tests showed that 7 ns is just fine, so set the
spi-cs-setup-ns dt property to 7.

With the increase of frequency the reads are now faster with ~37%.

Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Link: https://lore.kernel.org/r/20230328101517.1595738-3-tudor.ambarus@linaro.org
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>

arch/arm/boot/dts/at91-sama5d27_som1.dtsi

@@ -43,7 +43,8 @@
 					#size-cells = <1>;
 					compatible = "jedec,spi-nor";
 					reg = <0>;
-					spi-max-frequency = <80000000>;
+					spi-max-frequency = <104000000>;
+					spi-cs-setup-ns = <7>;
 					spi-tx-bus-width = <4>;
 					spi-rx-bus-width = <4>;
 					m25p,fast-read;