Add boot parameter in device\qcom\msm_xxx\BoardConfig.mk. xxx is the build variant name

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Build and flash boot.img:
make kernel
#put device into fastboot mode and update boot.img
fastboot flash boot boot.img
fastboot reboot
check cmdline to make sure change is successful.
adb root
$ adb shell cat /proc/cmdline
sched_enable_hmp=1 console=ttyHSL0,115200,n8 androidboot.console=ttyHSL0
androidboot.hardware=qcom msm_rtb.filter=0x237 ehci-hcd.park=3 lpm_levels.sleep_disabled=1
androidboot.bootdevice=7824900.sdhci earlycon=msm_hsl_uart,0x78B0000 trace_event=sched:,
workqueue:,irq:,cpufreq_interactive:,:thermal:* trace_buf_size=48M androidboot.emmc=true
androidboot.verifiedbootstate=orange androidboot.veritymode=enforcing androidboot.keymaster=1
androidboot.serialno=41544cca androidboot.baseband=msm mdss_mdp.panel=1:dsi:0:qcom,
mdss_dsi_r69006_1080p_cmd:1:none:cfg:single_dsi
1.2.1 Choose the right tracing categories
All the available events can be found with commands:
adb shell ls /d/tracing/events
almk
asoc
binder
block
cfg80211
cma
…
Interested events can be appended to boot command lines, separated with commar.
1.2.2 Enable user space tracing

If we are interested in the userspace tracing like am/wm/audio/input, can specify that by adding
property in system/build.prop
debug.atrace.tags.enableflags=0xFFFF
each bit in this value is corresponding to a category: (system/core/include/cutils/trace.h)
#define ATRACE_TAG_GRAPHICS (1<<1)
#define ATRACE_TAG_INPUT (1<<2)
….
#define ATRACE_TAG_POWER (1<<17)
So we can simply enable all the categories by set debug.atrace.tags.enableflags to 0xFFFF
#/add a new line in system/buildprop
debug.atrace.tags.enableflags=0xFFFF
#push to system/build.prop
adb root
adb remount
adb push build.prop /system/build.prop
adb shell chmod 755 /system/build.prop
adb shell sync
adb reboot
1.3 capture ftrace logs
Connect device with usb cable, and boot up, once boot up is finished, run the scripts to pull logs:
adb root
adb wait-for-device
adb shell echo 0 > /d/tracing/trace_on
adb pull /d/tracing/trace trace.txt