target: Speedup Target.write_value()

Avoid an execute() by doing the check in the same command. This also
allows to return early if the write is fast, and to extend for longer if
the write is slow. The speed at which you can observe a write in sysfs
depends on the backing kernel handlers, so there is a wide variety of
situations.

Author: douglas-raillard-arm

devlib/target.py

@@ -736,12 +736,36 @@ def batch_revertable_write_value(self, kwargs_list):
 
     def write_value(self, path, value, verify=True):
         value = str(value)
-        self.execute('echo {} > {}'.format(quote(value), quote(path)), check_exit_code=False, as_root=True)
+
         if verify:
-            output = self.read_value(path)
-            if not output == value:
-                message = 'Could not set the value of {} to "{}" (read "{}")'.format(path, value, output)
-                raise TargetStableError(message)
+            # Check in a loop for a while since updates to sysfs files can take
+            # some time to be observed, typically when a write triggers a
+            # lengthy kernel-side request, and the read is based on some piece
+            # of state that may take some time to be updated by the write
+            # request, such as hotplugging a CPU.
+            cmd = '''
+orig=$(cat {path} 2>/dev/null || printf "")
+printf "%s" {value} > {path}
+if [ {value} != "$orig" ]; then
+   trials=0
+   while [ "$(cat {path} 2>/dev/null)" != {value} ]; do
+       if [ $trials -ge 10 ]; then
+           cat {path}
+           exit 1
+       fi
+       sleep 0.01
+       trials=$((trials + 1))
+   done
+fi
+'''
+        else:
+            cmd = 'printf "%s" {value} > {path}'
+        cmd = cmd.format(path=quote(path), value=quote(value))
+
+        out = self.execute(cmd, check_exit_code=False, as_root=True)
+        if verify and out:
+            message = 'Could not set the value of {} to "{}" (read "{}")'.format(path, value, out)
+            raise TargetStableError(message)
 
     def reset(self):
         try: