Added the panthor_perf system initialization and unplug code to allow
for the handling of userspace sessions to be added in follow-up
patches.

Signed-off-by: Lukas Zapolskas <lukas.zapolskas@arm.com>
Reviewed-by: Adrián Larumbe <adrian.larumbe@collabora.com>
---
 drivers/gpu/drm/panthor/panthor_device.c |  2 +
 drivers/gpu/drm/panthor/panthor_device.h |  5 +-
 drivers/gpu/drm/panthor/panthor_perf.c   | 63 +++++++++++++++++++++++-
 drivers/gpu/drm/panthor/panthor_perf.h   |  1 +
 4 files changed, 69 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/panthor/panthor_device.c b/drivers/gpu/drm/panthor/panthor_device.c
index dc237da92340..3063ffbead45 100644
--- a/drivers/gpu/drm/panthor/panthor_device.c
+++ b/drivers/gpu/drm/panthor/panthor_device.c
@@ -120,6 +120,7 @@ void panthor_device_unplug(struct panthor_device *ptdev)
 	/* Now, try to cleanly shutdown the GPU before the device resources
 	 * get reclaimed.
 	 */
+	panthor_perf_unplug(ptdev);
 	panthor_sched_unplug(ptdev);
 	panthor_fw_unplug(ptdev);
 	panthor_mmu_unplug(ptdev);
@@ -323,6 +324,7 @@ int panthor_device_init(struct panthor_device *ptdev)
 
 err_disable_autosuspend:
 	pm_runtime_dont_use_autosuspend(ptdev->base.dev);
+	panthor_perf_unplug(ptdev);
 
 err_unplug_sched:
 	panthor_sched_unplug(ptdev);
diff --git a/drivers/gpu/drm/panthor/panthor_device.h b/drivers/gpu/drm/panthor/panthor_device.h
index 64b0048de6ac..e1a6250cecc8 100644
--- a/drivers/gpu/drm/panthor/panthor_device.h
+++ b/drivers/gpu/drm/panthor/panthor_device.h
@@ -28,8 +28,8 @@ struct panthor_hw;
 struct panthor_job;
 struct panthor_mmu;
 struct panthor_fw;
-struct panthor_perfcnt;
 struct panthor_pwr;
+struct panthor_perf;
 struct panthor_vm;
 struct panthor_vm_pool;
 
@@ -160,6 +160,9 @@ struct panthor_device {
 	/** @devfreq: Device frequency scaling management data. */
 	struct panthor_devfreq *devfreq;
 
+	/** @perf: Performance counter management data. */
+	struct panthor_perf *perf;
+
 	/** @unplug: Device unplug related fields. */
 	struct {
 		/** @lock: Lock used to serialize unplug operations. */
diff --git a/drivers/gpu/drm/panthor/panthor_perf.c b/drivers/gpu/drm/panthor/panthor_perf.c
index 842d62826ac3..3a65d6d326e8 100644
--- a/drivers/gpu/drm/panthor/panthor_perf.c
+++ b/drivers/gpu/drm/panthor/panthor_perf.c
@@ -4,6 +4,7 @@
 
 #include <linux/bitops.h>
 #include <drm/panthor_drm.h>
+#include <drm/drm_print.h>
 
 #include "panthor_device.h"
 #include "panthor_fw.h"
@@ -22,6 +23,19 @@
  */
 #define PANTHOR_HW_COUNTER_SIZE (sizeof(u32))
 
+struct panthor_perf {
+	/** @next_session: The ID of the next session. */
+	u32 next_session;
+
+	/** @session_range: The number of sessions supported at a time. */
+	struct xa_limit session_range;
+
+	/**
+	 * @sessions: Global map of sessions, accessed by their ID.
+	 */
+	struct xarray sessions;
+};
+
 struct panthor_perf_counter_block {
 	struct drm_panthor_perf_block_header header;
 	u64 counters[];
@@ -76,14 +90,61 @@ static void panthor_perf_info_init(struct panthor_device *const ptdev)
  * panthor_perf_init - Initialize the performance counter subsystem.
  * @ptdev: Panthor device
  *
+ * The performance counters require the FW interface to be available to setup the
+ * sampling ringbuffers, so this must be called only after FW is initialized.
+ *
  * Return: 0 on success, negative error code on failure.
  */
 int panthor_perf_init(struct panthor_device *ptdev)
 {
+	struct panthor_perf *perf __free(kfree) = NULL;
+	int ret = 0;
+
 	if (!ptdev)
 		return -EINVAL;
 
 	panthor_perf_info_init(ptdev);
 
-	return 0;
+	perf = kzalloc(sizeof(*perf), GFP_KERNEL);
+	if (ZERO_OR_NULL_PTR(perf))
+		return -ENOMEM;
+
+	xa_init_flags(&perf->sessions, XA_FLAGS_ALLOC);
+
+	perf->session_range = (struct xa_limit) {
+		.min = 0,
+		.max = 1,
+	};
+
+	drm_info(&ptdev->base, "Performance counter subsystem initialized");
+
+	ptdev->perf = no_free_ptr(perf);
+
+	return ret;
+}
+
+/**
+ * panthor_perf_unplug - Terminate the performance counter subsystem.
+ * @ptdev: Panthor device.
+ *
+ * This function will terminate the performance counter control structures and any remaining
+ * sessions, after waiting for any pending interrupts.
+ */
+void panthor_perf_unplug(struct panthor_device *ptdev)
+{
+	struct panthor_perf *perf = ptdev->perf;
+
+	if (!perf)
+		return;
+
+	if (!xa_empty(&perf->sessions)) {
+		drm_err(&ptdev->base,
+			"Performance counter sessions active when unplugging the driver!");
+	}
+
+	xa_destroy(&perf->sessions);
+
+	kfree(ptdev->perf);
+
+	ptdev->perf = NULL;
 }
diff --git a/drivers/gpu/drm/panthor/panthor_perf.h b/drivers/gpu/drm/panthor/panthor_perf.h
index 3c32c24c164c..e4805727b9e7 100644
--- a/drivers/gpu/drm/panthor/panthor_perf.h
+++ b/drivers/gpu/drm/panthor/panthor_perf.h
@@ -10,6 +10,7 @@
 struct panthor_device;
 
 int panthor_perf_init(struct panthor_device *ptdev);
+void panthor_perf_unplug(struct panthor_device *ptdev);
 
 #endif /* __PANTHOR_PERF_H__ */
 
-- 
2.33.0.dirty

On Mon, 15 Dec 2025 17:14:49 +0000
Lukas Zapolskas <lukas.zapolskas@arm.com> wrote:

> Added the panthor_perf system initialization and unplug code to allow
> for the handling of userspace sessions to be added in follow-up
> patches.
> 
> Signed-off-by: Lukas Zapolskas <lukas.zapolskas@arm.com>
> Reviewed-by: Adrián Larumbe <adrian.larumbe@collabora.com>
> ---
>  drivers/gpu/drm/panthor/panthor_device.c |  2 +
>  drivers/gpu/drm/panthor/panthor_device.h |  5 +-
>  drivers/gpu/drm/panthor/panthor_perf.c   | 63 +++++++++++++++++++++++-
>  drivers/gpu/drm/panthor/panthor_perf.h   |  1 +
>  4 files changed, 69 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_device.c b/drivers/gpu/drm/panthor/panthor_device.c
> index dc237da92340..3063ffbead45 100644
> --- a/drivers/gpu/drm/panthor/panthor_device.c
> +++ b/drivers/gpu/drm/panthor/panthor_device.c
> @@ -120,6 +120,7 @@ void panthor_device_unplug(struct panthor_device *ptdev)
>  	/* Now, try to cleanly shutdown the GPU before the device resources
>  	 * get reclaimed.
>  	 */
> +	panthor_perf_unplug(ptdev);
>  	panthor_sched_unplug(ptdev);
>  	panthor_fw_unplug(ptdev);
>  	panthor_mmu_unplug(ptdev);
> @@ -323,6 +324,7 @@ int panthor_device_init(struct panthor_device *ptdev)
>  
>  err_disable_autosuspend:
>  	pm_runtime_dont_use_autosuspend(ptdev->base.dev);
> +	panthor_perf_unplug(ptdev);
>  
>  err_unplug_sched:
>  	panthor_sched_unplug(ptdev);
> diff --git a/drivers/gpu/drm/panthor/panthor_device.h b/drivers/gpu/drm/panthor/panthor_device.h
> index 64b0048de6ac..e1a6250cecc8 100644
> --- a/drivers/gpu/drm/panthor/panthor_device.h
> +++ b/drivers/gpu/drm/panthor/panthor_device.h
> @@ -28,8 +28,8 @@ struct panthor_hw;
>  struct panthor_job;
>  struct panthor_mmu;
>  struct panthor_fw;
> -struct panthor_perfcnt;
>  struct panthor_pwr;
> +struct panthor_perf;
>  struct panthor_vm;
>  struct panthor_vm_pool;
>  
> @@ -160,6 +160,9 @@ struct panthor_device {
>  	/** @devfreq: Device frequency scaling management data. */
>  	struct panthor_devfreq *devfreq;
>  
> +	/** @perf: Performance counter management data. */
> +	struct panthor_perf *perf;
> +
>  	/** @unplug: Device unplug related fields. */
>  	struct {
>  		/** @lock: Lock used to serialize unplug operations. */
> diff --git a/drivers/gpu/drm/panthor/panthor_perf.c b/drivers/gpu/drm/panthor/panthor_perf.c
> index 842d62826ac3..3a65d6d326e8 100644
> --- a/drivers/gpu/drm/panthor/panthor_perf.c
> +++ b/drivers/gpu/drm/panthor/panthor_perf.c
> @@ -4,6 +4,7 @@
>  
>  #include <linux/bitops.h>
>  #include <drm/panthor_drm.h>
> +#include <drm/drm_print.h>
>  
>  #include "panthor_device.h"
>  #include "panthor_fw.h"
> @@ -22,6 +23,19 @@
>   */
>  #define PANTHOR_HW_COUNTER_SIZE (sizeof(u32))
>  
> +struct panthor_perf {
> +	/** @next_session: The ID of the next session. */
> +	u32 next_session;
> +
> +	/** @session_range: The number of sessions supported at a time. */
> +	struct xa_limit session_range;
> +
> +	/**
> +	 * @sessions: Global map of sessions, accessed by their ID.
> +	 */
> +	struct xarray sessions;
> +};
> +
>  struct panthor_perf_counter_block {
>  	struct drm_panthor_perf_block_header header;
>  	u64 counters[];
> @@ -76,14 +90,61 @@ static void panthor_perf_info_init(struct panthor_device *const ptdev)
>   * panthor_perf_init - Initialize the performance counter subsystem.
>   * @ptdev: Panthor device
>   *
> + * The performance counters require the FW interface to be available to setup the
> + * sampling ringbuffers, so this must be called only after FW is initialized.
> + *
>   * Return: 0 on success, negative error code on failure.
>   */
>  int panthor_perf_init(struct panthor_device *ptdev)
>  {
> +	struct panthor_perf *perf __free(kfree) = NULL;
> +	int ret = 0;
> +
>  	if (!ptdev)
>  		return -EINVAL;
>  
>  	panthor_perf_info_init(ptdev);
>  
> -	return 0;
> +	perf = kzalloc(sizeof(*perf), GFP_KERNEL);
> +	if (ZERO_OR_NULL_PTR(perf))
> +		return -ENOMEM;
> +
> +	xa_init_flags(&perf->sessions, XA_FLAGS_ALLOC);
> +
> +	perf->session_range = (struct xa_limit) {
> +		.min = 0,
> +		.max = 1,
> +	};
> +
> +	drm_info(&ptdev->base, "Performance counter subsystem initialized");
> +
> +	ptdev->perf = no_free_ptr(perf);

side-note: it's a bit of a personal opinion but those __free(kfree)
annotated vars that are then assigned to something or returned make me
nervous. AFAICT, there's no compile-time warning/error when you forget
to wrap the assigned/returned value with no_free_ptr()/return_ptr(),
which basically turn unlikely leaks (because the error path is rarely
taken) into almost certain UAF situations. I know it's something you
have to get right only once most of the time, but still, I find it quite
dangerous, especially when early bailouts with valid pointers are added
to an existing function. All this to say that, unless there's a very
good reason to use it, or if there exist tools to prove correctness at
compile time, I'd prefer to stay away from this pattern and stick to
manual kfree()s.

> +
> +	return ret;
> +}
> +
> +/**
> + * panthor_perf_unplug - Terminate the performance counter subsystem.
> + * @ptdev: Panthor device.
> + *
> + * This function will terminate the performance counter control structures and any remaining
> + * sessions, after waiting for any pending interrupts.
> + */
> +void panthor_perf_unplug(struct panthor_device *ptdev)
> +{
> +	struct panthor_perf *perf = ptdev->perf;
> +
> +	if (!perf)
> +		return;
> +
> +	if (!xa_empty(&perf->sessions)) {
> +		drm_err(&ptdev->base,
> +			"Performance counter sessions active when unplugging the driver!");
> +	}
> +
> +	xa_destroy(&perf->sessions);
> +
> +	kfree(ptdev->perf);
> +
> +	ptdev->perf = NULL;
>  }
> diff --git a/drivers/gpu/drm/panthor/panthor_perf.h b/drivers/gpu/drm/panthor/panthor_perf.h
> index 3c32c24c164c..e4805727b9e7 100644
> --- a/drivers/gpu/drm/panthor/panthor_perf.h
> +++ b/drivers/gpu/drm/panthor/panthor_perf.h
> @@ -10,6 +10,7 @@
>  struct panthor_device;
>  
>  int panthor_perf_init(struct panthor_device *ptdev);
> +void panthor_perf_unplug(struct panthor_device *ptdev);
>  
>  #endif /* __PANTHOR_PERF_H__ */
>  

On Mon, 15 Dec 2025 17:14:49 +0000
Lukas Zapolskas <lukas.zapolskas@arm.com> wrote:

> Added the panthor_perf system initialization and unplug code to allow
> for the handling of userspace sessions to be added in follow-up
> patches.
> 
> Signed-off-by: Lukas Zapolskas <lukas.zapolskas@arm.com>
> Reviewed-by: Adrián Larumbe <adrian.larumbe@collabora.com>
> ---
>  drivers/gpu/drm/panthor/panthor_device.c |  2 +
>  drivers/gpu/drm/panthor/panthor_device.h |  5 +-
>  drivers/gpu/drm/panthor/panthor_perf.c   | 63 +++++++++++++++++++++++-
>  drivers/gpu/drm/panthor/panthor_perf.h   |  1 +
>  4 files changed, 69 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_device.c b/drivers/gpu/drm/panthor/panthor_device.c
> index dc237da92340..3063ffbead45 100644
> --- a/drivers/gpu/drm/panthor/panthor_device.c
> +++ b/drivers/gpu/drm/panthor/panthor_device.c
> @@ -120,6 +120,7 @@ void panthor_device_unplug(struct panthor_device *ptdev)
>  	/* Now, try to cleanly shutdown the GPU before the device resources
>  	 * get reclaimed.
>  	 */
> +	panthor_perf_unplug(ptdev);
>  	panthor_sched_unplug(ptdev);
>  	panthor_fw_unplug(ptdev);
>  	panthor_mmu_unplug(ptdev);
> @@ -323,6 +324,7 @@ int panthor_device_init(struct panthor_device *ptdev)
>  
>  err_disable_autosuspend:
>  	pm_runtime_dont_use_autosuspend(ptdev->base.dev);
> +	panthor_perf_unplug(ptdev);
>  
>  err_unplug_sched:
>  	panthor_sched_unplug(ptdev);
> diff --git a/drivers/gpu/drm/panthor/panthor_device.h b/drivers/gpu/drm/panthor/panthor_device.h
> index 64b0048de6ac..e1a6250cecc8 100644
> --- a/drivers/gpu/drm/panthor/panthor_device.h
> +++ b/drivers/gpu/drm/panthor/panthor_device.h
> @@ -28,8 +28,8 @@ struct panthor_hw;
>  struct panthor_job;
>  struct panthor_mmu;
>  struct panthor_fw;
> -struct panthor_perfcnt;
>  struct panthor_pwr;
> +struct panthor_perf;
>  struct panthor_vm;
>  struct panthor_vm_pool;
>  
> @@ -160,6 +160,9 @@ struct panthor_device {
>  	/** @devfreq: Device frequency scaling management data. */
>  	struct panthor_devfreq *devfreq;
>  
> +	/** @perf: Performance counter management data. */
> +	struct panthor_perf *perf;
> +
>  	/** @unplug: Device unplug related fields. */
>  	struct {
>  		/** @lock: Lock used to serialize unplug operations. */
> diff --git a/drivers/gpu/drm/panthor/panthor_perf.c b/drivers/gpu/drm/panthor/panthor_perf.c
> index 842d62826ac3..3a65d6d326e8 100644
> --- a/drivers/gpu/drm/panthor/panthor_perf.c
> +++ b/drivers/gpu/drm/panthor/panthor_perf.c
> @@ -4,6 +4,7 @@
>  
>  #include <linux/bitops.h>
>  #include <drm/panthor_drm.h>
> +#include <drm/drm_print.h>
>  
>  #include "panthor_device.h"
>  #include "panthor_fw.h"
> @@ -22,6 +23,19 @@
>   */
>  #define PANTHOR_HW_COUNTER_SIZE (sizeof(u32))
>  
> +struct panthor_perf {
> +	/** @next_session: The ID of the next session. */
> +	u32 next_session;
> +
> +	/** @session_range: The number of sessions supported at a time. */
> +	struct xa_limit session_range;
> +
> +	/**
> +	 * @sessions: Global map of sessions, accessed by their ID.
> +	 */
> +	struct xarray sessions;
> +};
> +
>  struct panthor_perf_counter_block {
>  	struct drm_panthor_perf_block_header header;
>  	u64 counters[];
> @@ -76,14 +90,61 @@ static void panthor_perf_info_init(struct panthor_device *const ptdev)
>   * panthor_perf_init - Initialize the performance counter subsystem.
>   * @ptdev: Panthor device
>   *
> + * The performance counters require the FW interface to be available to setup the
> + * sampling ringbuffers, so this must be called only after FW is initialized.
> + *
>   * Return: 0 on success, negative error code on failure.
>   */
>  int panthor_perf_init(struct panthor_device *ptdev)
>  {
> +	struct panthor_perf *perf __free(kfree) = NULL;
> +	int ret = 0;
> +
>  	if (!ptdev)
>  		return -EINVAL;
>  
>  	panthor_perf_info_init(ptdev);
>  
> -	return 0;
> +	perf = kzalloc(sizeof(*perf), GFP_KERNEL);
> +	if (ZERO_OR_NULL_PTR(perf))
> +		return -ENOMEM;
> +
> +	xa_init_flags(&perf->sessions, XA_FLAGS_ALLOC);
> +
> +	perf->session_range = (struct xa_limit) {
> +		.min = 0,
> +		.max = 1,
> +	};
> +
> +	drm_info(&ptdev->base, "Performance counter subsystem initialized");
> +
> +	ptdev->perf = no_free_ptr(perf);
> +
> +	return ret;

	return 0;

and you can drop the ret.

> +}
> +
> +/**
> + * panthor_perf_unplug - Terminate the performance counter subsystem.
> + * @ptdev: Panthor device.
> + *
> + * This function will terminate the performance counter control structures and any remaining
> + * sessions, after waiting for any pending interrupts.
> + */
> +void panthor_perf_unplug(struct panthor_device *ptdev)
> +{
> +	struct panthor_perf *perf = ptdev->perf;
> +
> +	if (!perf)
> +		return;
> +
> +	if (!xa_empty(&perf->sessions)) {
> +		drm_err(&ptdev->base,
> +			"Performance counter sessions active when unplugging the driver!");
> +	}
> +
> +	xa_destroy(&perf->sessions);
> +
> +	kfree(ptdev->perf);
> +
> +	ptdev->perf = NULL;
>  }
> diff --git a/drivers/gpu/drm/panthor/panthor_perf.h b/drivers/gpu/drm/panthor/panthor_perf.h
> index 3c32c24c164c..e4805727b9e7 100644
> --- a/drivers/gpu/drm/panthor/panthor_perf.h
> +++ b/drivers/gpu/drm/panthor/panthor_perf.h
> @@ -10,6 +10,7 @@
>  struct panthor_device;
>  
>  int panthor_perf_init(struct panthor_device *ptdev);
> +void panthor_perf_unplug(struct panthor_device *ptdev);
>  
>  #endif /* __PANTHOR_PERF_H__ */
>  

On Mon, 15 Dec 2025 17:14:49 +0000
Lukas Zapolskas <lukas.zapolskas@arm.com> wrote:

> Added the panthor_perf system initialization and unplug code to allow
> for the handling of userspace sessions to be added in follow-up
> patches.
> 
> Signed-off-by: Lukas Zapolskas <lukas.zapolskas@arm.com>
> Reviewed-by: Adrián Larumbe <adrian.larumbe@collabora.com>
> ---
>  drivers/gpu/drm/panthor/panthor_device.c |  2 +
>  drivers/gpu/drm/panthor/panthor_device.h |  5 +-
>  drivers/gpu/drm/panthor/panthor_perf.c   | 63 +++++++++++++++++++++++-
>  drivers/gpu/drm/panthor/panthor_perf.h   |  1 +
>  4 files changed, 69 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_device.c b/drivers/gpu/drm/panthor/panthor_device.c
> index dc237da92340..3063ffbead45 100644
> --- a/drivers/gpu/drm/panthor/panthor_device.c
> +++ b/drivers/gpu/drm/panthor/panthor_device.c
> @@ -120,6 +120,7 @@ void panthor_device_unplug(struct panthor_device *ptdev)
>  	/* Now, try to cleanly shutdown the GPU before the device resources
>  	 * get reclaimed.
>  	 */
> +	panthor_perf_unplug(ptdev);
>  	panthor_sched_unplug(ptdev);
>  	panthor_fw_unplug(ptdev);
>  	panthor_mmu_unplug(ptdev);
> @@ -323,6 +324,7 @@ int panthor_device_init(struct panthor_device *ptdev)
>  
>  err_disable_autosuspend:
>  	pm_runtime_dont_use_autosuspend(ptdev->base.dev);
> +	panthor_perf_unplug(ptdev);
>  
>  err_unplug_sched:
>  	panthor_sched_unplug(ptdev);
> diff --git a/drivers/gpu/drm/panthor/panthor_device.h b/drivers/gpu/drm/panthor/panthor_device.h
> index 64b0048de6ac..e1a6250cecc8 100644
> --- a/drivers/gpu/drm/panthor/panthor_device.h
> +++ b/drivers/gpu/drm/panthor/panthor_device.h
> @@ -28,8 +28,8 @@ struct panthor_hw;
>  struct panthor_job;
>  struct panthor_mmu;
>  struct panthor_fw;
> -struct panthor_perfcnt;
>  struct panthor_pwr;
> +struct panthor_perf;
>  struct panthor_vm;
>  struct panthor_vm_pool;
>  
> @@ -160,6 +160,9 @@ struct panthor_device {
>  	/** @devfreq: Device frequency scaling management data. */
>  	struct panthor_devfreq *devfreq;
>  
> +	/** @perf: Performance counter management data. */
> +	struct panthor_perf *perf;
> +
>  	/** @unplug: Device unplug related fields. */
>  	struct {
>  		/** @lock: Lock used to serialize unplug operations. */
> diff --git a/drivers/gpu/drm/panthor/panthor_perf.c b/drivers/gpu/drm/panthor/panthor_perf.c
> index 842d62826ac3..3a65d6d326e8 100644
> --- a/drivers/gpu/drm/panthor/panthor_perf.c
> +++ b/drivers/gpu/drm/panthor/panthor_perf.c
> @@ -4,6 +4,7 @@
>  
>  #include <linux/bitops.h>
>  #include <drm/panthor_drm.h>
> +#include <drm/drm_print.h>
>  
>  #include "panthor_device.h"
>  #include "panthor_fw.h"
> @@ -22,6 +23,19 @@
>   */
>  #define PANTHOR_HW_COUNTER_SIZE (sizeof(u32))
>  
> +struct panthor_perf {
> +	/** @next_session: The ID of the next session. */
> +	u32 next_session;
> +
> +	/** @session_range: The number of sessions supported at a time. */
> +	struct xa_limit session_range;
> +
> +	/**
> +	 * @sessions: Global map of sessions, accessed by their ID.
> +	 */
> +	struct xarray sessions;

Unless there's a need to have global session ID (sessions can be shared
and accessed from different processes), I'd move the xarray to some
panthor_file_perf object and make this session ID per-FD. Actually, I'm
not even sure I see a use case for having more than one session
per-file, so it could even be just a panthor_perf_session pointer and a
lock in panthor_file:

struct panthor_file {
	...
	struct {
		struct mutex lock;
		struct panthor_perf_session *session;
	} perf;
};

If we want to restrict the total number of sessions, we can have an
atomic_t in panthor_perf, but I think what really matters is the
maximum number of active sessions, and I believe we already have a
counter for that (panthor_perf_sampler::enabled_clients).

> +};
> +
>  struct panthor_perf_counter_block {
>  	struct drm_panthor_perf_block_header header;
>  	u64 counters[];
> @@ -76,14 +90,61 @@ static void panthor_perf_info_init(struct panthor_device *const ptdev)
>   * panthor_perf_init - Initialize the performance counter subsystem.
>   * @ptdev: Panthor device
>   *
> + * The performance counters require the FW interface to be available to setup the
> + * sampling ringbuffers, so this must be called only after FW is initialized.
> + *
>   * Return: 0 on success, negative error code on failure.
>   */
>  int panthor_perf_init(struct panthor_device *ptdev)
>  {
> +	struct panthor_perf *perf __free(kfree) = NULL;
> +	int ret = 0;
> +
>  	if (!ptdev)
>  		return -EINVAL;
>  
>  	panthor_perf_info_init(ptdev);
>  
> -	return 0;
> +	perf = kzalloc(sizeof(*perf), GFP_KERNEL);
> +	if (ZERO_OR_NULL_PTR(perf))
> +		return -ENOMEM;
> +
> +	xa_init_flags(&perf->sessions, XA_FLAGS_ALLOC);
> +
> +	perf->session_range = (struct xa_limit) {
> +		.min = 0,
> +		.max = 1,
> +	};
> +
> +	drm_info(&ptdev->base, "Performance counter subsystem initialized");
> +
> +	ptdev->perf = no_free_ptr(perf);
> +
> +	return ret;
> +}
> +
> +/**
> + * panthor_perf_unplug - Terminate the performance counter subsystem.
> + * @ptdev: Panthor device.
> + *
> + * This function will terminate the performance counter control structures and any remaining
> + * sessions, after waiting for any pending interrupts.
> + */
> +void panthor_perf_unplug(struct panthor_device *ptdev)
> +{
> +	struct panthor_perf *perf = ptdev->perf;
> +
> +	if (!perf)
> +		return;
> +
> +	if (!xa_empty(&perf->sessions)) {
> +		drm_err(&ptdev->base,
> +			"Performance counter sessions active when unplugging the driver!");
> +	}
> +
> +	xa_destroy(&perf->sessions);
> +
> +	kfree(ptdev->perf);
> +
> +	ptdev->perf = NULL;
>  }
> diff --git a/drivers/gpu/drm/panthor/panthor_perf.h b/drivers/gpu/drm/panthor/panthor_perf.h
> index 3c32c24c164c..e4805727b9e7 100644
> --- a/drivers/gpu/drm/panthor/panthor_perf.h
> +++ b/drivers/gpu/drm/panthor/panthor_perf.h
> @@ -10,6 +10,7 @@
>  struct panthor_device;
>  
>  int panthor_perf_init(struct panthor_device *ptdev);
> +void panthor_perf_unplug(struct panthor_device *ptdev);
>  
>  #endif /* __PANTHOR_PERF_H__ */
>