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, 233 insertions, 0 deletions

diff --git a/Documentation/input/gameport-programming.rst b/Documentation/input/gameport-programming.rst
new file mode 100644
index 000000000..7d7063ad0
--- /dev/null
+++ b/Documentation/input/gameport-programming.rst
@@ -0,0 +1,233 @@
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Programming gameport drivers
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+A basic classic gameport
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+If the gameport doesn't provide more than the inb()/outb() functionality,
+the code needed to register it with the joystick drivers is simple::
+
+	struct gameport gameport;
+
+	gameport.io = MY_IO_ADDRESS;
+	gameport_register_port(&gameport);
+
+Make sure struct gameport is initialized to 0 in all other fields. The
+gameport generic code will take care of the rest.
+
+If your hardware supports more than one io address, and your driver can
+choose which one to program the hardware to, starting from the more exotic
+addresses is preferred, because the likelihood of clashing with the standard
+0x201 address is smaller.
+
+E.g. if your driver supports addresses 0x200, 0x208, 0x210 and 0x218, then
+0x218 would be the address of first choice.
+
+If your hardware supports a gameport address that is not mapped to ISA io
+space (is above 0x1000), use that one, and don't map the ISA mirror.
+
+Also, always request_region() on the whole io space occupied by the
+gameport. Although only one ioport is really used, the gameport usually
+occupies from one to sixteen addresses in the io space.
+
+Please also consider enabling the gameport on the card in the ->open()
+callback if the io is mapped to ISA space - this way it'll occupy the io
+space only when something really is using it. Disable it again in the
+->close() callback. You also can select the io address in the ->open()
+callback, so that it doesn't fail if some of the possible addresses are
+already occupied by other gameports.
+
+Memory mapped gameport
+~~~~~~~~~~~~~~~~~~~~~~
+
+When a gameport can be accessed through MMIO, this way is preferred, because
+it is faster, allowing more reads per second. Registering such a gameport
+isn't as easy as a basic IO one, but not so much complex::
+
+	struct gameport gameport;
+
+	void my_trigger(struct gameport *gameport)
+	{
+		my_mmio = 0xff;
+	}
+
+	unsigned char my_read(struct gameport *gameport)
+	{
+		return my_mmio;
+	}
+
+	gameport.read = my_read;
+	gameport.trigger = my_trigger;
+	gameport_register_port(&gameport);
+
+.. _gameport_pgm_cooked_mode:
+
+Cooked mode gameport
+~~~~~~~~~~~~~~~~~~~~
+
+There are gameports that can report the axis values as numbers, that means
+the driver doesn't have to measure them the old way - an ADC is built into
+the gameport. To register a cooked gameport::
+
+	struct gameport gameport;
+
+	int my_cooked_read(struct gameport *gameport, int *axes, int *buttons)
+	{
+		int i;
+
+		for (i = 0; i < 4; i++)
+			axes[i] = my_mmio[i];
+		buttons[0] = my_mmio[4];
+	}
+
+	int my_open(struct gameport *gameport, int mode)
+	{
+		return -(mode != GAMEPORT_MODE_COOKED);
+	}
+
+	gameport.cooked_read = my_cooked_read;
+	gameport.open = my_open;
+	gameport.fuzz = 8;
+	gameport_register_port(&gameport);
+
+The only confusing thing here is the fuzz value. Best determined by
+experimentation, it is the amount of noise in the ADC data. Perfect
+gameports can set this to zero, most common have fuzz between 8 and 32.
+See analog.c and input.c for handling of fuzz - the fuzz value determines
+the size of a gaussian filter window that is used to eliminate the noise
+in the data.
+
+More complex gameports
+~~~~~~~~~~~~~~~~~~~~~~
+
+Gameports can support both raw and cooked modes. In that case combine either
+examples 1+2 or 1+3. Gameports can support internal calibration - see below,
+and also lightning.c and analog.c on how that works. If your driver supports
+more than one gameport instance simultaneously, use the ->private member of
+the gameport struct to point to your data.
+
+Unregistering a gameport
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+Simple::
+
+    gameport_unregister_port(&gameport);
+
+The gameport structure
+~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+    struct gameport {
+
+	void *port_data;
+
+A private pointer for free use in the gameport driver. (Not the joystick
+driver!)
+
+::
+
+	char name[32];
+
+Driver's name as set by driver calling gameport_set_name(). Informational
+purpose only.
+
+::
+
+	char phys[32];
+
+gameport's physical name/description as set by driver calling gameport_set_phys().
+Informational purpose only.
+
+::
+
+	int io;
+
+I/O address for use with raw mode. You have to either set this, or ->read()
+to some value if your gameport supports raw mode.
+
+::
+
+	int speed;
+
+Raw mode speed of the gameport reads in thousands of reads per second.
+
+::
+
+	int fuzz;
+
+If the gameport supports cooked mode, this should be set to a value that
+represents the amount of noise in the data. See
+:ref:`gameport_pgm_cooked_mode`.
+
+::
+
+	void (*trigger)(struct gameport *);
+
+Trigger. This function should trigger the ns558 oneshots. If set to NULL,
+outb(0xff, io) will be used.
+
+::
+
+	unsigned char (*read)(struct gameport *);
+
+Read the buttons and ns558 oneshot bits. If set to NULL, inb(io) will be
+used instead.
+
+::
+
+	int (*cooked_read)(struct gameport *, int *axes, int *buttons);
+
+If the gameport supports cooked mode, it should point this to its cooked
+read function. It should fill axes[0..3] with four values of the joystick axes
+and buttons[0] with four bits representing the buttons.
+
+::
+
+	int (*calibrate)(struct gameport *, int *axes, int *max);
+
+Function for calibrating the ADC hardware. When called, axes[0..3] should be
+pre-filled by cooked data by the caller, max[0..3] should be pre-filled with
+expected maximums for each axis. The calibrate() function should set the
+sensitivity of the ADC hardware so that the maximums fit in its range and
+recompute the axes[] values to match the new sensitivity or re-read them from
+the hardware so that they give valid values.
+
+::
+
+	int (*open)(struct gameport *, int mode);
+
+Open() serves two purposes. First a driver either opens the port in raw or
+in cooked mode, the open() callback can decide which modes are supported.
+Second, resource allocation can happen here. The port can also be enabled
+here. Prior to this call, other fields of the gameport struct (namely the io
+member) need not to be valid.
+
+::
+
+	void (*close)(struct gameport *);
+
+Close() should free the resources allocated by open, possibly disabling the
+gameport.
+
+::
+
+	struct timer_list poll_timer;
+	unsigned int poll_interval;     /* in msecs */
+	spinlock_t timer_lock;
+	unsigned int poll_cnt;
+	void (*poll_handler)(struct gameport *);
+	struct gameport *parent, *child;
+	struct gameport_driver *drv;
+	struct mutex drv_mutex;		/* protects serio->drv so attributes can pin driver */
+	struct device dev;
+	struct list_head node;
+
+For internal use by the gameport layer.
+
+::
+
+    };
+
+Enjoy!