sched_ext: Maintain an online cid mask in the scheduler arenagrafted

Schedulers on the default cid mapping treat [0, nr_online_cids) as the
online set and restart on hotplug. Schedulers that install their own mapping
with scx_bpf_cid_override() have no way to learn which cids are online: the
count no longer identifies members and the CPU-form cpumask is unusable from
cid programs. This is an obvious hole in the cid API.

Add scx_bpf_online_cmask(), a kernel-maintained cmask in the scheduler's
arena, allocated alongside the per-CPU scratch masks and populated after the
cid mapping is finalized and before ops.init(), for child schedulers too.
The pointer stays valid through ops.exit() with no reference to take. It is
the arena offset as a void pointer, the same form struct_ops arena arguments
arrive in. The verifier types the void return as a scalar for the program's
arena cast.

The mask follows the SCX hotplug notifications: seeded from cpu_active_mask
and updated before ops.cid_online/offline() runs, so it lags cpu_online_mask
only inside a hotplug transition. Updates walk the scheduler list under the
lock that also serializes unlinking. Reads are live, not atomic snapshots.
Root initialization excludes hotplug.

v2: Reworded the getter kerneldoc (Andrea Righi).

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>

2 files changed, 184 insertions, 0 deletions

diff --git a/Documentation/gpu/imagination/index.rst b/Documentation/gpu/imagination/index.rst
new file mode 100644
index 000000000..0c1e247ce
--- /dev/null
+++ b/Documentation/gpu/imagination/index.rst
@@ -0,0 +1,13 @@
+=======================================
+drm/imagination PowerVR Graphics Driver
+=======================================
+
+.. kernel-doc:: drivers/gpu/drm/imagination/pvr_drv.c
+   :doc: PowerVR (Series 6 and later) and IMG Graphics Driver
+
+Contents
+========
+.. toctree::
+   :maxdepth: 2
+
+   uapi
diff --git a/Documentation/gpu/imagination/uapi.rst b/Documentation/gpu/imagination/uapi.rst
new file mode 100644
index 000000000..7502413d0
--- /dev/null
+++ b/Documentation/gpu/imagination/uapi.rst
@@ -0,0 +1,171 @@
+====
+UAPI
+====
+The sources associated with this section can be found in ``pvr_drm.h``.
+
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :doc: PowerVR UAPI
+
+OBJECT ARRAYS
+=============
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :identifiers: drm_pvr_obj_array
+
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :identifiers: DRM_PVR_OBJ_ARRAY
+
+IOCTLS
+======
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :doc: PowerVR IOCTL interface
+
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :identifiers: PVR_IOCTL
+
+DEV_QUERY
+---------
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :doc: PowerVR IOCTL DEV_QUERY interface
+
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :identifiers: drm_pvr_dev_query
+
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :identifiers: drm_pvr_ioctl_dev_query_args
+
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :identifiers: drm_pvr_dev_query_gpu_info
+                 drm_pvr_dev_query_runtime_info
+                 drm_pvr_dev_query_hwrt_info
+                 drm_pvr_dev_query_quirks
+                 drm_pvr_dev_query_enhancements
+
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :identifiers: drm_pvr_heap_id
+                 drm_pvr_heap
+                 drm_pvr_dev_query_heap_info
+
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :identifiers: drm_pvr_static_data_area_usage
+                 drm_pvr_static_data_area
+                 drm_pvr_dev_query_static_data_areas
+
+CREATE_BO
+---------
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :doc: PowerVR IOCTL CREATE_BO interface
+
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :identifiers: drm_pvr_ioctl_create_bo_args
+
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :doc: Flags for CREATE_BO
+
+GET_BO_MMAP_OFFSET
+------------------
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :doc: PowerVR IOCTL GET_BO_MMAP_OFFSET interface
+
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :identifiers: drm_pvr_ioctl_get_bo_mmap_offset_args
+
+CREATE_VM_CONTEXT and DESTROY_VM_CONTEXT
+----------------------------------------
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :doc: PowerVR IOCTL CREATE_VM_CONTEXT and DESTROY_VM_CONTEXT interfaces
+
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :identifiers: drm_pvr_ioctl_create_vm_context_args
+                 drm_pvr_ioctl_destroy_vm_context_args
+
+VM_MAP and VM_UNMAP
+-------------------
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :doc: PowerVR IOCTL VM_MAP and VM_UNMAP interfaces
+
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :identifiers: drm_pvr_ioctl_vm_map_args
+                 drm_pvr_ioctl_vm_unmap_args
+
+CREATE_CONTEXT and DESTROY_CONTEXT
+----------------------------------
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :doc: PowerVR IOCTL CREATE_CONTEXT and DESTROY_CONTEXT interfaces
+
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :identifiers: drm_pvr_ioctl_create_context_args
+
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :identifiers: drm_pvr_ctx_priority
+                 drm_pvr_ctx_type
+                 drm_pvr_static_render_context_state
+                 drm_pvr_static_render_context_state_format
+                 drm_pvr_reset_framework
+                 drm_pvr_reset_framework_format
+
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :identifiers: drm_pvr_ioctl_destroy_context_args
+
+CREATE_FREE_LIST and DESTROY_FREE_LIST
+--------------------------------------
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :doc: PowerVR IOCTL CREATE_FREE_LIST and DESTROY_FREE_LIST interfaces
+
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :identifiers: drm_pvr_ioctl_create_free_list_args
+
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :identifiers: drm_pvr_ioctl_destroy_free_list_args
+
+CREATE_HWRT_DATASET and DESTROY_HWRT_DATASET
+--------------------------------------------
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :doc: PowerVR IOCTL CREATE_HWRT_DATASET and DESTROY_HWRT_DATASET interfaces
+
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :identifiers: drm_pvr_ioctl_create_hwrt_dataset_args
+
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :identifiers: drm_pvr_create_hwrt_geom_data_args
+                 drm_pvr_create_hwrt_rt_data_args
+
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :identifiers: drm_pvr_ioctl_destroy_hwrt_dataset_args
+
+SUBMIT_JOBS
+-----------
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :doc: PowerVR IOCTL SUBMIT_JOBS interface
+
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :doc: Flags for the drm_pvr_sync_op object.
+
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :identifiers: drm_pvr_ioctl_submit_jobs_args
+
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :doc: Flags for SUBMIT_JOB ioctl geometry command.
+
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :doc: Flags for SUBMIT_JOB ioctl fragment command.
+
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :doc: Flags for SUBMIT_JOB ioctl compute command.
+
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :doc: Flags for SUBMIT_JOB ioctl transfer command.
+
+.. kernel-doc:: include/uapi/drm/pvr_drm.h
+   :identifiers: drm_pvr_sync_op
+                 drm_pvr_job_type
+                 drm_pvr_hwrt_data_ref
+                 drm_pvr_job
+
+Internal notes
+==============
+.. kernel-doc:: drivers/gpu/drm/imagination/pvr_device.h
+   :doc: IOCTL validation helpers
+
+.. kernel-doc:: drivers/gpu/drm/imagination/pvr_device.h
+   :identifiers: PVR_STATIC_ASSERT_64BIT_ALIGNED PVR_IOCTL_UNION_PADDING_CHECK
+                 pvr_ioctl_union_padding_check