ath79: reduce spi-max-frequency to 50 MHz

The introduction of ebf0d8dadeca ("ath79: add new ar934x spi driver")
made the SPI memory unusable on devices with very high spi-max-frequency
(104 MHz).

Here's how the actual clock is calculated: (AHB_CLK/((CLOCK_DIVIDER+1)*2))
where AHB_CLK is a fixed clock (e.g. 200MHz on AR9331) and CLOCK_DIVIDER
is the parameter we can set. Highest clock according to this formula is
AHB_CLK/2 (100MHz, but that didn't work in device tests).
The next possible value is AHB_CLK/4 (50MHz). Speeds between 50 MHz and
100 MHz will be rounded down, so using values higher than 50 MHz does
not provide any benefit.

Consequently, this patch reduces spi-max-frequency for all devices with
values higher than 50 MHz to 50 MHz (effectively, this only affects
devices with 104 MHz before this patch).

Tested on GL.inet GL-AR150:
Boot fails with 104 MHz but is successful with both 50 MHz and 80 MHz
(fast-read), where the latter two yield identical read speeds.

Fixes: ebf0d8dadeca ("ath79: add new ar934x spi driver")

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

1 files changed, 1 insertions, 1 deletions

diff --git a/target/linux/ath79/dts/ar9331_embeddedwireless_dorin.dts b/target/linux/ath79/dts/ar9331_embeddedwireless_dorin.dts
index 3489a972d4..4761b91093 100644
--- a/target/linux/ath79/dts/ar9331_embeddedwireless_dorin.dts
+++ b/target/linux/ath79/dts/ar9331_embeddedwireless_dorin.dts
@@ -68,7 +68,7 @@
 
 	flash@0 {
 		compatible = "jedec,spi-nor";
-		spi-max-frequency = <104000000>;
+		spi-max-frequency = <50000000>;
 		reg = <0>;
 
 		partitions {