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, 122 insertions, 0 deletions

diff --git a/drivers/usb/serial/empeg.c b/drivers/usb/serial/empeg.c
new file mode 100644
index 000000000..4937575ca
--- /dev/null
+++ b/drivers/usb/serial/empeg.c
@@ -0,0 +1,122 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * USB Empeg empeg-car player driver
+ *
+ *	Copyright (C) 2000, 2001
+ *	    Gary Brubaker (xavyer@ix.netcom.com)
+ *
+ *	Copyright (C) 1999 - 2001
+ *	    Greg Kroah-Hartman (greg@kroah.com)
+ *
+ * See Documentation/usb/usb-serial.rst for more information on using this
+ * driver
+ */
+
+#include <linux/kernel.h>
+#include <linux/errno.h>
+#include <linux/slab.h>
+#include <linux/tty.h>
+#include <linux/module.h>
+#include <linux/spinlock.h>
+#include <linux/usb.h>
+#include <linux/usb/serial.h>
+
+#define DRIVER_AUTHOR "Greg Kroah-Hartman <greg@kroah.com>, Gary Brubaker <xavyer@ix.netcom.com>"
+#define DRIVER_DESC "USB Empeg Mark I/II Driver"
+
+#define EMPEG_VENDOR_ID			0x084f
+#define EMPEG_PRODUCT_ID		0x0001
+
+/* function prototypes for an empeg-car player */
+static int  empeg_startup(struct usb_serial *serial);
+static void empeg_init_termios(struct tty_struct *tty);
+
+static const struct usb_device_id id_table[] = {
+	{ USB_DEVICE(EMPEG_VENDOR_ID, EMPEG_PRODUCT_ID) },
+	{ }					/* Terminating entry */
+};
+
+MODULE_DEVICE_TABLE(usb, id_table);
+
+static struct usb_serial_driver empeg_device = {
+	.driver = {
+		.name =		"empeg",
+	},
+	.id_table =		id_table,
+	.num_ports =		1,
+	.bulk_out_size =	256,
+	.throttle =		usb_serial_generic_throttle,
+	.unthrottle =		usb_serial_generic_unthrottle,
+	.attach =		empeg_startup,
+	.init_termios =		empeg_init_termios,
+};
+
+static struct usb_serial_driver * const serial_drivers[] = {
+	&empeg_device, NULL
+};
+
+static int empeg_startup(struct usb_serial *serial)
+{
+	int r;
+
+	if (serial->dev->actconfig->desc.bConfigurationValue != 1) {
+		dev_err(&serial->dev->dev, "active config #%d != 1 ??\n",
+			serial->dev->actconfig->desc.bConfigurationValue);
+		return -ENODEV;
+	}
+
+	r = usb_reset_configuration(serial->dev);
+
+	/* continue on with initialization */
+	return r;
+}
+
+static void empeg_init_termios(struct tty_struct *tty)
+{
+	struct ktermios *termios = &tty->termios;
+
+	/*
+	 * The empeg-car player wants these particular tty settings.
+	 * You could, for example, change the baud rate, however the
+	 * player only supports 115200 (currently), so there is really
+	 * no point in support for changes to the tty settings.
+	 * (at least for now)
+	 *
+	 * The default requirements for this device are:
+	 */
+	termios->c_iflag
+		&= ~(IGNBRK	/* disable ignore break */
+		| BRKINT	/* disable break causes interrupt */
+		| PARMRK	/* disable mark parity errors */
+		| ISTRIP	/* disable clear high bit of input characters */
+		| INLCR		/* disable translate NL to CR */
+		| IGNCR		/* disable ignore CR */
+		| ICRNL		/* disable translate CR to NL */
+		| IXON);	/* disable enable XON/XOFF flow control */
+
+	termios->c_oflag
+		&= ~OPOST;	/* disable postprocess output characters */
+
+	termios->c_lflag
+		&= ~(ECHO	/* disable echo input characters */
+		| ECHONL	/* disable echo new line */
+		| ICANON	/* disable erase, kill, werase, and rprnt special characters */
+		| ISIG		/* disable interrupt, quit, and suspend special characters */
+		| IEXTEN);	/* disable non-POSIX special characters */
+
+	termios->c_cflag
+		&= ~(CSIZE	/* no size */
+		| PARENB	/* disable parity bit */
+		| CBAUD);	/* clear current baud rate */
+
+	termios->c_cflag
+		|= CS8;		/* character size 8 bits */
+
+	tty_encode_baud_rate(tty, 115200, 115200);
+}
+
+module_usb_serial_driver(serial_drivers, id_table);
+
+MODULE_AUTHOR(DRIVER_AUTHOR);
+MODULE_DESCRIPTION(DRIVER_DESC);
+MODULE_LICENSE("GPL v2");