[openwrt/openwrt] ath79: mikrotik: fix beeper phantom noise on RB912

[LEDE Commits]

xback pushed a commit to openwrt/openwrt.git, branch openwrt-21.02:
https://git.openwrt.org/0ad49d368b33de12a20ee5c9adcf0273aac891fa

commit 0ad49d368b33de12a20ee5c9adcf0273aac891fa
Author: Koen Vandeputte <koen.vandeputte at ncentric.com>
AuthorDate: Mon Jun 28 10:57:21 2021 +0200

    ath79: mikrotik: fix beeper phantom noise on RB912
    
    Analysis done by Denis Kalashnikov:
    
    It seems that some ROS versions on some routerboard models have this bug:
    after silence boot (no output to uart, no beeps) beeper clicks when wireless traffic is.
    
    https://forum.mikrotik.com/viewtopic.php?f=3&t=92269
    https://forum.mikrotik.com/viewtopic.php?t=63399
    
    From these links:
    
    1)
    Hello, I have RB951G-2HnD and I noticed strange thing
    when I loaded the device with some wireless traffic it
    produced strange sound - like hissing, fizzing etc.
    
    2)
    Same problem still on 6.33, with silent boot enabled
    I hear buzzing noise on wireless load.
    
    3)
    The sound is fixed in v5.19, it was a bug that caused beeper to make clicks.
    
    It also got fixed in RouterOS:
    
    * What's new in 5.19 (2012-Jul-16 10:51):
    fix ticking sound on RB411UAHL;
    
    * What's new in 6.38.3 (2017-Feb-07 09:52):
    rb3011 - fixed noise from buzzer after silent boot;
    
    I've checked with an oscilloscope that:
    * When on the ssr beeper pin is 0,
      on the beeper itself is 1 (~5V),
      and when on the ssr beeper pin is 1,
      on the beeper is 0
      The beeper doesn't consume power,
      so 1 should be a default/idle value for the ssr beeper pin).
    * When there is wireless traffic (ping packets)
      in the background and the beeper clicks, I see
      pulses on the beeper itself,
      but no pulses on the ssr beeper pin (Q5 pin of 74hc595).
      When I manually toggle the ssr beeper pin I see pulses on both.
      So, it is likely that the phantom beeper clicks are caused by the EMI.
    
    Suggested-by: Denis Kalashnikov <denis281089 at gmail.com>
    Reviewed-by: Sergey Ryazanov <ryazanov.s.a at gmail.com>
    Signed-off-by: Koen Vandeputte <koen.vandeputte at ncentric.com>
    (cherry picked from commit a58bcc9e673db3c6aa39f2089d216d51c8356418)
---
 target/linux/ath79/dts/ar9342_mikrotik_routerboard-912uag-2hpnd.dts | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/target/linux/ath79/dts/ar9342_mikrotik_routerboard-912uag-2hpnd.dts b/target/linux/ath79/dts/ar9342_mikrotik_routerboard-912uag-2hpnd.dts
index cc5553acda..ee2c12b4e0 100644
--- a/target/linux/ath79/dts/ar9342_mikrotik_routerboard-912uag-2hpnd.dts
+++ b/target/linux/ath79/dts/ar9342_mikrotik_routerboard-912uag-2hpnd.dts
@@ -107,6 +107,12 @@
 	gpio-export {
 		compatible = "gpio-export";
 
+		beeper {
+			gpio-export,name = "beeper";
+			gpio-export,output = <1>;	/* Must be 1 to avoid EMI induced clicking noise */
+			gpios = <&ssr 5 GPIO_ACTIVE_HIGH>;
+		};
+
 		usb_power {
 			gpio-export,name = "power-usb";
 			gpio-export,output = <1>;