Re: [PATCH v4 15/15] coresight: Move CPU hotplug callbacks to core layer

[Leo Yan <leo.yan@arm.com>]

On Wed, Nov 19, 2025 at 03:19:55PM +0000, James Clark wrote:

[...]

> > > > +static int coresight_dying_cpu(unsigned int cpu)
> > > > +{
> > > > +	struct coresight_device *source = per_cpu(csdev_source, cpu);
> > > > +	struct coresight_path *path;
> > > > +
> > > > +	if (!source || !source->path)
> > > > +		return 0;
> > > > +
> > > > +	/*
> > > > +	 * The perf event layer will disable PMU events in the CPU hotplug.
> > > > +	 * CoreSight driver should never handle the CS_MODE_PERF case.
> > > > +	 */
> > > > +	if (coresight_get_mode(source) != CS_MODE_SYSFS)
> > > > +		return 0;
> > > > +
> > > > +	/*
> > > > +	 * Save 'source->path' here, as it will be cleared in
> > > > +	 * coresight_disable_source().
> > > > +	 */
> > > > +	path = source->path;
> > > > +
> > > > +	coresight_disable_source(source, NULL);
> > > > +	coresight_disable_path(path);
> > > > +	return 0;
> > > 
> > > If the user is expected to re-enable and this new state is visible, don't
> > > you need to use the regular coresight_disable_sysfs() function? It calls
> > > coresight_disable_source_sysfs() which updates a refcount.
> > 
> > Good point!  We only need to maintain refcnt for system tracers (e.g.,
> > STM).  The per-CPU tracer is only binary states (on or off), we can
> > simply use "mode" to track state and no need refcnt.
> > 
> > I will use a separate patch to refactor refcnt.
> 
> If you remove the refcount then wouldn't it break scripts that make multiple
> enable calls? I thought the same logic for the system sources applies to CPU
> sources.

I am not removing the refcount. Since we have csdev->mode to track the
device state, I just keep refcount == 0 for the CPU source instead of
refcount == 1.  This is transparnt for user space (no any change).

Before my change:

  echo 1 > /sys/bus/coresight/devices/tmc_etr0/enable_sink
  echo 1 > /sys/bus/coresight/devices/etm2/enable_source
  refcount = 1
  echo 1 > /sys/bus/coresight/devices/etm2/enable_source
  refcount = 1
  echo 1 > /sys/bus/coresight/devices/etm2/enable_source
  refcount = 1

  echo 0 > /sys/bus/coresight/devices/etm2/enable_source
  refcount = 0

After my change:

  echo 1 > /sys/bus/coresight/devices/tmc_etr0/enable_sink
  echo 1 > /sys/bus/coresight/devices/etm2/enable_source
  refcount = 0
  echo 1 > /sys/bus/coresight/devices/etm2/enable_source
  refcount = 0
  echo 1 > /sys/bus/coresight/devices/etm2/enable_source
  refcount = 0

  echo 0 > /sys/bus/coresight/devices/etm2/enable_source
  refcount = 0

As a result, the CPU PM flow doesn't need to bother refcount when to
disable the path.

Thanks,
Leo

> TBH I don't really know what problem the refcount and multiple balanced
> enable/disable calls solves. But even if we decide it's not needed, it's too
> late to change now.