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, 265 insertions, 0 deletions

diff --git a/Documentation/devicetree/bindings/dpll/dpll-device.yaml b/Documentation/devicetree/bindings/dpll/dpll-device.yaml
new file mode 100644
index 000000000..fb8d7a9a3
--- /dev/null
+++ b/Documentation/devicetree/bindings/dpll/dpll-device.yaml
@@ -0,0 +1,76 @@
+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/dpll/dpll-device.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Digital Phase-Locked Loop (DPLL) Device
+
+maintainers:
+  - Ivan Vecera <ivecera@redhat.com>
+
+description:
+  Digital Phase-Locked Loop (DPLL) device is used for precise clock
+  synchronization in networking and telecom hardware. The device can
+  have one or more channels (DPLLs) and one or more physical input and
+  output pins. Each DPLL channel can either produce pulse-per-clock signal
+  or drive ethernet equipment clock. The type of each channel can be
+  indicated by dpll-types property.
+
+properties:
+  $nodename:
+    pattern: "^dpll(@.*)?$"
+
+  "#address-cells":
+    const: 0
+
+  "#size-cells":
+    const: 0
+
+  dpll-types:
+    description: List of DPLL channel types, one per DPLL instance.
+    $ref: /schemas/types.yaml#/definitions/non-unique-string-array
+    items:
+      enum: [pps, eec]
+
+  input-pins:
+    type: object
+    description: DPLL input pins
+    unevaluatedProperties: false
+
+    properties:
+      "#address-cells":
+        const: 1
+      "#size-cells":
+        const: 0
+
+    patternProperties:
+      "^pin@[0-9a-f]+$":
+        $ref: /schemas/dpll/dpll-pin.yaml
+        unevaluatedProperties: false
+
+    required:
+      - "#address-cells"
+      - "#size-cells"
+
+  output-pins:
+    type: object
+    description: DPLL output pins
+    unevaluatedProperties: false
+
+    properties:
+      "#address-cells":
+        const: 1
+      "#size-cells":
+        const: 0
+
+    patternProperties:
+      "^pin@[0-9]+$":
+        $ref: /schemas/dpll/dpll-pin.yaml
+        unevaluatedProperties: false
+
+    required:
+      - "#address-cells"
+      - "#size-cells"
+
+additionalProperties: true
diff --git a/Documentation/devicetree/bindings/dpll/dpll-pin.yaml b/Documentation/devicetree/bindings/dpll/dpll-pin.yaml
new file mode 100644
index 000000000..1287a472f
--- /dev/null
+++ b/Documentation/devicetree/bindings/dpll/dpll-pin.yaml
@@ -0,0 +1,58 @@
+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/dpll/dpll-pin.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: DPLL Pin
+
+maintainers:
+  - Ivan Vecera <ivecera@redhat.com>
+
+description: |
+  The DPLL pin is either a physical input or output pin that is provided
+  by a DPLL( Digital Phase-Locked Loop) device. The pin is identified by
+  its physical order number that is stored in reg property and can have
+  an additional set of properties like supported (allowed) frequencies,
+  label, type and may support embedded sync.
+
+  Note that the pin in this context has nothing to do with pinctrl.
+
+properties:
+  reg:
+    description: Hardware index of the DPLL pin.
+    maxItems: 1
+
+  connection-type:
+    description: Connection type of the pin
+    $ref: /schemas/types.yaml#/definitions/string
+    enum: [ext, gnss, int, mux, synce]
+
+  esync-control:
+    description: Indicates whether the pin supports embedded sync functionality.
+    type: boolean
+
+  label:
+    description: String exposed as the pin board label
+    $ref: /schemas/types.yaml#/definitions/string
+
+  ref-sync-sources:
+    description: |
+      List of phandles to input pins that can serve as the sync source
+      in a Reference-Sync pair with this pin acting as the clock source.
+      A Ref-Sync pair consists of a clock reference and a low-frequency
+      sync signal.  The DPLL locks to the clock reference but
+      phase-aligns to the sync reference.
+      Only valid for input pins.  Each referenced pin must be a
+      different input pin on the same device.
+    $ref: /schemas/types.yaml#/definitions/phandle-array
+    items:
+      maxItems: 1
+
+  supported-frequencies-hz:
+    description: List of supported frequencies for this pin, expressed in Hz.
+
+required:
+  - reg
+
+additionalProperties: false
diff --git a/Documentation/devicetree/bindings/dpll/microchip,zl30731.yaml b/Documentation/devicetree/bindings/dpll/microchip,zl30731.yaml
new file mode 100644
index 000000000..fa5a8f8e3
--- /dev/null
+++ b/Documentation/devicetree/bindings/dpll/microchip,zl30731.yaml
@@ -0,0 +1,131 @@
+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/dpll/microchip,zl30731.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Microchip Azurite DPLL device
+
+maintainers:
+  - Ivan Vecera <ivecera@redhat.com>
+
+description:
+  Microchip Azurite DPLL (ZL3073x) is a family of DPLL devices that
+  provides up to 5 independent DPLL channels, up to 10 differential or
+  single-ended inputs and 10 differential or 20 single-ended outputs.
+  These devices support both I2C and SPI interfaces.
+
+properties:
+  compatible:
+    enum:
+      - microchip,zl30731
+      - microchip,zl30732
+      - microchip,zl30733
+      - microchip,zl30734
+      - microchip,zl30735
+
+  reg:
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+
+allOf:
+  - $ref: /schemas/dpll/dpll-device.yaml#
+  - $ref: /schemas/spi/spi-peripheral-props.yaml#
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    i2c {
+      #address-cells = <1>;
+      #size-cells = <0>;
+
+      dpll@70 {
+        compatible = "microchip,zl30732";
+        reg = <0x70>;
+        dpll-types = "pps", "eec";
+
+        input-pins {
+          #address-cells = <1>;
+          #size-cells = <0>;
+
+          sync0: pin@0 { /* REF0P - 1 PPS sync source */
+            reg = <0>;
+            connection-type = "ext";
+            label = "SMA1";
+            supported-frequencies-hz = /bits/ 64 <1>;
+          };
+
+          pin@1 { /* REF0N - clock source, can pair with sync0 */
+            reg = <1>;
+            connection-type = "ext";
+            label = "SMA2";
+            supported-frequencies-hz = /bits/ 64 <10000 10000000>;
+            ref-sync-sources = <&sync0>;
+          };
+        };
+
+        output-pins {
+          #address-cells = <1>;
+          #size-cells = <0>;
+
+          pin@3 { /* OUT1N */
+            reg = <3>;
+            connection-type = "gnss";
+            esync-control;
+            label = "Output 1";
+            supported-frequencies-hz = /bits/ 64 <1 10000>;
+          };
+        };
+      };
+    };
+  - |
+    spi {
+      #address-cells = <1>;
+      #size-cells = <0>;
+
+      dpll@70 {
+        compatible = "microchip,zl30731";
+        reg = <0x70>;
+        spi-max-frequency = <12500000>;
+
+        dpll-types = "pps";
+
+        input-pins {
+          #address-cells = <1>;
+          #size-cells = <0>;
+
+          sync1: pin@0 { /* REF0P - 1 PPS sync source */
+            reg = <0>;
+            connection-type = "gnss";
+            label = "GNSS_1PPS_IN";
+            supported-frequencies-hz = /bits/ 64 <1>;
+          };
+
+          pin@1 { /* REF0N - clock source */
+            reg = <1>;
+            connection-type = "gnss";
+            label = "GNSS_10M_IN";
+            supported-frequencies-hz = /bits/ 64 <10000000>;
+            ref-sync-sources = <&sync1>;
+          };
+        };
+
+        output-pins {
+          #address-cells = <1>;
+          #size-cells = <0>;
+
+          pin@3 { /* OUT1N */
+            reg = <3>;
+            connection-type = "gnss";
+            esync-control;
+            label = "Output 1";
+            supported-frequencies-hz = /bits/ 64 <1 10000>;
+          };
+        };
+      };
+    };
+...