skip hw reboot cause if warm/fast reboot found from the proc cmdline (#13378)

#### Why I did it
Backport #13246 to 202012 branch.

In case of warm/fast reboot, the hardware reboot cause will NOT be cleared because CPLD will not be touched in this flow. To not confuse the reboot cause determine logic, the leftover hardware reboot cause shall be skipped by the platform API, platform API will return the 'REBOOT_CAUSE_NON_HARDWARE' instead of the "hardware" reboot cause.

#### How I did it

Check the proc cmdline to see whether the last reboot is a warm or fast reboot, if yes skip checking the leftover hardware reboot cause.

#### How to verify it

a. Manual test:
> 1. Perform a power loss
> 2. Perform a warm/fast reboot
> 3. check the reboot cause should be "warm-reboot" or "fast-reboot" instead of "power loss"

b. Run reboot cause related regression test.

Author: keboliu

platform/mellanox/mlnx-platform-api/sonic_platform/chassis.py

@@ -13,6 +13,7 @@
     from sonic_py_common.logger import Logger
     from os import listdir
     from os.path import isfile, join
+    from . import utils
     import sys
     import io
     import re
@@ -38,6 +39,10 @@
 
 REBOOT_CAUSE_FILE_LENGTH = 1
 
+REBOOT_TYPE_KEXEC_FILE = "/proc/cmdline"
+REBOOT_TYPE_KEXEC_PATTERN_WARM = ".*SONIC_BOOT_TYPE=(warm|fastfast).*"
+REBOOT_TYPE_KEXEC_PATTERN_FAST = ".*SONIC_BOOT_TYPE=(fast|fast-reboot).*"
+
 # Global logger class instance
 logger = Logger()
 
@@ -350,6 +355,17 @@ def _verify_reboot_cause(self, filename):
         '''
         return bool(int(self._read_generic_file(join(REBOOT_CAUSE_ROOT, filename), REBOOT_CAUSE_FILE_LENGTH).rstrip('\n')))
 
+    def _parse_warmfast_reboot_from_proc_cmdline(self):
+        if isfile(REBOOT_TYPE_KEXEC_FILE):
+            with open(REBOOT_TYPE_KEXEC_FILE) as cause_file:
+                cause_file_kexec = cause_file.readline()
+            m = re.search(REBOOT_TYPE_KEXEC_PATTERN_WARM, cause_file_kexec)
+            if m and m.group(1):
+                return 'warm-reboot'
+            m = re.search(REBOOT_TYPE_KEXEC_PATTERN_FAST, cause_file_kexec)
+            if m and m.group(1):
+                return 'fast-reboot'
+        return None
 
     def initialize_reboot_cause(self):
         self.reboot_major_cause_dict = {
@@ -388,6 +404,13 @@ def get_reboot_cause(self):
             is "REBOOT_CAUSE_HARDWARE_OTHER", the second string can be used
             to pass a description of the reboot cause.
         """
+        # To avoid the leftover hardware reboot cause confusing the reboot cause determine service
+        # Skip the hardware reboot cause check if warm/fast reboot cause found from cmdline
+        if utils.is_host():
+            reboot_cause = self._parse_warmfast_reboot_from_proc_cmdline()
+            if reboot_cause:
+                return self.REBOOT_CAUSE_NON_HARDWARE, ''
+
         #read reboot causes files in the following order
         if not self.reboot_cause_initialized:
             self.initialize_reboot_cause()