fwctl: Basic ioctl dispatch for the character device

Each file descriptor gets a chunk of per-FD driver specific context that
allows the driver to attach a device specific struct to. The core code
takes care of the memory lifetime for this structure.

The ioctl dispatch and design is based on what was built for iommufd. The
ioctls have a struct which has a combined in/out behavior with a typical
'zero pad' scheme for future extension and backwards compatibility.

Like iommufd some shared logic does most of the ioctl marshaling and
compatibility work and table dispatches to some function pointers for
each unique ioctl.

This approach has proven to work quite well in the iommufd and rdma
subsystems.

Allocate an ioctl number space for the subsystem.

Link: https://patch.msgid.link/r/2-v5-642aa0c94070+4447f-fwctl_jgg@nvidia.com
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Shannon Nelson <shannon.nelson@amd.com>
Tested-by: Dave Jiang <dave.jiang@intel.com>
Tested-by: Shannon Nelson <shannon.nelson@amd.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

1 files changed, 38 insertions, 0 deletions

diff --git a/include/uapi/fwctl/fwctl.h b/include/uapi/fwctl/fwctl.h
new file mode 100644
index 000000000000..8f5fe821cf28
--- /dev/null
+++ b/include/uapi/fwctl/fwctl.h
@@ -0,0 +1,38 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+/* Copyright (c) 2024-2025, NVIDIA CORPORATION & AFFILIATES.
+ */
+#ifndef _UAPI_FWCTL_H
+#define _UAPI_FWCTL_H
+
+#define FWCTL_TYPE 0x9A
+
+/**
+ * DOC: General ioctl format
+ *
+ * The ioctl interface follows a general format to allow for extensibility. Each
+ * ioctl is passed a structure pointer as the argument providing the size of
+ * the structure in the first u32. The kernel checks that any structure space
+ * beyond what it understands is 0. This allows userspace to use the backward
+ * compatible portion while consistently using the newer, larger, structures.
+ *
+ * ioctls use a standard meaning for common errnos:
+ *
+ *  - ENOTTY: The IOCTL number itself is not supported at all
+ *  - E2BIG: The IOCTL number is supported, but the provided structure has
+ *    non-zero in a part the kernel does not understand.
+ *  - EOPNOTSUPP: The IOCTL number is supported, and the structure is
+ *    understood, however a known field has a value the kernel does not
+ *    understand or support.
+ *  - EINVAL: Everything about the IOCTL was understood, but a field is not
+ *    correct.
+ *  - ENOMEM: Out of memory.
+ *  - ENODEV: The underlying device has been hot-unplugged and the FD is
+ *            orphaned.
+ *
+ * As well as additional errnos, within specific ioctls.
+ */
+enum {
+	FWCTL_CMD_BASE = 0,
+};
+
+#endif