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>

3 files changed, 126 insertions, 0 deletions

diff --git a/Documentation/devicetree/bindings/trigger-source/adi,util-sigma-delta-spi.yaml b/Documentation/devicetree/bindings/trigger-source/adi,util-sigma-delta-spi.yaml
new file mode 100644
index 000000000..ea4661795
--- /dev/null
+++ b/Documentation/devicetree/bindings/trigger-source/adi,util-sigma-delta-spi.yaml
@@ -0,0 +1,49 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+# Copyright (c) 2025 Analog Devices, Inc.
+# Copyright (c) 2025 BayLibre, SAS
+
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/trigger-source/adi,util-sigma-delta-spi.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Analog Devices Util Sigma-Delta SPI IP Core
+
+maintainers:
+  - David Lechner <dlechner@baylibre.com>
+
+description:
+  The Util Sigma-Delta SPI is an FPGA IP core from Analog Devices that provides
+  a SPI offload trigger from the RDY signal of the combined DOUT/RDY pin of
+  the sigma-delta family of ADCs.
+  https://analogdevicesinc.github.io/hdl/library/util_sigma_delta_spi/index.html
+
+properties:
+  compatible:
+    const: adi,util-sigma-delta-spi
+
+  reg:
+    maxItems: 1
+
+  clocks:
+    maxItems: 1
+
+  '#trigger-source-cells':
+    const: 0
+
+required:
+  - compatible
+  - reg
+  - clocks
+  - '#trigger-source-cells'
+
+additionalProperties: false
+
+examples:
+  - |
+    trigger@40000 {
+        reg = <0x40000 0x1000>;
+        compatible = "adi,util-sigma-delta-spi";
+        clocks = <&clk 0>;
+        #trigger-source-cells = <0>;
+    };
diff --git a/Documentation/devicetree/bindings/trigger-source/gpio-trigger.yaml b/Documentation/devicetree/bindings/trigger-source/gpio-trigger.yaml
new file mode 100644
index 000000000..1331d153e
--- /dev/null
+++ b/Documentation/devicetree/bindings/trigger-source/gpio-trigger.yaml
@@ -0,0 +1,40 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/trigger-source/gpio-trigger.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Generic trigger source using GPIO
+
+description: A GPIO used as a trigger source.
+
+maintainers:
+  - Jonathan Santos <Jonathan.Santos@analog.com>
+
+properties:
+  compatible:
+    const: gpio-trigger
+
+  '#trigger-source-cells':
+    const: 0
+
+  gpios:
+    maxItems: 1
+    description: GPIO to be used as a trigger source.
+
+required:
+  - compatible
+  - '#trigger-source-cells'
+  - gpios
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/gpio/gpio.h>
+
+    trigger {
+        compatible = "gpio-trigger";
+        #trigger-source-cells = <0>;
+        gpios = <&gpio1 15 GPIO_ACTIVE_HIGH>;
+    };
diff --git a/Documentation/devicetree/bindings/trigger-source/pwm-trigger.yaml b/Documentation/devicetree/bindings/trigger-source/pwm-trigger.yaml
new file mode 100644
index 000000000..1eac20031
--- /dev/null
+++ b/Documentation/devicetree/bindings/trigger-source/pwm-trigger.yaml
@@ -0,0 +1,37 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/trigger-source/pwm-trigger.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Generic trigger source using PWM
+
+description: Remaps a PWM channel as a trigger source.
+
+maintainers:
+  - David Lechner <dlechner@baylibre.com>
+
+properties:
+  compatible:
+    const: pwm-trigger
+
+  '#trigger-source-cells':
+    const: 0
+
+  pwms:
+    maxItems: 1
+
+required:
+  - compatible
+  - '#trigger-source-cells'
+  - pwms
+
+additionalProperties: false
+
+examples:
+  - |
+    trigger {
+        compatible = "pwm-trigger";
+        #trigger-source-cells = <0>;
+        pwms = <&pwm 0 1000000 0>;
+    };