Re: [PATCH v2 14/17] coresight: trbe: Make sure we have enough space

[Suzuki K Poulose]

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@xxxxxxx>
> > Cc: Mathieu Poirier <mathieu.poirier@xxxxxxxxxx>
> > Cc: Mike Leach <mike.leach@xxxxxxxxxx>
> > Cc: Leo Yan <leo.yan@xxxxxxxxxx>
> > Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> > ---
> >   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