pi-bluetooth: Fix bluetooth data drop on rpi3 v1.2

The rpi3 v1.2 lacks hardware flow control for the uart used for
bluetooth. This fact coupled with an apparent change in scheduling
introduced between the 4.14 and 4.19 kernels made the SoC drop UART
data. See for details:
https://github.com/raspberrypi/firmware/issues/
1150#issuecomment-518654921

The workaround for this is therefore to lower the baudrate so that we
decrease the chance of having too much lost data.

This change here should be revisited when meta-raspberrypi switches to
kernel 5.x as I have tested the UART data drop on kernel 5.4.38 and did
notice way less data drop with the default 921600 baudrate and playing
audio over bt was a good experience overall with that kernel. (tested
using balenaSound v2.1.7).

Note that this change will only decrease the baudrate for bt equipped
rpi variants that lack hw flow control (i.e. just the rpi3 v1.2).
See for details:
https://github.com/raspberrypi/firmware/issues/
1150#issuecomment-603116313

Fixes: #476
Changelog-entry: Fix bluetooth data drop on rpi3 v1.2
Signed-off-by: Florin Sarbu <florin@balena.io>

layers/meta-balena-raspberrypi/recipes-connectivity/pi-bluetooth/files/0001-Lower-the-bt-uart-baudrate-to-460800-for-rpi3-v1.2.patch

@@ -0,0 +1,42 @@
+From 731ee9b752c3df73fb7ab7143fd99f81d35e1ef8 Mon Sep 17 00:00:00 2001
+From: Florin Sarbu <florin@balena.io>
+Date: Fri, 8 May 2020 16:38:31 +0200
+Subject: [PATCH] Lower the bt uart baudrate to 460800 for rpi3 v1.2
+
+The rpi3 v1.2 lacks hardware flow control for the uart used for
+bluetooth. This fact coupled with an apparent change in scheduling
+introduced between the 4.14 and 4.19 kernels made the SoC drop UART
+data. See for details:
+https://github.com/raspberrypi/firmware/issues/
+1150#issuecomment-518654921
+
+The workaround for this is therefore to lower the baudrate so that we
+decrease the chance of having too much lost data.
+
+This change here should be revisited when meta-raspberrypi switches to
+kernel 5.x as I have tested the UART data drop on kernel 5.4.38 and did
+notice way less data drop with the default 921600 baudrate and playing
+audio over bt was a good experience overall with that kernel.
+
+Upstream-Status: Pending
+Signed-off-by: Florin Sarbu <florin@balena.io>
+---
+ usr/bin/btuart | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/usr/bin/btuart b/usr/bin/btuart
+index eb8cf37..3b4c682 100755
+--- a/usr/bin/btuart
++++ b/usr/bin/btuart
+@@ -15,7 +15,7 @@ if [ "$uart0" = "$serial1" ] ; then
+ 	if [ "$uart0_pins" = "16" ] ; then
+ 		$HCIATTACH /dev/serial1 bcm43xx 3000000 flow - $BDADDR
+ 	else
+-		$HCIATTACH /dev/serial1 bcm43xx 921600 noflow - $BDADDR
++		$HCIATTACH /dev/serial1 bcm43xx 460800 noflow - $BDADDR
+ 	fi
+ else
+ 	$HCIATTACH /dev/serial1 bcm43xx 460800 noflow - $BDADDR
+-- 
+2.7.4
+

layers/meta-balena-raspberrypi/recipes-connectivity/pi-bluetooth/pi-bluetooth_%.bbappend

@@ -1,3 +1,7 @@
+FILESEXTRAPATHS_append := ":${THISDIR}/files"
+
+SRC_URI_append = " file://0001-Lower-the-bt-uart-baudrate-to-460800-for-rpi3-v1.2.patch"
+
 do_install_append () {
     # Move udev rules into /lib as /etc/udev/rules.d is bind mounted for custom rules
     install -d ${D}${base_libdir}/udev/rules.d