Re: [PATCH 05/10] drivers/perf: Use PERF_PMU_CAP_NO_SAMPLING consistently

[James Clark <james.clark@arm.com>]

On 12/03/2024 17:34, Robin Murphy wrote:
> Our system PMUs fundamentally cannot support the current notion of
> sampling events, so now that the core capability has been clarified,
> apply it consistently and purge yet more boilerplate.
> 
> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
> ---
>  drivers/perf/alibaba_uncore_drw_pmu.c     |  6 +-----
>  drivers/perf/amlogic/meson_ddr_pmu_core.c |  3 ++-
>  drivers/perf/arm-cci.c                    |  3 ++-
>  drivers/perf/arm-ccn.c                    | 12 +-----------
>  drivers/perf/arm-cmn.c                    |  3 ++-
>  drivers/perf/arm_cspmu/arm_cspmu.c        | 17 ++++-------------
>  drivers/perf/arm_dmc620_pmu.c             |  4 ++--
>  drivers/perf/arm_dsu_pmu.c                | 12 +-----------
>  drivers/perf/arm_smmuv3_pmu.c             |  6 +-----
>  drivers/perf/cxl_pmu.c                    |  3 ++-
>  drivers/perf/dwc_pcie_pmu.c               |  5 +----
>  drivers/perf/fsl_imx8_ddr_perf.c          |  3 ++-
>  drivers/perf/fsl_imx9_ddr_perf.c          |  3 ++-
>  drivers/perf/hisilicon/hisi_pcie_pmu.c    |  4 ++--
>  drivers/perf/hisilicon/hisi_uncore_pmu.c  |  3 ++-
>  drivers/perf/hisilicon/hns3_pmu.c         |  4 ++--
>  drivers/perf/marvell_cn10k_ddr_pmu.c      |  6 +-----
>  drivers/perf/qcom_l2_pmu.c                |  7 +------
>  drivers/perf/qcom_l3_pmu.c                |  7 +------
>  drivers/perf/thunderx2_pmu.c              |  4 ++--
>  drivers/perf/xgene_pmu.c                  |  4 ++--
>  21 files changed, 36 insertions(+), 83 deletions(-)
> 
[...]
>  
> diff --git a/drivers/perf/arm-ccn.c b/drivers/perf/arm-ccn.c
> index ce26bb773a56..4114349e62dd 100644
> --- a/drivers/perf/arm-ccn.c
> +++ b/drivers/perf/arm-ccn.c
> @@ -713,7 +713,6 @@ static void arm_ccn_pmu_event_release(struct perf_event *event)
>  static int arm_ccn_pmu_event_init(struct perf_event *event)
>  {
>  	struct arm_ccn *ccn;
> -	struct hw_perf_event *hw = &event->hw;
>  	u32 node_xp, type, event_id;
>  	int valid;
>  	int i;
> @@ -721,16 +720,6 @@ static int arm_ccn_pmu_event_init(struct perf_event *event)
>  
>  	ccn = pmu_to_arm_ccn(event->pmu);
>  
> -	if (hw->sample_period) {
> -		dev_dbg(ccn->dev, "Sampling not supported!\n");
> -		return -EOPNOTSUPP;
> -	}
> -
> -	if (has_branch_stack(event)) {
> -		dev_dbg(ccn->dev, "Can't exclude execution levels!\n");
> -		return -EINVAL;
> -	}
> -

[...]

> diff --git a/drivers/perf/arm_dsu_pmu.c b/drivers/perf/arm_dsu_pmu.c
> index f5ea5acaf2f3..3424d165795c 100644
> --- a/drivers/perf/arm_dsu_pmu.c
> +++ b/drivers/perf/arm_dsu_pmu.c
> @@ -544,23 +544,12 @@ static int dsu_pmu_event_init(struct perf_event *event)
>  {
>  	struct dsu_pmu *dsu_pmu = to_dsu_pmu(event->pmu);
>  
> -	/* We don't support sampling */
> -	if (is_sampling_event(event)) {
> -		dev_dbg(dsu_pmu->pmu.dev, "Can't support sampling events\n");
> -		return -EOPNOTSUPP;
> -	}
> -
>  	/* We cannot support task bound events */
>  	if (event->cpu < 0 || event->attach_state & PERF_ATTACH_TASK) {
>  		dev_dbg(dsu_pmu->pmu.dev, "Can't support per-task counters\n");
>  		return -EINVAL;
>  	}
>  
> -	if (has_branch_stack(event)) {
> -		dev_dbg(dsu_pmu->pmu.dev, "Can't support filtering\n");
> -		return -EINVAL;
> -	}
> -

I'm assuming that this and the other has_branch_stack() check were
removed because branch stacks don't actually do anything unless sampling
is enabled?

It's a small difference that there is now no error message if you ask
for branch stacks, but it wouldn't have done anything anyway? I suppose
this error message was also not applied very consistently across the
different devices.

James