[PATCH v3 24/31] coresight: Add per-CPU path pointer

[Mike Leach]

Hi Leo

On Mon, 15 Sept 2025 at 11:34, Leo Yan <leo.yan at arm.com> wrote:
>
> Introduce per-CPU path pointers, save activated paths into the
> structure. This will be used by later changes for controlling the path
> during CPU idle.
>
> The path pointer is assigned before setting the source device mode to
> active, and it is cleared after the device is changed to an inactive
> mode. So safe access to path pointers is guaranteed when the device is
> in an active mode.
>
> The path enabling has been refactored: a new _coresight_enable_path()
> function is introduced to enable components in the path. The existing
> coresight_enable_path() function now calls this new helper and updates
> the per-CPU pointers accordingly.
>
> Signed-off-by: Leo Yan <leo.yan at arm.com>
> ---
>  drivers/hwtracing/coresight/coresight-core.c | 48 ++++++++++++++++++++++++++--
>  1 file changed, 46 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/hwtracing/coresight/coresight-core.c b/drivers/hwtracing/coresight/coresight-core.c
> index 94803cacf1a374ae8a3b9513d7d029d5c77d5925..904123279ee70531787feea98e5f9fc502470f88 100644
> --- a/drivers/hwtracing/coresight/coresight-core.c
> +++ b/drivers/hwtracing/coresight/coresight-core.c
> @@ -37,6 +37,27 @@ DEFINE_MUTEX(coresight_mutex);
>  static DEFINE_PER_CPU(struct coresight_device *, csdev_sink);
>  static DEFINE_PER_CPU(struct coresight_device *, csdev_source);
>
> +/*
> + * Activated path pointer for a per-CPU source device. When enabling a path,
> + * the path pointer is first assigned, followed by a synchronous SMP call on
> + * the target CPU to transition the device mode from DISABLED to an enabled
> + * state. Conversely, during the disable flow, an SMP call on the target CPU
> + * transitions the device mode to DISABLED, after which the path pointer is
> + * cleared.
> + *
> + *  per_cpu(csdev_cpu_path, csdev->cpu) = path
> + *  coresight_take_mode(csdev, CS_MODE_SYSFS or CS_MODE_PERF)
> + *
> + *  // Safe to access per_cpu(csdev_cpu_path, cpu);
> + *
> + *  coresight_set_mode(csdev, CS_MODE_DISABLED)
> + *  per_cpu(csdev_cpu_path, csdev->cpu) = NULL
> + *
> + * As a result, the device mode is used to determine whether it is safe
> + * to access the path pointer.
> + */
> +static DEFINE_PER_CPU(struct coresight_path *, csdev_cpu_path);

This replicates the per-cpu tracer_path variable in coresight-sysfs.c
- seems there should be a way to combine them.
This is also set / unset on enable / disable

> +
>  /**
>   * struct coresight_node - elements of a path, from source to sink
>   * @csdev:     Address of an element.
> @@ -510,6 +531,12 @@ static void coresight_disable_path_from(struct coresight_path *path,
>
>  void coresight_disable_path(struct coresight_path *path)
>  {
> +       struct coresight_device *source;
> +
> +       source = coresight_get_source(path);
> +       if (coresight_is_percpu_source(source))
> +               per_cpu(csdev_cpu_path, source->cpu) = NULL;
> +
>         coresight_disable_path_from(path, NULL);
>  }
>  EXPORT_SYMBOL_GPL(coresight_disable_path);
> @@ -533,8 +560,8 @@ static int coresight_enable_helpers(struct coresight_device *csdev,
>         return 0;
>  }
>
> -int coresight_enable_path(struct coresight_path *path, enum cs_mode mode,
> -                         void *sink_data)
> +static int _coresight_enable_path(struct coresight_path *path,
> +                                 enum cs_mode mode, void *sink_data)
>  {
>         int ret = 0;
>         u32 type;
> @@ -601,6 +628,23 @@ int coresight_enable_path(struct coresight_path *path, enum cs_mode mode,
>         goto out;
>  }
>
> +int coresight_enable_path(struct coresight_path *path, enum cs_mode mode,
> +                         void *sink_data)
> +{
> +       int ret;
> +       struct coresight_device *source;
> +
> +       ret = _coresight_enable_path(path, mode, sink_data);
> +       if (ret)
> +               return ret;
> +
> +       source = coresight_get_source(path);
> +       if (coresight_is_percpu_source(source))
> +               per_cpu(csdev_cpu_path, source->cpu) = path;


why is this in a refactored function for enable() but not disable().
Seems that the original coresight_enable_path() creates a 'source'
variable, so just the two lines

if (coresight_is_percpu_source(source))
              per_cpu(csdev_cpu_path, source->cpu) = path;

added to the original function acheives the same thing with less churn

Regards

Mike


> +
> +       return 0;
> +}
> +
>  struct coresight_device *coresight_get_sink(struct coresight_path *path)
>  {
>         struct coresight_device *csdev;
>
> --
> 2.34.1
>


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