Re: [PATCH v8 3/8] perf: imx_perf: let the driver manage the counter usage rather the user

[Will Deacon <will@kernel.org>]

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?

Will