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Date: Wed, 10 Apr 2024 16:45:47 +0100
From: Will Deacon <will@kernel.org>
To: Xu Yang <xu.yang_2@nxp.com>
Cc: Frank Li <frank.li@nxp.com>,
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Subject: Re: [EXT] Re: [PATCH v8 3/8] perf: imx_perf: let the driver manage
 the counter usage rather the user
Message-ID: <20240410154546.GA25225@willie-the-truck>
References: <20240322063930.749126-1-xu.yang_2@nxp.com>
 <20240322063930.749126-3-xu.yang_2@nxp.com>
 <20240409152627.GA23621@willie-the-truck>
 <DU2PR04MB882209413AB62531713166B38C062@DU2PR04MB8822.eurprd04.prod.outlook.com>
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User-Agent: Mutt/1.10.1 (2018-07-13)

On Wed, Apr 10, 2024 at 07:39:46AM +0000, Xu Yang wrote:
> > On Fri, Mar 22, 2024 at 02:39:25PM +0800, Xu Yang wrote:
> > > In current design, the user of perf app needs to input counter ID to count
> > > events. However, this is not user-friendly since the user needs to lookup
> > > the map table to find the counter. Instead of letting the user to input
> > > the counter, let this driver to manage the counters in this patch.
> > 
> > I think we still have to support the old interface so that we don't break
> > those existing users (even if the driver just ignores whatever counter ID
> > is provided in a backwards-compatible way).
> > 
> > > This will be implemented by:
> > >  1. allocate counter 0 for cycle event.
> > >  2. find unused counter from 1-10 for reference events.
> > >  3. allocate specific counter for counter-specific events.
> > >
> > > In this patch, counter attribute is removed too. To mark counter-specific
> > > events, counter ID will be encoded into perf_pmu_events_attr.id.
> > >
> > > Reviewed-by: Frank Li <Frank.Li@nxp.com>
> > > Signed-off-by: Xu Yang <xu.yang_2@nxp.com>
> > >
> > > ---
> > > Changes in v6:
> > >  - new patch
> > > Changes in v7:
> > >  - no changes
> > > Changes in v8:
> > >  - add Rb tag
> > > ---
> > >  drivers/perf/fsl_imx9_ddr_perf.c | 168 ++++++++++++++++++-------------
> > >  1 file changed, 99 insertions(+), 69 deletions(-)
> > >
> > > diff --git a/drivers/perf/fsl_imx9_ddr_perf.c b/drivers/perf/fsl_imx9_ddr_perf.c
> > > index 0017f2c9ef91..b728719b494c 100644
> > > --- a/drivers/perf/fsl_imx9_ddr_perf.c
> > > +++ b/drivers/perf/fsl_imx9_ddr_perf.c
> > > @@ -245,14 +249,12 @@ static const struct attribute_group ddr_perf_events_attr_group = {
> > >       .attrs = ddr_perf_events_attrs,
> > >  };
> > >
> > > -PMU_FORMAT_ATTR(event, "config:0-7");
> > > -PMU_FORMAT_ATTR(counter, "config:8-15");
> > > +PMU_FORMAT_ATTR(event, "config:0-15");
> > 
> > Sadly, this is a user-visible change so I think it will break old tools,
> > won't it?
> 
> For imx ddr pmu, most of the people will use metrics rather event itself,
> and we have speficy the format of event parameters in metrics table.
> The parameters is also updated in this patchset.
> 
> And to easy use for user, the counter should be hidden (transparent) to
> user after I had talk with Frank. Then, the user need't to look up the event
> table to find which counter to use. 
> 
> So this patchset will basically not break the usage of perf tools and will
> improve practicality.

Sorry, but I don't agree. The original commit adding this driver
(55691f99d417) gives the following examples in the commit message:

For example:
      perf stat -a -I 1000 -e imx9_ddr0/eddrtq_pm_rd_trans_filt,counter=2,axi_mask=ID_MASK,axi_id=ID/
      perf stat -a -I 1000 -e imx9_ddr0/eddrtq_pm_wr_trans_filt,counter=3,axi_mask=ID_MASK,axi_id=ID/
      perf stat -a -I 1000 -e imx9_ddr0/eddrtq_pm_rd_beat_filt,counter=4,axi_mask=ID_MASK,axi_id=ID/

I don't think these will work any more if we apply this patch.

Will