coresight: trbe: Do not truncate buffer on IRQ

The TRBE driver marks the AUX buffer as TRUNCATED when we get an IRQ
on FILL event. This has rather unwanted side-effect of the event
being disabled when there may be more space in the ring buffer.

So, instead of TRUNCATE we need a different flag to indicate
that the trace may have lost a few bytes (i.e from the point of
generating the FILL event until the IRQ is consumed). Anyways, the
userspace must use the size from RECORD_AUX headers to restrict
the "trace" decoding.

Using PARTIAL flag causes the perf tool to generate the
following warning:

  Warning:
  AUX data had gaps in it XX times out of YY!

  Are you running a KVM guest in the background?

which is pointlessly scary for a user. The other remaining options
are :
  - COLLISION - Use by SPE to indicate samples collided
  - Add a new flag - Specifically for CoreSight, doesn't sound
    so good, if we can re-use something.

Given that we don't already use the "COLLISION" flag, the above
behavior can be notified using this flag for CoreSight.

Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: James Clark <james.clark@arm.com>
Cc: Mike Leach <mike.leach@linaro.org>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20210923143919.2944311-4-suzuki.poulose@arm.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>

diff --git a/drivers/hwtracing/coresight/coresight-trbe.c b/drivers/hwtracing/coresight/coresight-trbe.c
index 47120d2..0a9106c 100644 (file)
--- a/drivers/hwtracing/coresight/coresight-trbe.c
+++ b/drivers/hwtracing/coresight/coresight-trbe.c
@@ -120,6 +120,25 @@ static void trbe_reset_local(void)
        write_sysreg_s(0, SYS_TRBSR_EL1);
 }
 
+static void trbe_report_wrap_event(struct perf_output_handle *handle)
+{
+       /*
+        * Mark the buffer to indicate that there was a WRAP event by
+        * setting the COLLISION flag. This indicates to the user that
+        * the TRBE trace collection was stopped without stopping the
+        * ETE and thus there might be some amount of trace that was
+        * lost between the time the WRAP was detected and the IRQ
+        * was consumed by the CPU.
+        *
+        * Setting the TRUNCATED flag would move the event to STOPPED
+        * state unnecessarily, even when there is space left in the
+        * ring buffer. Using the COLLISION flag doesn't have this side
+        * effect. We only set TRUNCATED flag when there is no space
+        * left in the ring buffer.
+        */
+       perf_aux_output_flag(handle, PERF_AUX_FLAG_COLLISION);
+}
+
 static void trbe_stop_and_truncate_event(struct perf_output_handle *handle)
 {
        struct trbe_buf *buf = etm_perf_sink_config(handle);
@@ -612,7 +631,7 @@ static unsigned long arm_trbe_update_buffer(struct coresight_device *csdev,
                 * for correct size. Also, mark the buffer truncated.
                 */
                write = get_trbe_limit_pointer();
-               perf_aux_output_flag(handle, PERF_AUX_FLAG_TRUNCATED);
+               trbe_report_wrap_event(handle);
        }
 
        offset = write - base;
@@ -708,11 +727,7 @@ static void trbe_handle_overflow(struct perf_output_handle *handle)
        if (buf->snapshot)
                handle->head += size;
 
-       /*
-        * Mark the buffer as truncated, as we have stopped the trace
-        * collection upon the WRAP event, without stopping the source.
-        */
-       perf_aux_output_flag(handle, PERF_AUX_FLAG_TRUNCATED);
+       trbe_report_wrap_event(handle);
        perf_aux_output_end(handle, size);
        event_data = perf_aux_output_begin(handle, event);
        if (!event_data) {