On 22/09/2021 10:58, Anshuman Khandual wrote:

On 9/21/21 7:11 PM, Suzuki K Poulose wrote:

...

The TRBE driver makes sure that there is enough space for a meaningful run, otherwise pads the given space and restarts the offset calculation once. But there is no guarantee that we may find space or hit "no space".

So what happens currently when it neither finds the required minimum buffer space for a meaningful run nor does it hit the "no space" scenario ?

It tries once today and assumes that it will either hit :

- No space OR - Enough space

which is reasonable, given the minimum space needed is a few bytes. But this may no longer be true with other erratum workaround.

...

Make sure that we repeat the step until, either :

• We have the minimum space

OR

• There is NO space at all.

Cc: Anshuman Khandual anshuman.khandual@arm.com Cc: Mathieu Poirier mathieu.poirier@linaro.org Cc: Mike Leach mike.leach@linaro.org Cc: Leo Yan leo.yan@linaro.org Signed-off-by: Suzuki K Poulose suzuki.poulose@arm.com

---

drivers/hwtracing/coresight/coresight-trbe.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/hwtracing/coresight/coresight-trbe.c b/drivers/hwtracing/coresight/coresight-trbe.c index 3373f4e2183b..02f9e00e2091 100644 --- a/drivers/hwtracing/coresight/coresight-trbe.c +++ b/drivers/hwtracing/coresight/coresight-trbe.c @@ -451,10 +451,14 @@ static unsigned long trbe_normal_offset(struct perf_output_handle *handle) * If the head is too close to the limit and we don't * have space for a meaningful run, we rather pad it * and start fresh.

• *
  

• * We might have to do this more than once to make sure
  

• * we have enough required space.
  

OR no space at all, as explained in the commit message. Hence this comment needs an update.

...

*/

• if (limit && ((limit - head) < trbe_min_trace_buf_size(handle))) {

• while (limit && ((limit - head) < trbe_min_trace_buf_size(handle))) { trbe_pad_buf(handle, limit - head); limit = __trbe_normal_offset(handle);

• head = PERF_IDX2OFF(handle->head, buf);
  

Should the loop be bound with a retry limit as well ?

No. We will eventually hit No-space as we keep on padding the buffer.

Suzuki