Adds a method to select a suitable sink connected to a given source.

In cases where no sink is defined, the coresight_find_default_sink routine can search from a given source, through the child connections until a suitable sink is found.

The suitability is defined in by the sink coresight_dev_subtype on the CoreSight device, and the distance from the source by counting connections.

Higher value subtype is preferred - where these are equal, shorter distance from source is used as a tie-break.

This allows for default sink to be discovered were none is specified (e.g. perf command line)

Signed-off-by: Mike Leach mike.leach@linaro.org Suggested-by: Suzuki K Poulose suzuki.poulose@arm.com Reviewed-by: Leo Yan leo.yan@linaro.org Reviewed-by: Mathieu Poirier mathieu.poirier@linaro.org --- drivers/hwtracing/coresight/coresight-priv.h | 2 + drivers/hwtracing/coresight/coresight.c | 166 +++++++++++++++++++ include/linux/coresight.h | 3 + 3 files changed, 171 insertions(+)

diff --git a/drivers/hwtracing/coresight/coresight-priv.h b/drivers/hwtracing/coresight/coresight-priv.h index 36c943ae94d5..f2dc625ea585 100644 --- a/drivers/hwtracing/coresight/coresight-priv.h +++ b/drivers/hwtracing/coresight/coresight-priv.h @@ -150,6 +150,8 @@ int coresight_enable_path(struct list_head *path, u32 mode, void *sink_data); struct coresight_device *coresight_get_sink(struct list_head *path); struct coresight_device *coresight_get_enabled_sink(bool reset); struct coresight_device *coresight_get_sink_by_id(u32 id); +struct coresight_device * +coresight_find_default_sink(struct coresight_device *csdev); struct list_head *coresight_build_path(struct coresight_device *csdev, struct coresight_device *sink); void coresight_release_path(struct list_head *path); diff --git a/drivers/hwtracing/coresight/coresight.c b/drivers/hwtracing/coresight/coresight.c index f3efbb3b2b4d..e9c90f2de34a 100644 --- a/drivers/hwtracing/coresight/coresight.c +++ b/drivers/hwtracing/coresight/coresight.c @@ -769,6 +769,171 @@ void coresight_release_path(struct list_head *path) path = NULL; }

+/* return true if the device is a suitable type for a default sink */ +static inline bool coresight_is_def_sink_type(struct coresight_device *csdev) +{ + /* sink & correct subtype */ + if (((csdev->type == CORESIGHT_DEV_TYPE_SINK) || + (csdev->type == CORESIGHT_DEV_TYPE_LINKSINK)) && + (csdev->subtype.sink_subtype >= CORESIGHT_DEV_SUBTYPE_SINK_BUFFER)) + return true; + return false; +} + +/** + * coresight_select_best_sink - return the best sink for use as default from + * the two provided. + * + * @sink: current best sink. + * @depth: search depth where current sink was found. + * @new_sink: new sink for comparison with current sink. + * @new_depth: search depth where new sink was found. + * + * Sinks prioritised according to coresight_dev_subtype_sink, with only + * subtypes CORESIGHT_DEV_SUBTYPE_SINK_BUFFER or higher being used. + * + * Where two sinks of equal priority are found, the sink closest to the + * source is used (smallest search depth). + * + * return @new_sink & update @depth if better than @sink, else return @sink. + */ +static struct coresight_device * +coresight_select_best_sink(struct coresight_device *sink, int *depth, + struct coresight_device *new_sink, int new_depth) +{ + bool update = false; + + if (!sink) { + /* first found at this level */ + update = true; + } else if (new_sink->subtype.sink_subtype > + sink->subtype.sink_subtype) { + /* found better sink */ + update = true; + } else if ((new_sink->subtype.sink_subtype == + sink->subtype.sink_subtype) && + (*depth > new_depth)) { + /* found same but closer sink */ + update = true; + } + + if (update) + *depth = new_depth; + return update ? new_sink : sink; +} + +/** + * coresight_find_sink - recursive function to walk trace connections from + * source to find a suitable default sink. + * + * @csdev: source / current device to check. + * @depth: [in] search depth of calling dev, [out] depth of found sink. + * + * This will walk the connection path from a source (ETM) till a suitable + * sink is encountered and return that sink to the original caller. + * + * If current device is a plain sink return that & depth, otherwise recursively + * call child connections looking for a sink. Select best possible using + * coresight_select_best_sink. + * + * return best sink found, or NULL if not found at this node or child nodes. + */ +static struct coresight_device * +coresight_find_sink(struct coresight_device *csdev, int *depth) +{ + int i, curr_depth = *depth + 1, found_depth = 0; + struct coresight_device *found_sink = NULL; + + if (coresight_is_def_sink_type(csdev)) { + found_depth = curr_depth; + found_sink = csdev; + if (csdev->type == CORESIGHT_DEV_TYPE_SINK) + goto return_def_sink; + /* look past LINKSINK for something better */ + } + + /* + * Not a sink we want - or possible child sink may be better. + * recursively explore each port found on this element. + */ + for (i = 0; i < csdev->pdata->nr_outport; i++) { + struct coresight_device *child_dev, *sink = NULL; + int child_depth = curr_depth; + + child_dev = csdev->pdata->conns[i].child_dev; + if (child_dev) + sink = coresight_find_sink(child_dev, &child_depth); + + if (sink) + found_sink = coresight_select_best_sink(found_sink, + &found_depth, + sink, + child_depth); + } + +return_def_sink: + /* return found sink and depth */ + if (found_sink) + *depth = found_depth; + return found_sink; +} + +/** + * coresight_find_default_sink: Find a sink suitable for use as a + * default sink. + * + * @csdev: starting source to find a connected sink. + * + * Walks connections graph looking for a suitable sink to enable for the + * supplied source. Uses CoreSight device subtypes and distance from source + * to select the best sink. + * + * If a sink is found, then the default sink for this device is set and + * will be automatically used in future. + * + * Used in cases where the CoreSight user (perf / sysfs) has not selected a + * sink. + */ +struct coresight_device * +coresight_find_default_sink(struct coresight_device *csdev) +{ + int depth = 0; + + /* look for a default sink if we have not found for this device */ + if (!csdev->def_sink) + csdev->def_sink = coresight_find_sink(csdev, &depth); + return csdev->def_sink; +} + +static int coresight_remove_sink_ref(struct device *dev, void *data) +{ + struct coresight_device *sink = data; + struct coresight_device *source = to_coresight_device(dev); + + if (source->def_sink == sink) + source->def_sink = NULL; + return 0; +} + +/** + * coresight_clear_default_sink: Remove all default sink references to the + * supplied sink. + * + * If supplied device is a sink, then check all the bus devices and clear + * out all the references to this sink from the coresight_device def_sink + * parameter. + * + * @csdev: coresight sink - remove references to this from all sources. + */ +static void coresight_clear_default_sink(struct coresight_device *csdev) +{ + if ((csdev->type == CORESIGHT_DEV_TYPE_SINK) || + (csdev->type == CORESIGHT_DEV_TYPE_LINKSINK)) { + bus_for_each_dev(&coresight_bustype, NULL, csdev, + coresight_remove_sink_ref); + } +} + /** coresight_validate_source - make sure a source has the right credentials * @csdev: the device structure for a source. * @function: the function this was called from. @@ -1358,6 +1523,7 @@ void coresight_unregister(struct coresight_device *csdev) etm_perf_del_symlink_sink(csdev); /* Remove references of that device in the topology */ coresight_remove_conns(csdev); + coresight_clear_default_sink(csdev); coresight_release_platform_data(csdev, csdev->pdata); device_unregister(&csdev->dev); } diff --git a/include/linux/coresight.h b/include/linux/coresight.h index 84dc695e87d4..58fffdecdbfd 100644 --- a/include/linux/coresight.h +++ b/include/linux/coresight.h @@ -48,6 +48,7 @@ enum coresight_dev_subtype_sink { CORESIGHT_DEV_SUBTYPE_SINK_NONE, CORESIGHT_DEV_SUBTYPE_SINK_PORT, CORESIGHT_DEV_SUBTYPE_SINK_BUFFER, + CORESIGHT_DEV_SUBTYPE_SINK_SYSMEM, };

enum coresight_dev_subtype_link { @@ -182,6 +183,7 @@ struct coresight_sysfs_link { * happens when a source has been selected and a path is enabled * from source to that sink. * @ea: Device attribute for sink representation under PMU directory. + * @def_sink: cached reference to default sink found for this device. * @ect_dev: Associated cross trigger device. Not part of the trace data * path or connections. * @nr_links: number of sysfs links created to other components from this @@ -200,6 +202,7 @@ struct coresight_device { /* sink specific fields */ bool activated; /* true only if a sink is part of a path */ struct dev_ext_attribute *ea; + struct coresight_device *def_sink; /* cross trigger handling */ struct coresight_device *ect_dev; /* sysfs links between components */

Add default sink selection to the perf trace handling in the etm driver. Uses the select default sink infrastructure to select a sink for the perf session, if no other sink is specified.

Signed-off-by: Mike Leach mike.leach@linaro.org Reviewed-by: Mathieu Poirier mathieu.poirier@linaro.org --- .../hwtracing/coresight/coresight-etm-perf.c | 17 ++++++++++++++--- 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/drivers/hwtracing/coresight/coresight-etm-perf.c b/drivers/hwtracing/coresight/coresight-etm-perf.c index 84f1dcb69827..1a3169e69bb1 100644 --- a/drivers/hwtracing/coresight/coresight-etm-perf.c +++ b/drivers/hwtracing/coresight/coresight-etm-perf.c @@ -226,9 +226,6 @@ static void *etm_setup_aux(struct perf_event *event, void **pages, sink = coresight_get_enabled_sink(true); }

- if (!sink) - goto err; - mask = &event_data->mask;

/* @@ -253,6 +250,16 @@ static void *etm_setup_aux(struct perf_event *event, void **pages, continue; }

+ /* + * No sink provided - look for a default sink for one of the + * devices. At present we only support topology where all CPUs + * use the same sink [N:1], so only need to find one sink. The + * coresight_build_path later will remove any CPU that does not + * attach to the sink, or if we have not found a sink. + */ + if (!sink) + sink = coresight_find_default_sink(csdev); + /* * Building a path doesn't enable it, it simply builds a * list of devices from source to sink that can be @@ -267,6 +274,10 @@ static void *etm_setup_aux(struct perf_event *event, void **pages, *etm_event_cpu_path_ptr(event_data, cpu) = path; }

+ /* no sink found for any CPU - cannot trace */ + if (!sink) + goto err; + /* If we don't have any CPUs ready for tracing, abort */ cpu = cpumask_first(mask); if (cpu >= nr_cpu_ids)

Updates the CoreSight documentation to cover the use of default sinks for both perf and sysfs operations.

Signed-off-by: Mike Leach mike.leach@linaro.org --- Documentation/trace/coresight/coresight.rst | 48 +++++++++++++++------ 1 file changed, 34 insertions(+), 14 deletions(-)

diff --git a/Documentation/trace/coresight/coresight.rst b/Documentation/trace/coresight/coresight.rst index 0b73acb44efa..917d89f74c2e 100644 --- a/Documentation/trace/coresight/coresight.rst +++ b/Documentation/trace/coresight/coresight.rst @@ -341,17 +341,18 @@ provide details on using both methods. 1) Using the sysFS interface:

Before trace collection can start, a coresight sink needs to be identified. -There is no limit on the amount of sinks (nor sources) that can be enabled at -any given moment. As a generic operation, all device pertaining to the sink -class will have an "active" entry in sysfs:: +There is no limit on the amount of sources and sinks that can be enabled at +any given moment. However, any source will only trace into a single sink. +As a generic operation, all device pertaining to the sink class will have an +"active" entry in sysfs::

root:/sys/bus/coresight/devices# ls - replicator 20030000.tpiu 2201c000.ptm 2203c000.etm 2203e000.etm - 20010000.etb 20040000.funnel 2201d000.ptm 2203d000.etm - root:/sys/bus/coresight/devices# ls 20010000.etb + replicator0 tpiu0 ptm0 etm2 etm3 + etb0 funnel0 ptm1 etm4 + root:/sys/bus/coresight/devices# ls etb0 enable_sink status trigger_cntr - root:/sys/bus/coresight/devices# echo 1 > 20010000.etb/enable_sink - root:/sys/bus/coresight/devices# cat 20010000.etb/enable_sink + root:/sys/bus/coresight/devices# echo 1 > etb0/enable_sink + root:/sys/bus/coresight/devices# cat etb0/enable_sink 1 root:/sys/bus/coresight/devices#

@@ -360,10 +361,10 @@ comparator with "_stext" and "_etext", essentially tracing any instruction that falls within that range. As such "enabling" a source will immediately trigger a trace capture::

- root:/sys/bus/coresight/devices# echo 1 > 2201c000.ptm/enable_source - root:/sys/bus/coresight/devices# cat 2201c000.ptm/enable_source + root:/sys/bus/coresight/devices# echo 1 > ptm0/enable_source + root:/sys/bus/coresight/devices# cat ptm0/enable_source 1 - root:/sys/bus/coresight/devices# cat 20010000.etb/status + root:/sys/bus/coresight/devices# cat etb0/status Depth: 0x2000 Status: 0x1 RAM read ptr: 0x0 @@ -376,13 +377,22 @@ trigger a trace capture::

Trace collection is stopped the same way::

- root:/sys/bus/coresight/devices# echo 0 > 2201c000.ptm/enable_source + root:/sys/bus/coresight/devices# echo 0 > ptm0/enable_source root:/sys/bus/coresight/devices#

+If no sink is enabled before the source is enabled, then a default sink will +be selected and enabled automatically. Once the source is disabled, then the +sink used can be read from <source>/last_sink.:: + + root:/sys/bus/coresight/devices# echo 1 > ptm0/enable_source + root:/sys/bus/coresight/devices# echo 0 > ptm0/enable_source + root:/sys/bus/coresight/devices# cat ptm0/last_sink + etb0 + root:/sys/bus/coresight/devices# echo 0 > etb0/enable_sink + The content of the ETB buffer can be harvested directly from /dev::

- root:/sys/bus/coresight/devices# dd if=/dev/20010000.etb \ - of=~/cstrace.bin + root:/sys/bus/coresight/devices# dd if=/dev/etb0 of=~/cstrace.bin 64+0 records in 64+0 records out 32768 bytes (33 kB) copied, 0.00125258 s, 26.2 MB/s @@ -490,6 +500,16 @@ The syntax within the forward slashes '/' is important. The '@' character tells the parser that a sink is about to be specified and that this is the sink to use for the trace session.

+Alternatively, if no sink name is given between the //, then the CoreSight +system will select a default sink:: + + root@linaro-nano:~# perf record -e cs_etm//u --per-thread program + +The system selects a sink by searching connection path from the source ETM to +any sink that is on the path.The system will prefer ETR devices over ETB/ETF, +and where two of the same type are found, the closest to the ETM, in terms of +connection links. + More information on the above and other example on how to use Coresight with the perf tools can be found in the "HOWTO.md" file of the openCSD gitHub repository [#third]_.

Hi Mathieu,

On Mon, 29 Jun 2020 at 18:47, Mathieu Poirier mathieu.poirier@linaro.org wrote:

Hi Mike,

I have applied patches 1 to 3 of this set. Please see below for comments on this patch.

On Tue, Jun 16, 2020 at 05:40:05PM +0100, Mike Leach wrote:

...

When enabling a trace source using sysfs, allow the CoreSight system to auto-select a default sink if none has been enabled by the user.

Uses the sink select algorithm that uses the default select priorities set when sinks are registered with the system. At present this will prefer ETR over ETB / ETF.

Adds a new attribute 'last_sink' to source CoreSight devices. This is set when a source is enabled using sysfs, to the sink that the device will trace into. This applies for both user enabled and default enabled sinks.

Signed-off-by: Mike Leach mike.leach@linaro.org

drivers/hwtracing/coresight/coresight.c | 39 +++++++++++++++++++++++-- include/linux/coresight.h | 3 ++ 2 files changed, 40 insertions(+), 2 deletions(-)

diff --git a/drivers/hwtracing/coresight/coresight.c b/drivers/hwtracing/coresight/coresight.c index e9c90f2de34a..db39e0b56994 100644 --- a/drivers/hwtracing/coresight/coresight.c +++ b/drivers/hwtracing/coresight/coresight.c @@ -934,6 +934,16 @@ static void coresight_clear_default_sink(struct coresight_device *csdev) } }

+static void coresight_set_last_sink_name(struct coresight_device *source,

•                                  struct coresight_device *sink)
  

+{

• /* remove current value and set new one if *sink not NULL */
  

• kfree(source->last_sink);
  

• source->last_sink = NULL;
  

• if (sink)
  

•         source->last_sink = kstrdup(dev_name(&sink->dev), GFP_KERNEL);
  

+}

/** coresight_validate_source - make sure a source has the right credentials

• @csdev: the device structure for a source.
• @function: the function this was called from.

@@ -994,8 +1004,15 @@ int coresight_enable(struct coresight_device *csdev) */ sink = coresight_get_enabled_sink(false); if (!sink) {

•         ret = -EINVAL;
  

•         goto out;
  

•         /* look for a default sink if nothing enabled */
  

•         sink = coresight_find_default_sink(csdev);
  

•         if (!sink) {
  

•                 ret = -EINVAL;
  

•                 goto out;
  

•         }
  

•         /* mark the default as enabled */
  

•         sink->activated = true;
  

•         dev_info(&sink->dev, "Enabled default sink.");
  

I'm very ambivalent about extending the automatic sink selection to the sysfs interface, mainly because of the new sysfs entry it requires.

That's interesting - this was added to overcome Suzuki's objection that it wasn't possible to determine which sink was in use!

However, I think it is important to allow this as once we see systems with many cores + many sinks, determining the correct sink to enable becomes much more difficult.

You said yourself, albeit in relation to perf, that for 1:1 systems, sink selection should be implicit. This is something I completely agree with, and hence the automatic selection algorithm that was chosen to ensure that this is the case. Is there any reason not to make the same assertion for sysfs?

Further, this allows sysfs users to write board agnostic tests (similar to the one Leo wrote for perf) - effectively all we need to do to test the coresight function on a board is iterate through the cpus / etms without worrying about the sink in use, then name of which can be read from the etm and then data read out.

As an aside - last_sink also shows which sink was used should you happen to explicitly enable two sinks in the etm path (e.g. etf & etr).

I find it clunky that users don't have to specify the sink to use but have to explicitly disable it after the trace session.

Sure - but is it not just as clunky to have to figure out which sink attaches to your etm in the first place? (yes there are topolgy links now but this is not the most straighforward thing to use) Ultimately, if you are only using sysfs, you never actually need to disable the sink to read back data if you don't want to. I am not sure there are many people who use both syfs and perf in the same session to collect trace - and these are the ones who would need to be careful about disabling the sink.

From a debug tools perspective, anything we can do to make things

easier for users - especially at so little extra cost, can only be a good thing.

We could automatically disable the sink after a trace session but that would break with the current sysfs heuristic where sinks have to be explicitly enabled and disabled.

Or possibly only auto-disable if the sink was selected by default in the first place. Another short cut would be to allow writing 0 to the last_sink attribute to disable the named sink.

Regards

Mike

Thanks, Mathieu

...

  }

  path = coresight_build_path(csdev, sink);

@@ -1033,6 +1050,9 @@ int coresight_enable(struct coresight_device *csdev) break; }

• /* record name of sink used for this session */
  

• coresight_set_last_sink_name(csdev, sink);
  

out: mutex_unlock(&coresight_mutex); return ret; @@ -1145,6 +1165,19 @@ static ssize_t enable_source_store(struct device *dev, } static DEVICE_ATTR_RW(enable_source);

+static ssize_t last_sink_show(struct device *dev,

•                       struct device_attribute *attr, char *buf)
  

+{

• struct coresight_device *csdev = to_coresight_device(dev);
  

• ssize_t size = 0;
  

• if (csdev->last_sink)
  

•         size = scnprintf(buf, PAGE_SIZE, "%s\n", csdev->last_sink);
  

• return size;
  

+} +static DEVICE_ATTR_RO(last_sink);

static struct attribute *coresight_sink_attrs[] = { &dev_attr_enable_sink.attr, NULL, @@ -1153,6 +1186,7 @@ ATTRIBUTE_GROUPS(coresight_sink);

static struct attribute *coresight_source_attrs[] = { &dev_attr_enable_source.attr,

• &dev_attr_last_sink.attr,
  NULL,
  

}; ATTRIBUTE_GROUPS(coresight_source); @@ -1524,6 +1558,7 @@ void coresight_unregister(struct coresight_device *csdev) /* Remove references of that device in the topology */ coresight_remove_conns(csdev); coresight_clear_default_sink(csdev);

• coresight_set_last_sink_name(csdev, NULL);
  coresight_release_platform_data(csdev, csdev->pdata);
  device_unregister(&csdev->dev);
  

} diff --git a/include/linux/coresight.h b/include/linux/coresight.h index 58fffdecdbfd..fc320dd2cedc 100644 --- a/include/linux/coresight.h +++ b/include/linux/coresight.h @@ -184,6 +184,8 @@ struct coresight_sysfs_link {

•       from source to that sink.
  

• @ea: Device attribute for sink representation under PMU directory.
• @def_sink: cached reference to default sink found for this device.

• • @last_sink: Name of last sink used for this source to trace into. Set when

• •       enabling a source using sysfs - only set on a source device.
    

  • @ect_dev: Associated cross trigger device. Not part of the trace data

  •       path or connections.
    

  • @nr_links: number of sysfs links created to other components from this

@@ -203,6 +205,7 @@ struct coresight_device { bool activated; /* true only if a sink is part of a path */ struct dev_ext_attribute *ea; struct coresight_device *def_sink;

• const char *last_sink;
  /* cross trigger handling */
  struct coresight_device *ect_dev;
  /* sysfs links between components */
  

-- 2.17.1

-- Mike Leach Principal Engineer, ARM Ltd. Manchester Design Centre. UK

Hi Suzuki,

On Wed, 1 Jul 2020 at 23:19, Suzuki K Poulose suzuki.poulose@arm.com wrote:

Hi Mike, Mathieu,

On 07/01/2020 05:40 PM, Mike Leach wrote:

...

Hi Mathieu,

On Mon, 29 Jun 2020 at 18:47, Mathieu Poirier mathieu.poirier@linaro.org wrote:

...

Hi Mike,

I have applied patches 1 to 3 of this set. Please see below for comments on this patch.

On Tue, Jun 16, 2020 at 05:40:05PM +0100, Mike Leach wrote:

...

When enabling a trace source using sysfs, allow the CoreSight system to auto-select a default sink if none has been enabled by the user.

Uses the sink select algorithm that uses the default select priorities set when sinks are registered with the system. At present this will prefer ETR over ETB / ETF.

Adds a new attribute 'last_sink' to source CoreSight devices. This is set when a source is enabled using sysfs, to the sink that the device will trace into. This applies for both user enabled and default enabled sinks.

Signed-off-by: Mike Leach mike.leach@linaro.org

drivers/hwtracing/coresight/coresight.c | 39 +++++++++++++++++++++++-- include/linux/coresight.h | 3 ++ 2 files changed, 40 insertions(+), 2 deletions(-)

diff --git a/drivers/hwtracing/coresight/coresight.c b/drivers/hwtracing/coresight/coresight.c index e9c90f2de34a..db39e0b56994 100644 --- a/drivers/hwtracing/coresight/coresight.c +++ b/drivers/hwtracing/coresight/coresight.c @@ -934,6 +934,16 @@ static void coresight_clear_default_sink(struct coresight_device *csdev) } }

+static void coresight_set_last_sink_name(struct coresight_device *source,

•                                  struct coresight_device *sink)
  

+{

• /* remove current value and set new one if *sink not NULL */
  

• kfree(source->last_sink);
  

• source->last_sink = NULL;
  

• if (sink)
  

•         source->last_sink = kstrdup(dev_name(&sink->dev), GFP_KERNEL);
  

+}

• /** coresight_validate_source - make sure a source has the right credentials
  • @csdev: the device structure for a source.
  • @function: the function this was called from.

@@ -994,8 +1004,15 @@ int coresight_enable(struct coresight_device *csdev) */ sink = coresight_get_enabled_sink(false); if (!sink) {

•         ret = -EINVAL;
  

•         goto out;
  

•         /* look for a default sink if nothing enabled */
  

•         sink = coresight_find_default_sink(csdev);
  

•         if (!sink) {
  

•                 ret = -EINVAL;
  

•                 goto out;
  

•         }
  

•         /* mark the default as enabled */
  

•         sink->activated = true;
  

•         dev_info(&sink->dev, "Enabled default sink.");
  

I'm very ambivalent about extending the automatic sink selection to the sysfs interface, mainly because of the new sysfs entry it requires.

That's interesting - this was added to overcome Suzuki's objection that it wasn't possible to determine which sink was in use!

I personally don't prefer the auto selection for sysfs mode. And that was one of the arguments to support it.

...

However, I think it is important to allow this as once we see systems with many cores + many sinks, determining the correct sink to enable becomes much more difficult.

You said yourself, albeit in relation to perf, that for 1:1 systems, sink selection should be implicit. This is something I completely agree with, and hence the automatic selection algorithm that was chosen to ensure that this is the case. Is there any reason not to make the same assertion for sysfs?

Further, this allows sysfs users to write board agnostic tests (similar to the one Leo wrote for perf) - effectively all we need to do to test the coresight function on a board is iterate through the cpus / etms without worrying about the sink in use, then name of which can be read from the etm and then data read out.

The tests could use the "connections" exposed via the sysfs to figure out the appropriate sink for a given source.

...

As an aside - last_sink also shows which sink was used should you happen to explicitly enable two sinks in the etm path (e.g. etf & etr).

...

I find it clunky that users don't have to specify the sink to use but have to explicitly disable it after the trace session.

Sure - but is it not just as clunky to have to figure out which sink attaches to your etm in the first place? (yes there are topolgy links now but this is not the most straighforward thing to use) Ultimately, if you are only using sysfs, you never actually need to disable the sink to read back data if you don't want to. I am not sure there are many people who use both syfs and perf in the same session to collect trace - and these are the ones who would need to be careful about disabling the sink.

The problem lies exactly there. Just like we don't know how many actual sysfs mode users are there, who consume the trace data and use it in a production environment compared to a bring up situation (verifying that the board topology is detected fine and the components are working fine), there could be users of the perf on these systems.

This is an issue irrespective of how the trace sink is turned on, be it automatically or explicitly. Given that it is possible to read the sink data without disabling the sink - the chances are it could happen either way.

Debugging such cases where someone forgot to disable the trace can be a painful process. Like I have said from the beginning, this is not worth the benefit that we get from this code (i.e, figuring out which sink is closer to a source in sysfs mode, when there is an existing infrastructure, i.e, "connections" already available for this).

Actually all connections can tell you is the number sinks available to the etm on the path - not which would be selected by the current priority algorithm - unless the user is willing to dig into the driver source code and figure out the priority mechanism.

Regards

Mike

Cheers Suzuki