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>

1 files changed, 139 insertions, 0 deletions

diff --git a/drivers/comedi/comedi_usb.c b/drivers/comedi/comedi_usb.c
new file mode 100644
index 000000000..75d65171c
--- /dev/null
+++ b/drivers/comedi/comedi_usb.c
@@ -0,0 +1,139 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * comedi_usb.c
+ * Comedi USB driver specific functions.
+ *
+ * COMEDI - Linux Control and Measurement Device Interface
+ * Copyright (C) 1997-2000 David A. Schleef <ds@schleef.org>
+ */
+
+#include <linux/module.h>
+#include <linux/comedi/comedi_usb.h>
+
+/**
+ * comedi_to_usb_interface() - Return USB interface attached to COMEDI device
+ * @dev: COMEDI device.
+ *
+ * Assuming @dev->hw_dev is non-%NULL, it is assumed to be pointing to a
+ * a &struct device embedded in a &struct usb_interface.
+ *
+ * Return: Attached USB interface if @dev->hw_dev is non-%NULL.
+ * Return %NULL if @dev->hw_dev is %NULL.
+ */
+struct usb_interface *comedi_to_usb_interface(struct comedi_device *dev)
+{
+	return dev->hw_dev ? to_usb_interface(dev->hw_dev) : NULL;
+}
+EXPORT_SYMBOL_GPL(comedi_to_usb_interface);
+
+/**
+ * comedi_to_usb_dev() - Return USB device attached to COMEDI device
+ * @dev: COMEDI device.
+ *
+ * Assuming @dev->hw_dev is non-%NULL, it is assumed to be pointing to a
+ * a &struct device embedded in a &struct usb_interface.
+ *
+ * Return: USB device to which the USB interface belongs if @dev->hw_dev is
+ * non-%NULL.  Return %NULL if @dev->hw_dev is %NULL.
+ */
+struct usb_device *comedi_to_usb_dev(struct comedi_device *dev)
+{
+	struct usb_interface *intf = comedi_to_usb_interface(dev);
+
+	return intf ? interface_to_usbdev(intf) : NULL;
+}
+EXPORT_SYMBOL_GPL(comedi_to_usb_dev);
+
+/**
+ * comedi_usb_auto_config() - Configure/probe a USB COMEDI driver
+ * @intf: USB interface.
+ * @driver: Registered COMEDI driver.
+ * @context: Driver specific data, passed to comedi_auto_config().
+ *
+ * Typically called from the usb_driver (*probe) function.  Auto-configure a
+ * COMEDI device, using a pointer to the &struct device embedded in *@intf as
+ * the hardware device.  The @context value gets passed through to @driver's
+ * "auto_attach" handler.  The "auto_attach" handler may call
+ * comedi_to_usb_interface() on the passed in COMEDI device to recover @intf.
+ *
+ * Return: The result of calling comedi_auto_config() (%0 on success, or
+ * a negative error number on failure).
+ */
+int comedi_usb_auto_config(struct usb_interface *intf,
+			   struct comedi_driver *driver,
+			   unsigned long context)
+{
+	return comedi_auto_config(&intf->dev, driver, context);
+}
+EXPORT_SYMBOL_GPL(comedi_usb_auto_config);
+
+/**
+ * comedi_usb_auto_unconfig() - Unconfigure/disconnect a USB COMEDI device
+ * @intf: USB interface.
+ *
+ * Typically called from the usb_driver (*disconnect) function.
+ * Auto-unconfigure a COMEDI device attached to this USB interface, using a
+ * pointer to the &struct device embedded in *@intf as the hardware device.
+ * The COMEDI driver's "detach" handler will be called during unconfiguration
+ * of the COMEDI device.
+ *
+ * Note that the COMEDI device may have already been unconfigured using the
+ * %COMEDI_DEVCONFIG ioctl, in which case this attempt to unconfigure it
+ * again should be ignored.
+ */
+void comedi_usb_auto_unconfig(struct usb_interface *intf)
+{
+	comedi_auto_unconfig(&intf->dev);
+}
+EXPORT_SYMBOL_GPL(comedi_usb_auto_unconfig);
+
+/**
+ * comedi_usb_driver_register() - Register a USB COMEDI driver
+ * @comedi_driver: COMEDI driver to be registered.
+ * @usb_driver: USB driver to be registered.
+ *
+ * This function is called from the module_init() of USB COMEDI driver modules
+ * to register the COMEDI driver and the USB driver.  Do not call it directly,
+ * use the module_comedi_usb_driver() helper macro instead.
+ *
+ * Return: %0 on success, or a negative error number on failure.
+ */
+int comedi_usb_driver_register(struct comedi_driver *comedi_driver,
+			       struct usb_driver *usb_driver)
+{
+	int ret;
+
+	ret = comedi_driver_register(comedi_driver);
+	if (ret < 0)
+		return ret;
+
+	ret = usb_register(usb_driver);
+	if (ret < 0) {
+		comedi_driver_unregister(comedi_driver);
+		return ret;
+	}
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(comedi_usb_driver_register);
+
+/**
+ * comedi_usb_driver_unregister() - Unregister a USB COMEDI driver
+ * @comedi_driver: COMEDI driver to be registered.
+ * @usb_driver: USB driver to be registered.
+ *
+ * This function is called from the module_exit() of USB COMEDI driver modules
+ * to unregister the USB driver and the COMEDI driver.  Do not call it
+ * directly, use the module_comedi_usb_driver() helper macro instead.
+ */
+void comedi_usb_driver_unregister(struct comedi_driver *comedi_driver,
+				  struct usb_driver *usb_driver)
+{
+	usb_deregister(usb_driver);
+	comedi_driver_unregister(comedi_driver);
+}
+EXPORT_SYMBOL_GPL(comedi_usb_driver_unregister);
+
+MODULE_AUTHOR("https://www.comedi.org");
+MODULE_DESCRIPTION("Comedi USB interface module");
+MODULE_LICENSE("GPL");