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>

2 files changed, 42 insertions, 0 deletions

diff --git a/Documentation/PCI/controller/index.rst b/Documentation/PCI/controller/index.rst
new file mode 100644
index 000000000..c2ce9ccdc
--- /dev/null
+++ b/Documentation/PCI/controller/index.rst
@@ -0,0 +1,10 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+===========================================
+PCI Native Host Bridge and Endpoint Drivers
+===========================================
+
+.. toctree::
+   :maxdepth: 2
+
+   rcar-pcie-firmware
diff --git a/Documentation/PCI/controller/rcar-pcie-firmware.rst b/Documentation/PCI/controller/rcar-pcie-firmware.rst
new file mode 100644
index 000000000..67d3bf66e
--- /dev/null
+++ b/Documentation/PCI/controller/rcar-pcie-firmware.rst
@@ -0,0 +1,32 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=================================================
+Firmware of PCIe controller for Renesas R-Car V4H
+=================================================
+
+Renesas R-Car V4H (r8a779g0) has a PCIe controller, requiring a specific
+firmware download during startup.
+
+However, Renesas currently cannot distribute the firmware free of charge.
+
+The firmware file "104_PCIe_fw_addr_data_ver1.05.txt" (note that the file name
+might be different between different datasheet revisions) can be found in the
+datasheet encoded as text, and as such, the file's content must be converted
+back to binary form. This can be achieved using the following example script:
+
+.. code-block:: sh
+
+	$ awk '/^\s*0x[0-9A-Fa-f]{4}\s+0x[0-9A-Fa-f]{4}/ { print substr($2,5,2) substr($2,3,2) }' \
+		104_PCIe_fw_addr_data_ver1.05.txt | \
+			xxd -p -r > rcar_gen4_pcie.bin
+
+Once the text content has been converted into a binary firmware file, verify
+its checksum as follows:
+
+.. code-block:: sh
+
+	$ sha1sum rcar_gen4_pcie.bin
+	1d0bd4b189b4eb009f5d564b1f93a79112994945  rcar_gen4_pcie.bin
+
+The resulting binary file called "rcar_gen4_pcie.bin" should be placed in the
+"/lib/firmware" directory before the driver runs.