Re: [PATCH v2 3/4] coresight: etm4x: Report the external outputs the PMU counts

[James Clark <james.clark@linaro.org>]

On 13/10/2026 00:05, Amir Ayupov wrote:
> Some CPUs count the trace unit's external outputs as the PMU common
> events TRCEXTOUT0-3 (0x4010-0x4013), which lets a configuration drive a
> PMU event from trace unit events. Other CPUs, Neoverse N1 for example,
> have no such events, and a feature relying on them would silently do
> nothing there.
> 
> Detect at probe, on the trace unit's CPU, which outputs the PMU counts:
> the trace unit has an external output for each of its events
> (TRCIDR0.NUMEVENT), and from PMUv3p1, PMCEID0_EL0[63:32] reports which of
> the common events 0x4000-0x401f are implemented. Per the ETE
> specification, an implemented TRCEXTOUTn event counts external output n;
> only whether that output is also an external input is IMPLEMENTATION
> DEFINED.
> 
> Add a capability match flag for each output,
> CS_CFG_MATCH_CAP_PMU_EXTOUT(n), set it on ETMs whose PMU counts that
> output, and show it in the feature's configfs 'matches' attribute. With
> capabilities matched separately from the device class, a feature that
> requires one only loads on those ETMs.
> 
> Assisted-by: Devmate:Opus-5.5
> Signed-off-by: Amir Ayupov <aaupov@fb.com>
> ---
>   .../hwtracing/coresight/coresight-config.h    |  4 +++
>   .../hwtracing/coresight/coresight-etm4x-cfg.c |  5 ++-
>   .../coresight/coresight-etm4x-core.c          | 32 +++++++++++++++++++
>   drivers/hwtracing/coresight/coresight-etm4x.h |  3 ++
>   .../coresight/coresight-syscfg-configfs.c     |  7 ++++
>   5 files changed, 50 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/hwtracing/coresight/coresight-config.h b/drivers/hwtracing/coresight/coresight-config.h
> index af6ef96b9ae01..10e95f5ecc8d9 100644
> --- a/drivers/hwtracing/coresight/coresight-config.h
> +++ b/drivers/hwtracing/coresight/coresight-config.h
> @@ -35,6 +35,10 @@
>    * a device that matches one of its classes and has all of its capabilities.
>    */
>   #define CS_CFG_MATCH_CAP_MASK		0x0000ff00
> +/* trace unit external output n is countable by the PMU as TRCEXTOUTn */
> +#define CS_CFG_MATCH_CAP_PMU_EXTOUT_SHIFT	8
> +#define CS_CFG_MATCH_CAP_PMU_EXTOUT_NR	4	/* TRCEXTOUT0-3 */
> +#define CS_CFG_MATCH_CAP_PMU_EXTOUT(n)	BIT(CS_CFG_MATCH_CAP_PMU_EXTOUT_SHIFT + (n))
>   
>   /* flags defining device instance matching - used in config match desc data. */
>   #define CS_CFG_MATCH_INST_ANY		0x80000000 /* any instance of a class */
> diff --git a/drivers/hwtracing/coresight/coresight-etm4x-cfg.c b/drivers/hwtracing/coresight/coresight-etm4x-cfg.c
> index e1a59b4345052..09a7a6b86968c 100644
> --- a/drivers/hwtracing/coresight/coresight-etm4x-cfg.c
> +++ b/drivers/hwtracing/coresight/coresight-etm4x-cfg.c
> @@ -174,9 +174,12 @@ static int etm4_cfg_load_feature(struct coresight_device *csdev,
>   
>   int etm4_cscfg_register(struct coresight_device *csdev)
>   {
> +	struct etmv4_drvdata *drvdata = dev_get_drvdata(csdev->dev.parent);
>   	struct cscfg_csdev_feat_ops ops;
> +	u32 match_flags = CS_CFG_ETM4_MATCH_FLAGS;
>   
>   	ops.load_feat = &etm4_cfg_load_feature;
> +	match_flags |= (u32)drvdata->pmu_extout << CS_CFG_MATCH_CAP_PMU_EXTOUT_SHIFT;
>   
> -	return cscfg_register_csdev(csdev, CS_CFG_ETM4_MATCH_FLAGS, &ops);
> +	return cscfg_register_csdev(csdev, match_flags, &ops);
>   }
> diff --git a/drivers/hwtracing/coresight/coresight-etm4x-core.c b/drivers/hwtracing/coresight/coresight-etm4x-core.c
> index 2247ad55d4442..1e78774025d13 100644
> --- a/drivers/hwtracing/coresight/coresight-etm4x-core.c
> +++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c
> @@ -30,6 +30,7 @@
>   #include <linux/uaccess.h>
>   #include <linux/perf_event.h>
>   #include <linux/perf/arm_pmu.h>
> +#include <linux/perf/arm_pmuv3.h>
>   #include <linux/platform_device.h>
>   #include <linux/pm_runtime.h>
>   #include <linux/property.h>
> @@ -1368,6 +1369,35 @@ static void etm4_fixup_wrong_ccitmin(struct etmv4_drvdata *drvdata)
>   	}
>   }
>   
> +/*
> + * Find the external outputs of this trace unit that the PMU counts as the
> + * common events TRCEXTOUT0-3. The trace unit has an external output for each
> + * of its events, and PMCEID0_EL0[63:32] reports the common events
> + * 0x4000-0x401f from PMUv3p1.
> + *
> + * Must run on the CPU that owns the trace unit.
> + */
> +static u8 etm4_pmu_extout(struct etmv4_drvdata *drvdata)
> +{
> +	int pmuver = read_pmuver();
> +	u64 pmceid;
> +	u8 extout = 0;
> +	int n;
> +
> +	if (!pmuv3_implemented(pmuver) || pmuver < ID_AA64DFR0_EL1_PMUVer_V3P1)
> +		return 0;
> +
> +	pmceid = read_pmceid0() >> 32;
> +	/* nr_event is TRCIDR0.NUMEVENT, the number of events minus one */
> +	for (n = 0; n <= drvdata->nr_event; n++) {
> +		if (pmceid & BIT_ULL(ARMV8_PMUV3_PERFCTR_TRCEXTOUT0 + n -
> +				     ARMV8_PMUV3_EXT_COMMON_EVENT_BASE))
> +			extout |= BIT(n);
> +	}

Maybe an pmuv3_event_implemented() function in arm_pmuv3.c would be a 
better fit for this. It can do something like:

   if (event > ARMV8_PMUV3_EXT_COMMON_EVENT_BASE)
      return (event - ARMV8_PMUV3_EXT_COMMON_EVENT_BASE) & pmceid_IDhi;
   else
      return event & pmceid_IDlo;

etc.



> +
> +	return extout;
> +}
> +
>   static void etm4_init_arch_data(void *info)
>   {
>   	u32 etmidr0;
> @@ -1522,6 +1552,8 @@ static void etm4_init_arch_data(void *info)
>   	/* NUMCNTR, bits[30:28] number of counters available for tracing */
>   	drvdata->nr_cntr = FIELD_GET(TRCIDR5_NUMCNTR_MASK, etmidr5);
>   
> +	drvdata->pmu_extout = etm4_pmu_extout(drvdata);
> +
>   	coresight_clear_self_claim_tag_unlocked(csa);
>   	etm4_cs_lock(drvdata, csa);
>   	cpu_detect_trace_filtering(drvdata);
> diff --git a/drivers/hwtracing/coresight/coresight-etm4x.h b/drivers/hwtracing/coresight/coresight-etm4x.h
> index df9e0748d71ec..11d95a4518b88 100644
> --- a/drivers/hwtracing/coresight/coresight-etm4x.h
> +++ b/drivers/hwtracing/coresight/coresight-etm4x.h
> @@ -979,6 +979,8 @@ struct etmv4_save_state {
>    * @nrseqstate: The number of sequencer states that are implemented.
>    * @nr_seq_ctrls: The number of sequence state transition control registers.
>    * @nr_event:	Indicates how many events the trace unit support.
> + * @pmu_extout:	Bitmap of the external outputs n that the PMU counts as
> + *		TRCEXTOUTn.
>    * @nr_resource:The number of resource selection pairs available for tracing.
>    * @nr_ss_cmp:	Number of single-shot comparator controls that are available.
>    * @trcid:	value of the current ID for this component.
> @@ -1045,6 +1047,7 @@ struct etmv4_drvdata {
>   	u8				nrseqstate;
>   	u8				nr_seq_ctrls;
>   	u8				nr_event;
> +	u8				pmu_extout;
>   	u8				nr_resource;
>   	u8				nr_ss_cmp;
>   	u8				trcid;
> diff --git a/drivers/hwtracing/coresight/coresight-syscfg-configfs.c b/drivers/hwtracing/coresight/coresight-syscfg-configfs.c
> index 2b40e556be872..7893d40f2e9ea 100644
> --- a/drivers/hwtracing/coresight/coresight-syscfg-configfs.c
> +++ b/drivers/hwtracing/coresight/coresight-syscfg-configfs.c
> @@ -243,6 +243,7 @@ static ssize_t cscfg_feat_matches_show(struct config_item *item, char *page)
>   							struct cscfg_fs_feature, group);
>   	u32 match_flags = fs_feat->feat_desc->match_flags;
>   	int used = 0;
> +	int n;
>   
>   	if (match_flags & CS_CFG_MATCH_CLASS_SRC_ALL)
>   		used = scnprintf(page, PAGE_SIZE, "SRC_ALL ");
> @@ -250,6 +251,12 @@ static ssize_t cscfg_feat_matches_show(struct config_item *item, char *page)
>   	if (match_flags & CS_CFG_MATCH_CLASS_SRC_ETM4)
>   		used += scnprintf(page + used, PAGE_SIZE - used, "SRC_ETMV4 ");
>   
> +	for (n = 0; n < CS_CFG_MATCH_CAP_PMU_EXTOUT_NR; n++) {
> +		if (match_flags & CS_CFG_MATCH_CAP_PMU_EXTOUT(n))
> +			used += scnprintf(page + used, PAGE_SIZE - used,
> +					  "PMU_EXTOUT%d ", n);
> +	}
> +
>   	used += scnprintf(page + used, PAGE_SIZE - used, "\n");
>   	return used;
>   }