[openwrt/openwrt] ramips: reduce spi-max-frequency for Xiaomi MI Router 4AG

[LEDE Commits]

hauke pushed a commit to openwrt/openwrt.git, branch openwrt-21.02:
https://git.openwrt.org/d33f4f856566bf7a5b44d6169fd673719bbdfcc6

commit d33f4f856566bf7a5b44d6169fd673719bbdfcc6
Author: David Bentham <db260179 at gmail.com>
AuthorDate: Sun Apr 4 13:08:43 2021 +0100

    ramips: reduce spi-max-frequency for Xiaomi MI Router 4AG
    
    Reduce spi-max-frequency for Xiaomi MI Router 4AG model
    
    Xiaomi MI Router 4AG MTD uses two flash chips (no specific on router versions when produced from factory) - GD25Q128C and W25Q128BV.
    
    These flash chips are capable of high frequency, but due to poor board design or manufacture process.
    
    We are seeing the following errors in the linux kernel bootup:
    
    `spi-nor spi0.0: unrecognized JEDEC id bytes: cc 60 1c cc 60 1c
     spi-nor: probe of spi0.0 failed with error -2`
    
    This causes the partitions not to be detected
    
    `VFS: Cannot open root device "(null)" or unknown-block(0,0): error -6`
    
    Then creates a bootloop and a bricked router.
    
    The solution to limit this race condition is to reduce the frequency from 80 mhz to 50 mhz.
    
    Signed-off-by: David Bentham <db260179 at gmail.com>
    (cherry picked from commit 17e690017d76c49070bc99d6a376b1926259c5ff)
---
 target/linux/ramips/dts/mt7621_xiaomi_mi-router-4a-3g-v2.dtsi | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/target/linux/ramips/dts/mt7621_xiaomi_mi-router-4a-3g-v2.dtsi b/target/linux/ramips/dts/mt7621_xiaomi_mi-router-4a-3g-v2.dtsi
index c86bdadf55..cc5abf09b6 100644
--- a/target/linux/ramips/dts/mt7621_xiaomi_mi-router-4a-3g-v2.dtsi
+++ b/target/linux/ramips/dts/mt7621_xiaomi_mi-router-4a-3g-v2.dtsi
@@ -49,7 +49,7 @@
 	flash at 0 {
 		compatible = "jedec,spi-nor";
 		reg = <0>;
-		spi-max-frequency = <80000000>;
+		spi-max-frequency = <50000000>;
 		m25p,fast-read;
 
 		partitions {