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, 225 insertions, 0 deletions

diff --git a/drivers/firmware/sysfb.c b/drivers/firmware/sysfb.c
new file mode 100644
index 000000000..8833582c1
--- /dev/null
+++ b/drivers/firmware/sysfb.c
@@ -0,0 +1,225 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Generic System Framebuffers
+ * Copyright (c) 2012-2013 David Herrmann <dh.herrmann@gmail.com>
+ */
+
+/*
+ * Simple-Framebuffer support
+ * Create a platform-device for any available boot framebuffer. The
+ * simple-framebuffer platform device is already available on DT systems, so
+ * this module parses the global "screen_info" object and creates a suitable
+ * platform device compatible with the "simple-framebuffer" DT object. If
+ * the framebuffer is incompatible, we instead create a legacy
+ * "vesa-framebuffer", "efi-framebuffer" or "platform-framebuffer" device and
+ * pass the screen_info as platform_data. This allows legacy drivers
+ * to pick these devices up without messing with simple-framebuffer drivers.
+ * The global "screen_info" is still valid at all times.
+ *
+ * If CONFIG_SYSFB_SIMPLEFB is not selected, never register "simple-framebuffer"
+ * platform devices, but only use legacy framebuffer devices for
+ * backwards compatibility.
+ *
+ * TODO: We set the dev_id field of all platform-devices to 0. This allows
+ * other OF/DT parsers to create such devices, too. However, they must
+ * start at offset 1 for this to work.
+ */
+
+#include <linux/err.h>
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/mm.h>
+#include <linux/pci.h>
+#include <linux/platform_data/simplefb.h>
+#include <linux/platform_device.h>
+#include <linux/screen_info.h>
+#include <linux/sysfb.h>
+
+static struct platform_device *pd;
+static DEFINE_MUTEX(disable_lock);
+static bool disabled;
+
+static struct device *sysfb_parent_dev(const struct screen_info *si);
+
+static bool sysfb_unregister(void)
+{
+	if (IS_ERR_OR_NULL(pd))
+		return false;
+
+	platform_device_unregister(pd);
+	pd = NULL;
+
+	return true;
+}
+
+/**
+ * sysfb_disable() - disable the Generic System Framebuffers support
+ * @dev:	the device to check if non-NULL
+ *
+ * This disables the registration of system framebuffer devices that match the
+ * generic drivers that make use of the system framebuffer set up by firmware.
+ *
+ * It also unregisters a device if this was already registered by sysfb_init().
+ *
+ * Context: The function can sleep. A @disable_lock mutex is acquired to serialize
+ *          against sysfb_init(), that registers a system framebuffer device.
+ */
+void sysfb_disable(struct device *dev)
+{
+	struct screen_info *si = &sysfb_primary_display.screen;
+	struct device *parent;
+
+	mutex_lock(&disable_lock);
+	parent = sysfb_parent_dev(si);
+	if (!dev || !parent || dev == parent) {
+		sysfb_unregister();
+		disabled = true;
+	}
+	mutex_unlock(&disable_lock);
+}
+EXPORT_SYMBOL_GPL(sysfb_disable);
+
+/**
+ * sysfb_handles_screen_info() - reports if sysfb handles the global screen_info
+ *
+ * Callers can use sysfb_handles_screen_info() to determine whether the Generic
+ * System Framebuffers (sysfb) can handle the global screen_info data structure
+ * or not. Drivers might need this information to know if they have to setup the
+ * system framebuffer, or if they have to delegate this action to sysfb instead.
+ *
+ * Returns:
+ * True if sysfb handles the global screen_info data structure.
+ */
+bool sysfb_handles_screen_info(void)
+{
+	const struct screen_info *si = &sysfb_primary_display.screen;
+
+	return !!screen_info_video_type(si);
+}
+EXPORT_SYMBOL_GPL(sysfb_handles_screen_info);
+
+#if defined(CONFIG_PCI)
+static bool sysfb_pci_dev_is_enabled(struct pci_dev *pdev)
+{
+	/*
+	 * TODO: Try to integrate this code into the PCI subsystem
+	 */
+	int ret;
+	u16 command;
+
+	ret = pci_read_config_word(pdev, PCI_COMMAND, &command);
+	if (ret != PCIBIOS_SUCCESSFUL)
+		return false;
+	if (!(command & PCI_COMMAND_MEMORY))
+		return false;
+	return true;
+}
+#else
+static bool sysfb_pci_dev_is_enabled(struct pci_dev *pdev)
+{
+	return false;
+}
+#endif
+
+static struct device *sysfb_parent_dev(const struct screen_info *si)
+{
+	struct pci_dev *pdev;
+
+	pdev = screen_info_pci_dev(si);
+	if (IS_ERR(pdev)) {
+		return ERR_CAST(pdev);
+	} else if (pdev) {
+		if (!sysfb_pci_dev_is_enabled(pdev)) {
+			pci_dev_put(pdev);
+			return ERR_PTR(-ENODEV);
+		}
+		return &pdev->dev;
+	}
+
+	return NULL;
+}
+
+static __init int sysfb_init(void)
+{
+	struct sysfb_display_info *dpy = &sysfb_primary_display;
+	struct screen_info *si = &dpy->screen;
+	struct device *parent;
+	unsigned int type;
+	struct simplefb_platform_data mode;
+	const char *name;
+	bool compatible;
+	int ret = 0;
+
+	screen_info_apply_fixups();
+
+	mutex_lock(&disable_lock);
+	if (disabled)
+		goto unlock_mutex;
+
+	sysfb_apply_efi_quirks(si);
+
+	parent = sysfb_parent_dev(si);
+	if (IS_ERR(parent)) {
+		ret = PTR_ERR(parent);
+		goto unlock_mutex;
+	}
+
+	/* try to create a simple-framebuffer device */
+	compatible = sysfb_parse_mode(si, &mode);
+	if (compatible) {
+		pd = sysfb_create_simplefb(si, &mode, parent);
+		if (!IS_ERR(pd))
+			goto put_device;
+	}
+
+	type = screen_info_video_type(si);
+
+	/* if the FB is incompatible, create a legacy framebuffer device */
+	switch (type) {
+	case VIDEO_TYPE_EGAC:
+		name = "ega-framebuffer";
+		break;
+	case VIDEO_TYPE_VGAC:
+		name = "vga-framebuffer";
+		break;
+	case VIDEO_TYPE_VLFB:
+		name = "vesa-framebuffer";
+		break;
+	case VIDEO_TYPE_EFI:
+		name = "efi-framebuffer";
+		break;
+	default:
+		name = "platform-framebuffer";
+		break;
+	}
+
+	pd = platform_device_alloc(name, 0);
+	if (!pd) {
+		ret = -ENOMEM;
+		goto put_device;
+	}
+
+	pd->dev.parent = parent;
+
+	sysfb_set_efifb_fwnode(si, pd);
+
+	ret = platform_device_add_data(pd, dpy, sizeof(*dpy));
+	if (ret)
+		goto err;
+
+	ret = platform_device_add(pd);
+	if (ret)
+		goto err;
+
+	goto put_device;
+err:
+	platform_device_put(pd);
+put_device:
+	put_device(parent);
+unlock_mutex:
+	mutex_unlock(&disable_lock);
+	return ret;
+}
+
+/* must execute after PCI subsystem for EFI quirks */
+device_initcall(sysfb_init);