linux-kernel - Re: [PATCH v2 12/17] coresight: trbe: Add a helper to fetch cpudata from perf handle

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Date:   Wed, 6 Oct 2021 11:15:36 -0600
From:   Mathieu Poirier <mathieu.poirier@...aro.org>
To:     Suzuki K Poulose <suzuki.poulose@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        maz@...nel.org, catalin.marinas@....com, mark.rutland@....com,
        james.morse@....com, anshuman.khandual@....com, leo.yan@...aro.org,
        mike.leach@...aro.org, will@...nel.org, lcherian@...vell.com,
        coresight@...ts.linaro.org
Subject: Re: [PATCH v2 12/17] coresight: trbe: Add a helper to fetch cpudata
 from perf handle

On Tue, Oct 05, 2021 at 11:35:13PM +0100, Suzuki K Poulose wrote:
> Hi Mathieu
> 
> On 04/10/2021 18:42, Mathieu Poirier wrote:
> > On Tue, Sep 21, 2021 at 02:41:16PM +0100, Suzuki K Poulose wrote:
> > > Add a helper to get the CPU specific data for TRBE instance, from
> > > a given perf handle. This also adds extra checks to make sure that
> > > the event associated with the handle is "bound" to the CPU and is
> > > active on the TRBE.
> > > 
> > > Cc: Anshuman Khandual <anshuman.khandual@....com>
> > > Cc: Mike Leach <mike.leach@...aro.org>
> > > Cc: Mathieu Poirier <mathieu.poirier@...aro.org>
> > > Cc: Leo Yan <leo.yan@...aro.org>
> > > Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
> > > ---
> > >   drivers/hwtracing/coresight/coresight-trbe.c | 12 ++++++++++--
> > >   1 file changed, 10 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/drivers/hwtracing/coresight/coresight-trbe.c b/drivers/hwtracing/coresight/coresight-trbe.c
> > > index 983dd5039e52..797d978f9fa7 100644
> > > --- a/drivers/hwtracing/coresight/coresight-trbe.c
> > > +++ b/drivers/hwtracing/coresight/coresight-trbe.c
> > > @@ -268,6 +268,15 @@ static unsigned long trbe_snapshot_offset(struct perf_output_handle *handle)
> > >   	return buf->nr_pages * PAGE_SIZE;
> > >   }
> > > +static inline struct trbe_cpudata *
> > > +trbe_handle_to_cpudata(struct perf_output_handle *handle)
> > > +{
> > > +	struct trbe_buf *buf = etm_perf_sink_config(handle);
> > > +
> > > +	BUG_ON(!buf || !buf->cpudata);
> > > +	return buf->cpudata;
> > > +}
> > > +
> > >   /*
> > >    * TRBE Limit Calculation
> > >    *
> > > @@ -533,8 +542,7 @@ static enum trbe_fault_action trbe_get_fault_act(struct perf_output_handle *hand
> > >   {
> > >   	int ec = get_trbe_ec(trbsr);
> > >   	int bsc = get_trbe_bsc(trbsr);
> > > -	struct trbe_buf *buf = etm_perf_sink_config(handle);
> > > -	struct trbe_cpudata *cpudata = buf->cpudata;
> > > +	struct trbe_cpudata *cpudata = trbe_handle_to_cpudata(handle);
> > 
> > There is two other places where this pattern is present:  is_perf_trbe() and
> > __trbe_normal_offset().
> 
> I skipped them, as they have to get access to the "trbe_buf" anyways.
> So the step by step, made sense. But I could replace them too to make it
> transparent.
> 
> What do you think ?

Humm...  I don't think there is a right way or a wrong way here.  If we move
forward with this patchset we have two ways of getting to buf->cpudata.  One
using trbe_handle_to_cpudata() and another one as laid out in is_perf_trbe() and
__trbe_normal_offset(), each with an equal number of occurences (2 for each).

I am usually not fond of small functions like trbe_handle_to_cpudata() and to me
keeping the current heuristic in trbe_get_fault_act() would have been just fine.
I agree with the argument that trbe_handle_to_cpudata() provides more checks but
is it really worth it if they aren't done everywhere?

In short I would get rid of trbe_handle_to_cpudata() entirely and live without
the extra checks... But I'm not strongly opinionated on this either.

> 
> Suzuki
> 
>