[PATCH v3 01/31] coresight: Change device mode to atomic type

[Mike Leach]

On Mon, 15 Sept 2025 at 11:33, Leo Yan <leo.yan at arm.com> wrote:
>
> The device mode is defined as local type. This type cannot promise
> SMP-safe access.
>
> Change to atomic type and impose relax ordering, which ensures the
> SMP-safe synchronisation and the ordering between the mode setting and
> relevant operations.
>
> Fixes: 22fd532eaa0c ("coresight: etm3x: adding operation mode for etm_enable()")
> Signed-off-by: Leo Yan <leo.yan at arm.com>
> ---
>  include/linux/coresight.h | 25 +++++++++++--------------
>  1 file changed, 11 insertions(+), 14 deletions(-)
>
> diff --git a/include/linux/coresight.h b/include/linux/coresight.h
> index 6de59ce8ef8ca45c29e2f09c1b979eb7686b685f..3e5e5acd0c7fcde7d312d440da4355faaf682c7b 100644
> --- a/include/linux/coresight.h
> +++ b/include/linux/coresight.h
> @@ -251,15 +251,11 @@ struct coresight_trace_id_map {
>   *             by @coresight_ops.
>   * @access:    Device i/o access abstraction for this device.
>   * @dev:       The device entity associated to this component.
> - * @mode:      This tracer's mode, i.e sysFS, Perf or disabled. This is
> - *             actually an 'enum cs_mode', but is stored in an atomic type.
> - *             This is always accessed through local_read() and local_set(),
> - *             but wherever it's done from within the Coresight device's lock,
> - *             a non-atomic read would also work. This is the main point of
> - *             synchronisation between code happening inside the sysfs mode's
> - *             coresight_mutex and outside when running in Perf mode. A compare
> - *             and exchange swap is done to atomically claim one mode or the
> - *             other.
> + * @mode:      The device mode, i.e sysFS, Perf or disabled. This is actually
> + *             an 'enum cs_mode' but stored in an atomic type. Access is always
> + *             through atomic APIs, ensuring SMP-safe synchronisation between
> + *             racing from sysFS and Perf mode. A compare-and-exchange
> + *             operation is done to atomically claim one mode or the other.
>   * @refcnt:    keep track of what is in use. Only access this outside of the
>   *             device's spinlock when the coresight_mutex held and mode ==
>   *             CS_MODE_SYSFS. Otherwise it must be accessed from inside the
> @@ -288,7 +284,7 @@ struct coresight_device {
>         const struct coresight_ops *ops;
>         struct csdev_access access;
>         struct device dev;
> -       local_t mode;
> +       atomic_t mode;
>         int refcnt;
>         bool orphan;
>         /* sink specific fields */
> @@ -621,13 +617,14 @@ static inline bool coresight_is_percpu_sink(struct coresight_device *csdev)
>  static inline bool coresight_take_mode(struct coresight_device *csdev,
>                                        enum cs_mode new_mode)
>  {
> -       return local_cmpxchg(&csdev->mode, CS_MODE_DISABLED, new_mode) ==
> -              CS_MODE_DISABLED;
> +       int curr = CS_MODE_DISABLED;
> +
> +       return atomic_try_cmpxchg_acquire(&csdev->mode, &curr, new_mode);
>  }
>
>  static inline enum cs_mode coresight_get_mode(struct coresight_device *csdev)
>  {
> -       return local_read(&csdev->mode);
> +       return atomic_read_acquire(&csdev->mode);
>  }
>
>  static inline void coresight_set_mode(struct coresight_device *csdev,
> @@ -643,7 +640,7 @@ static inline void coresight_set_mode(struct coresight_device *csdev,
>         WARN(new_mode != CS_MODE_DISABLED && current_mode != CS_MODE_DISABLED &&
>              current_mode != new_mode, "Device already in use\n");
>
> -       local_set(&csdev->mode, new_mode);
> +       atomic_set_release(&csdev->mode, new_mode);
>  }
>
>  struct coresight_device *coresight_register(struct coresight_desc *desc);
>
> --
> 2.34.1
>

Reviewed-by: Mike Leach <mike.leach at linaro.org>

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