ftrace: Separate top_buffer_size from buffer_size

Since we now set the top buffer size to be the same as the devlib buffer
size, this effectively halves the maximum available buffer size that can
be set while using devlib. Whatever size is passed as `buffer_size` will
be allocated twice, even if the top buffer is hardly used at all.

This commit separates them into `buffer_size` and `top_buffer_size`. If
the latter is not passed, the behaviour will not change compared to now.

Fixes: e0c53d0999
Signed-off-by: Kajetan Puchalski <kajetan.puchalski@arm.com>

devlib/collector/ftrace.py

@@ -60,6 +60,7 @@ class FtraceCollector(CollectorBase):
                  tracer=None,
                  trace_children_functions=False,
                  buffer_size=None,
+                 top_buffer_size=None,
                  buffer_size_step=1000,
                  tracing_path=None,
                  automark=True,
@@ -77,6 +78,7 @@ class FtraceCollector(CollectorBase):
         self.tracer = tracer
         self.trace_children_functions = trace_children_functions
         self.buffer_size = buffer_size
+        self.top_buffer_size = top_buffer_size
         self.tracing_path = self._resolve_tracing_path(target, tracing_path)
         self.automark = automark
         self.autoreport = autoreport
@@ -245,10 +247,11 @@ class FtraceCollector(CollectorBase):
         # parameter, but unfortunately some events still end up there (e.g.
         # print event). So we still need to set that size, otherwise the buffer
         # might be too small and some event lost.
-        if self.buffer_size is not None:
+        top_buffer_size = self.top_buffer_size if self.top_buffer_size else self.buffer_size
+        if top_buffer_size:
             self.target.write_value(
                 self.target.path.join(self.tracing_path, 'buffer_size_kb'),
-                self.buffer_size
+                top_buffer_size,
             )
 
         if self.functions: