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, 63 insertions, 0 deletions

diff --git a/drivers/video/fbdev/sbuslib.h b/drivers/video/fbdev/sbuslib.h
new file mode 100644
index 000000000..e9af2dc93
--- /dev/null
+++ b/drivers/video/fbdev/sbuslib.h
@@ -0,0 +1,63 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/* sbuslib.h: SBUS fb helper library interfaces */
+#ifndef _SBUSLIB_H
+#define _SBUSLIB_H
+
+struct device_node;
+struct fb_info;
+struct fb_var_screeninfo;
+struct vm_area_struct;
+
+struct sbus_mmap_map {
+	unsigned long voff;
+	unsigned long poff;
+	unsigned long size;
+};
+
+#define SBUS_MMAP_FBSIZE(n) (-n)
+#define SBUS_MMAP_EMPTY	0x80000000
+
+extern void sbusfb_fill_var(struct fb_var_screeninfo *var,
+			    struct device_node *dp, int bpp);
+extern int sbusfb_mmap_helper(const struct sbus_mmap_map *map,
+			      unsigned long physbase, unsigned long fbsize,
+			      unsigned long iospace,
+			      struct vm_area_struct *vma);
+int sbusfb_ioctl_helper(unsigned long cmd, unsigned long arg,
+			struct fb_info *info,
+			int type, int fb_depth, unsigned long fb_size);
+int sbusfb_compat_ioctl(struct fb_info *info, unsigned int cmd,
+			unsigned long arg);
+
+/*
+ * Initialize struct fb_ops for SBUS I/O.
+ */
+
+#define __FB_DEFAULT_SBUS_OPS_RDWR(__prefix) \
+	.fb_read	= fb_io_read, \
+	.fb_write	= fb_io_write
+
+#define __FB_DEFAULT_SBUS_OPS_DRAW(__prefix) \
+	.fb_fillrect	= cfb_fillrect, \
+	.fb_copyarea	= cfb_copyarea, \
+	.fb_imageblit	= cfb_imageblit
+
+#ifdef CONFIG_COMPAT
+#define __FB_DEFAULT_SBUS_OPS_IOCTL(__prefix) \
+	.fb_ioctl		= __prefix ## _sbusfb_ioctl, \
+	.fb_compat_ioctl	= sbusfb_compat_ioctl
+#else
+#define __FB_DEFAULT_SBUS_OPS_IOCTL(__prefix) \
+	.fb_ioctl	= __prefix ## _sbusfb_ioctl
+#endif
+
+#define __FB_DEFAULT_SBUS_OPS_MMAP(__prefix) \
+	.fb_mmap	= __prefix ## _sbusfb_mmap
+
+#define FB_DEFAULT_SBUS_OPS(__prefix) \
+	__FB_DEFAULT_SBUS_OPS_RDWR(__prefix), \
+	__FB_DEFAULT_SBUS_OPS_DRAW(__prefix), \
+	__FB_DEFAULT_SBUS_OPS_IOCTL(__prefix), \
+	__FB_DEFAULT_SBUS_OPS_MMAP(__prefix)
+
+#endif /* _SBUSLIB_H */