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From:   Tudor Ambarus <tudor.ambarus@linaro.org>
To:     nicolas.ferre@microchip.com, claudiu.beznea@microchip.com
Cc:     alexandre.belloni@bootlin.com, robh+dt@kernel.org,
        krzysztof.kozlowski+dt@linaro.org, devicetree@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        tudor.ambarus@linaro.org,
        Tudor Ambarus <tudor.ambarus@microchip.com>
Subject: [PATCH v3 2/4] ARM: dts: at91-sama5d27_som1: Set sst26vf064b SPI NOR flash at its maximum frequency
Date:   Tue, 28 Mar 2023 10:15:15 +0000
Message-Id: <20230328101517.1595738-3-tudor.ambarus@linaro.org>
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From: Tudor Ambarus <tudor.ambarus@microchip.com>

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>
---
 arch/arm/boot/dts/at91-sama5d27_som1.dtsi | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/arm/boot/dts/at91-sama5d27_som1.dtsi b/arch/arm/boot/dts/at91-sama5d27_som1.dtsi
index 8aa9e8dea337..95ecb7d040a8 100644
--- a/arch/arm/boot/dts/at91-sama5d27_som1.dtsi
+++ b/arch/arm/boot/dts/at91-sama5d27_som1.dtsi
@@ -43,7 +43,8 @@ flash@0 {
 					#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;
-- 
2.40.0.348.gf938b09366-goog