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, 99 insertions, 0 deletions

diff --git a/drivers/reset/reset-gpio.c b/drivers/reset/reset-gpio.c
new file mode 100644
index 000000000..bf95296a5
--- /dev/null
+++ b/drivers/reset/reset-gpio.c
@@ -0,0 +1,99 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/auxiliary_bus.h>
+#include <linux/gpio/consumer.h>
+#include <linux/module.h>
+#include <linux/property.h>
+#include <linux/reset-controller.h>
+
+struct reset_gpio_priv {
+	struct reset_controller_dev rc;
+	struct gpio_desc *reset;
+};
+
+static inline struct reset_gpio_priv
+*rc_to_reset_gpio(struct reset_controller_dev *rc)
+{
+	return container_of(rc, struct reset_gpio_priv, rc);
+}
+
+static int reset_gpio_assert(struct reset_controller_dev *rc, unsigned long id)
+{
+	struct reset_gpio_priv *priv = rc_to_reset_gpio(rc);
+
+	return gpiod_set_value_cansleep(priv->reset, 1);
+}
+
+static int reset_gpio_deassert(struct reset_controller_dev *rc,
+			       unsigned long id)
+{
+	struct reset_gpio_priv *priv = rc_to_reset_gpio(rc);
+
+	return gpiod_set_value_cansleep(priv->reset, 0);
+}
+
+static int reset_gpio_status(struct reset_controller_dev *rc, unsigned long id)
+{
+	struct reset_gpio_priv *priv = rc_to_reset_gpio(rc);
+
+	return gpiod_get_value_cansleep(priv->reset);
+}
+
+static const struct reset_control_ops reset_gpio_ops = {
+	.assert = reset_gpio_assert,
+	.deassert = reset_gpio_deassert,
+	.status = reset_gpio_status,
+};
+
+static int reset_gpio_fwnode_xlate(struct reset_controller_dev *rcdev,
+				   const struct fwnode_reference_args *reset_spec)
+{
+	return reset_spec->args[0];
+}
+
+static int reset_gpio_probe(struct auxiliary_device *adev,
+			    const struct auxiliary_device_id *id)
+{
+	struct device *dev = &adev->dev;
+	struct reset_gpio_priv *priv;
+
+	priv = devm_kzalloc(dev, sizeof(*priv), GFP_KERNEL);
+	if (!priv)
+		return -ENOMEM;
+
+	priv->reset = devm_gpiod_get(dev, "reset", GPIOD_OUT_HIGH);
+	if (IS_ERR(priv->reset))
+		return dev_err_probe(dev, PTR_ERR(priv->reset),
+				     "Could not get reset gpios\n");
+
+	priv->rc.ops = &reset_gpio_ops;
+	priv->rc.owner = THIS_MODULE;
+	priv->rc.dev = dev;
+
+	/* Cells to match GPIO specifier, but it's not really used */
+	priv->rc.fwnode_reset_n_cells = 2;
+	priv->rc.fwnode_xlate = reset_gpio_fwnode_xlate;
+	priv->rc.nr_resets = 1;
+
+	return devm_reset_controller_register(dev, &priv->rc);
+}
+
+static const struct auxiliary_device_id reset_gpio_ids[] = {
+	{ .name = "reset.gpio" },
+	{}
+};
+MODULE_DEVICE_TABLE(auxiliary, reset_gpio_ids);
+
+static struct auxiliary_driver reset_gpio_driver = {
+	.probe		= reset_gpio_probe,
+	.id_table	= reset_gpio_ids,
+	.driver	= {
+		.name = "reset-gpio",
+		.suppress_bind_attrs = true,
+	},
+};
+module_auxiliary_driver(reset_gpio_driver);
+
+MODULE_AUTHOR("Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>");
+MODULE_DESCRIPTION("Generic GPIO reset driver");
+MODULE_LICENSE("GPL");