Merge tag 'net-next-6.3' of git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-nextgrafted

Pull networking updates from Jakub Kicinski:
 "Core:

   - Add dedicated kmem_cache for typical/small skb->head, avoid having
     to access struct page at kfree time, and improve memory use.

   - Introduce sysctl to set default RPS configuration for new netdevs.

   - Define Netlink protocol specification format which can be used to
     describe messages used by each family and auto-generate parsers.
     Add tools for generating kernel data structures and uAPI headers.

   - Expose all net/core sysctls inside netns.

   - Remove 4s sleep in netpoll if carrier is instantly detected on
     boot.

   - Add configurable limit of MDB entries per port, and port-vlan.

   - Continue populating drop reasons throughout the stack.

   - Retire a handful of legacy Qdiscs and classifiers.

  Protocols:

   - Support IPv4 big TCP (TSO frames larger than 64kB).

   - Add IP_LOCAL_PORT_RANGE socket option, to control local port range
     on socket by socket basis.

   - Track and report in procfs number of MPTCP sockets used.

   - Support mixing IPv4 and IPv6 flows in the in-kernel MPTCP path
     manager.

   - IPv6: don't check net.ipv6.route.max_size and rely on garbage
     collection to free memory (similarly to IPv4).

   - Support Penultimate Segment Pop (PSP) flavor in SRv6 (RFC8986).

   - ICMP: add per-rate limit counters.

   - Add support for user scanning requests in ieee802154.

   - Remove static WEP support.

   - Support minimal Wi-Fi 7 Extremely High Throughput (EHT) rate
     reporting.

   - WiFi 7 EHT channel puncturing support (client & AP).

  BPF:

   - Add a rbtree data structure following the "next-gen data structure"
     precedent set by recently added linked list, that is, by using
     kfunc + kptr instead of adding a new BPF map type.

   - Expose XDP hints via kfuncs with initial support for RX hash and
     timestamp metadata.

   - Add BPF_F_NO_TUNNEL_KEY extension to bpf_skb_set_tunnel_key to
     better support decap on GRE tunnel devices not operating in collect
     metadata.

   - Improve x86 JIT's codegen for PROBE_MEM runtime error checks.

   - Remove the need for trace_printk_lock for bpf_trace_printk and
     bpf_trace_vprintk helpers.

   - Extend libbpf's bpf_tracing.h support for tracing arguments of
     kprobes/uprobes and syscall as a special case.

   - Significantly reduce the search time for module symbols by
     livepatch and BPF.

   - Enable cpumasks to be used as kptrs, which is useful for tracing
     programs tracking which tasks end up running on which CPUs in
     different time intervals.

   - Add support for BPF trampoline on s390x and riscv64.

   - Add capability to export the XDP features supported by the NIC.

   - Add __bpf_kfunc tag for marking kernel functions as kfuncs.

   - Add cgroup.memory=nobpf kernel parameter option to disable BPF
     memory accounting for container environments.

  Netfilter:

   - Remove the CLUSTERIP target. It has been marked as obsolete for
     years, and we still have WARN splats wrt races of the out-of-band
     /proc interface installed by this target.

   - Add 'destroy' commands to nf_tables. They are identical to the
     existing 'delete' commands, but do not return an error if the
     referenced object (set, chain, rule...) did not exist.

  Driver API:

   - Improve cpumask_local_spread() locality to help NICs set the right
     IRQ affinity on AMD platforms.

   - Separate C22 and C45 MDIO bus transactions more clearly.

   - Introduce new DCB table to control DSCP rewrite on egress.

   - Support configuration of Physical Layer Collision Avoidance (PLCA)
     Reconciliation Sublayer (RS) (802.3cg-2019). Modern version of
     shared medium Ethernet.

   - Support for MAC Merge layer (IEEE 802.3-2018 clause 99). Allowing
     preemption of low priority frames by high priority frames.

   - Add support for controlling MACSec offload using netlink SET.

   - Rework devlink instance refcounts to allow registration and
     de-registration under the instance lock. Split the code into
     multiple files, drop some of the unnecessarily granular locks and
     factor out common parts of netlink operation handling.

   - Add TX frame aggregation parameters (for USB drivers).

   - Add a new attr TCA_EXT_WARN_MSG to report TC (offload) warning
     messages with notifications for debug.

   - Allow offloading of UDP NEW connections via act_ct.

   - Add support for per action HW stats in TC.

   - Support hardware miss to TC action (continue processing in SW from
     a specific point in the action chain).

   - Warn if old Wireless Extension user space interface is used with
     modern cfg80211/mac80211 drivers. Do not support Wireless
     Extensions for Wi-Fi 7 devices at all. Everyone should switch to
     using nl80211 interface instead.

   - Improve the CAN bit timing configuration. Use extack to return
     error messages directly to user space, update the SJW handling,
     including the definition of a new default value that will benefit
     CAN-FD controllers, by increasing their oscillator tolerance.

  New hardware / drivers:

   - Ethernet:
      - nVidia BlueField-3 support (control traffic driver)
      - Ethernet support for imx93 SoCs
      - Motorcomm yt8531 gigabit Ethernet PHY
      - onsemi NCN26000 10BASE-T1S PHY (with support for PLCA)
      - Microchip LAN8841 PHY (incl. cable diagnostics and PTP)
      - Amlogic gxl MDIO mux

   - WiFi:
      - RealTek RTL8188EU (rtl8xxxu)
      - Qualcomm Wi-Fi 7 devices (ath12k)

   - CAN:
      - Renesas R-Car V4H

  Drivers:

   - Bluetooth:
      - Set Per Platform Antenna Gain (PPAG) for Intel controllers.

   - Ethernet NICs:
      - Intel (1G, igc):
         - support TSN / Qbv / packet scheduling features of i226 model
      - Intel (100G, ice):
         - use GNSS subsystem instead of TTY
         - multi-buffer XDP support
         - extend support for GPIO pins to E823 devices
      - nVidia/Mellanox:
         - update the shared buffer configuration on PFC commands
         - implement PTP adjphase function for HW offset control
         - TC support for Geneve and GRE with VF tunnel offload
         - more efficient crypto key management method
         - multi-port eswitch support
      - Netronome/Corigine:
         - add DCB IEEE support
         - support IPsec offloading for NFP3800
      - Freescale/NXP (enetc):
         - support XDP_REDIRECT for XDP non-linear buffers
         - improve reconfig, avoid link flap and waiting for idle
         - support MAC Merge layer
      - Other NICs:
         - sfc/ef100: add basic devlink support for ef100
         - ionic: rx_push mode operation (writing descriptors via MMIO)
         - bnxt: use the auxiliary bus abstraction for RDMA
         - r8169: disable ASPM and reset bus in case of tx timeout
         - cpsw: support QSGMII mode for J721e CPSW9G
         - cpts: support pulse-per-second output
         - ngbe: add an mdio bus driver
         - usbnet: optimize usbnet_bh() by avoiding unnecessary queuing
         - r8152: handle devices with FW with NCM support
         - amd-xgbe: support 10Mbps, 2.5GbE speeds and rx-adaptation
         - virtio-net: support multi buffer XDP
         - virtio/vsock: replace virtio_vsock_pkt with sk_buff
         - tsnep: XDP support

   - Ethernet high-speed switches:
      - nVidia/Mellanox (mlxsw):
         - add support for latency TLV (in FW control messages)
      - Microchip (sparx5):
         - separate explicit and implicit traffic forwarding rules, make
           the implicit rules always active
         - add support for egress DSCP rewrite
         - IS0 VCAP support (Ingress Classification)
         - IS2 VCAP filters (protos, L3 addrs, L4 ports, flags, ToS
           etc.)
         - ES2 VCAP support (Egress Access Control)
         - support for Per-Stream Filtering and Policing (802.1Q,
           8.6.5.1)

   - Ethernet embedded switches:
      - Marvell (mv88e6xxx):
         - add MAB (port auth) offload support
         - enable PTP receive for mv88e6390
      - NXP (ocelot):
         - support MAC Merge layer
         - support for the the vsc7512 internal copper phys
      - Microchip:
         - lan9303: convert to PHYLINK
         - lan966x: support TC flower filter statistics
         - lan937x: PTP support for KSZ9563/KSZ8563 and LAN937x
         - lan937x: support Credit Based Shaper configuration
         - ksz9477: support Energy Efficient Ethernet
      - other:
         - qca8k: convert to regmap read/write API, use bulk operations
         - rswitch: Improve TX timestamp accuracy

   - Intel WiFi (iwlwifi):
      - EHT (Wi-Fi 7) rate reporting
      - STEP equalizer support: transfer some STEP (connection to radio
        on platforms with integrated wifi) related parameters from the
        BIOS to the firmware.

   - Qualcomm 802.11ax WiFi (ath11k):
      - IPQ5018 support
      - Fine Timing Measurement (FTM) responder role support
      - channel 177 support

   - MediaTek WiFi (mt76):
      - per-PHY LED support
      - mt7996: EHT (Wi-Fi 7) support
      - Wireless Ethernet Dispatch (WED) reset support
      - switch to using page pool allocator

   - RealTek WiFi (rtw89):
      - support new version of Bluetooth co-existance

   - Mobile:
      - rmnet: support TX aggregation"

* tag 'net-next-6.3' of git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next: (1872 commits)
  page_pool: add a comment explaining the fragment counter usage
  net: ethtool: fix __ethtool_dev_mm_supported() implementation
  ethtool: pse-pd: Fix double word in comments
  xsk: add linux/vmalloc.h to xsk.c
  sefltests: netdevsim: wait for devlink instance after netns removal
  selftest: fib_tests: Always cleanup before exit
  net/mlx5e: Align IPsec ASO result memory to be as required by hardware
  net/mlx5e: TC, Set CT miss to the specific ct action instance
  net/mlx5e: Rename CHAIN_TO_REG to MAPPED_OBJ_TO_REG
  net/mlx5: Refactor tc miss handling to a single function
  net/mlx5: Kconfig: Make tc offload depend on tc skb extension
  net/sched: flower: Support hardware miss to tc action
  net/sched: flower: Move filter handle initialization earlier
  net/sched: cls_api: Support hardware miss to tc action
  net/sched: Rename user cookie and act cookie
  sfc: fix builds without CONFIG_RTC_LIB
  sfc: clean up some inconsistent indentings
  net/mlx4_en: Introduce flexible array to silence overflow warning
  net: lan966x: Fix possible deadlock inside PTP
  net/ulp: Remove redundant ->clone() test in inet_clone_ulp().
  ...

1 files changed, 561 insertions, 0 deletions

diff --git a/Documentation/kbuild/modules.rst b/Documentation/kbuild/modules.rst
new file mode 100644
index 000000000..a1f3eb7a4
--- /dev/null
+++ b/Documentation/kbuild/modules.rst
@@ -0,0 +1,561 @@
+=========================
+Building External Modules
+=========================
+
+This document describes how to build an out-of-tree kernel module.
+
+.. Table of Contents
+
+	=== 1 Introduction
+	=== 2 How to Build External Modules
+	   --- 2.1 Command Syntax
+	   --- 2.2 Options
+	   --- 2.3 Targets
+	   --- 2.4 Building Separate Files
+	=== 3. Creating a Kbuild File for an External Module
+	   --- 3.1 Shared Makefile
+	   --- 3.2 Separate Kbuild file and Makefile
+	   --- 3.3 Binary Blobs
+	   --- 3.4 Building Multiple Modules
+	=== 4. Include Files
+	   --- 4.1 Kernel Includes
+	   --- 4.2 Single Subdirectory
+	   --- 4.3 Several Subdirectories
+	=== 5. Module Installation
+	   --- 5.1 INSTALL_MOD_PATH
+	   --- 5.2 INSTALL_MOD_DIR
+	=== 6. Module Versioning
+	   --- 6.1 Symbols From the Kernel (vmlinux + modules)
+	   --- 6.2 Symbols and External Modules
+	   --- 6.3 Symbols From Another External Module
+	=== 7. Tips & Tricks
+	   --- 7.1 Testing for CONFIG_FOO_BAR
+
+
+
+1. Introduction
+===============
+
+"kbuild" is the build system used by the Linux kernel. Modules must use
+kbuild to stay compatible with changes in the build infrastructure and
+to pick up the right flags to "gcc." Functionality for building modules
+both in-tree and out-of-tree is provided. The method for building
+either is similar, and all modules are initially developed and built
+out-of-tree.
+
+Covered in this document is information aimed at developers interested
+in building out-of-tree (or "external") modules. The author of an
+external module should supply a makefile that hides most of the
+complexity, so one only has to type "make" to build the module. This is
+easily accomplished, and a complete example will be presented in
+section 3.
+
+
+2. How to Build External Modules
+================================
+
+To build external modules, you must have a prebuilt kernel available
+that contains the configuration and header files used in the build.
+Also, the kernel must have been built with modules enabled. If you are
+using a distribution kernel, there will be a package for the kernel you
+are running provided by your distribution.
+
+An alternative is to use the "make" target "modules_prepare." This will
+make sure the kernel contains the information required. The target
+exists solely as a simple way to prepare a kernel source tree for
+building external modules.
+
+NOTE: "modules_prepare" will not build Module.symvers even if
+CONFIG_MODVERSIONS is set; therefore, a full kernel build needs to be
+executed to make module versioning work.
+
+2.1 Command Syntax
+==================
+
+	The command to build an external module is::
+
+		$ make -C <path_to_kernel_src> M=$PWD
+
+	The kbuild system knows that an external module is being built
+	due to the "M=<dir>" option given in the command.
+
+	To build against the running kernel use::
+
+		$ make -C /lib/modules/`uname -r`/build M=$PWD
+
+	Then to install the module(s) just built, add the target
+	"modules_install" to the command::
+
+		$ make -C /lib/modules/`uname -r`/build M=$PWD modules_install
+
+2.2 Options
+===========
+
+	($KDIR refers to the path of the kernel source directory.)
+
+	make -C $KDIR M=$PWD
+
+	-C $KDIR
+		The directory where the kernel source is located.
+		"make" will actually change to the specified directory
+		when executing and will change back when finished.
+
+	M=$PWD
+		Informs kbuild that an external module is being built.
+		The value given to "M" is the absolute path of the
+		directory where the external module (kbuild file) is
+		located.
+
+2.3 Targets
+===========
+
+	When building an external module, only a subset of the "make"
+	targets are available.
+
+	make -C $KDIR M=$PWD [target]
+
+	The default will build the module(s) located in the current
+	directory, so a target does not need to be specified. All
+	output files will also be generated in this directory. No
+	attempts are made to update the kernel source, and it is a
+	precondition that a successful "make" has been executed for the
+	kernel.
+
+	modules
+		The default target for external modules. It has the
+		same functionality as if no target was specified. See
+		description above.
+
+	modules_install
+		Install the external module(s). The default location is
+		/lib/modules/<kernel_release>/extra/, but a prefix may
+		be added with INSTALL_MOD_PATH (discussed in section 5).
+
+	clean
+		Remove all generated files in the module directory only.
+
+	help
+		List the available targets for external modules.
+
+2.4 Building Separate Files
+===========================
+
+	It is possible to build single files that are part of a module.
+	This works equally well for the kernel, a module, and even for
+	external modules.
+
+	Example (The module foo.ko, consist of bar.o and baz.o)::
+
+		make -C $KDIR M=$PWD bar.lst
+		make -C $KDIR M=$PWD baz.o
+		make -C $KDIR M=$PWD foo.ko
+		make -C $KDIR M=$PWD ./
+
+
+3. Creating a Kbuild File for an External Module
+================================================
+
+In the last section we saw the command to build a module for the
+running kernel. The module is not actually built, however, because a
+build file is required. Contained in this file will be the name of
+the module(s) being built, along with the list of requisite source
+files. The file may be as simple as a single line::
+
+	obj-m := <module_name>.o
+
+The kbuild system will build <module_name>.o from <module_name>.c,
+and, after linking, will result in the kernel module <module_name>.ko.
+The above line can be put in either a "Kbuild" file or a "Makefile."
+When the module is built from multiple sources, an additional line is
+needed listing the files::
+
+	<module_name>-y := <src1>.o <src2>.o ...
+
+NOTE: Further documentation describing the syntax used by kbuild is
+located in Documentation/kbuild/makefiles.rst.
+
+The examples below demonstrate how to create a build file for the
+module 8123.ko, which is built from the following files::
+
+	8123_if.c
+	8123_if.h
+	8123_pci.c
+	8123_bin.o_shipped	<= Binary blob
+
+3.1 Shared Makefile
+-------------------
+
+	An external module always includes a wrapper makefile that
+	supports building the module using "make" with no arguments.
+	This target is not used by kbuild; it is only for convenience.
+	Additional functionality, such as test targets, can be included
+	but should be filtered out from kbuild due to possible name
+	clashes.
+
+	Example 1::
+
+		--> filename: Makefile
+		ifneq ($(KERNELRELEASE),)
+		# kbuild part of makefile
+		obj-m  := 8123.o
+		8123-y := 8123_if.o 8123_pci.o 8123_bin.o
+
+		else
+		# normal makefile
+		KDIR ?= /lib/modules/`uname -r`/build
+
+		default:
+			$(MAKE) -C $(KDIR) M=$$PWD
+
+		# Module specific targets
+		genbin:
+			echo "X" > 8123_bin.o_shipped
+
+		endif
+
+	The check for KERNELRELEASE is used to separate the two parts
+	of the makefile. In the example, kbuild will only see the two
+	assignments, whereas "make" will see everything except these
+	two assignments. This is due to two passes made on the file:
+	the first pass is by the "make" instance run on the command
+	line; the second pass is by the kbuild system, which is
+	initiated by the parameterized "make" in the default target.
+
+3.2 Separate Kbuild File and Makefile
+-------------------------------------
+
+	In newer versions of the kernel, kbuild will first look for a
+	file named "Kbuild," and only if that is not found, will it
+	then look for a makefile. Utilizing a "Kbuild" file allows us
+	to split up the makefile from example 1 into two files:
+
+	Example 2::
+
+		--> filename: Kbuild
+		obj-m  := 8123.o
+		8123-y := 8123_if.o 8123_pci.o 8123_bin.o
+
+		--> filename: Makefile
+		KDIR ?= /lib/modules/`uname -r`/build
+
+		default:
+			$(MAKE) -C $(KDIR) M=$$PWD
+
+		# Module specific targets
+		genbin:
+			echo "X" > 8123_bin.o_shipped
+
+	The split in example 2 is questionable due to the simplicity of
+	each file; however, some external modules use makefiles
+	consisting of several hundred lines, and here it really pays
+	off to separate the kbuild part from the rest.
+
+	The next example shows a backward compatible version.
+
+	Example 3::
+
+		--> filename: Kbuild
+		obj-m  := 8123.o
+		8123-y := 8123_if.o 8123_pci.o 8123_bin.o
+
+		--> filename: Makefile
+		ifneq ($(KERNELRELEASE),)
+		# kbuild part of makefile
+		include Kbuild
+
+		else
+		# normal makefile
+		KDIR ?= /lib/modules/`uname -r`/build
+
+		default:
+			$(MAKE) -C $(KDIR) M=$$PWD
+
+		# Module specific targets
+		genbin:
+			echo "X" > 8123_bin.o_shipped
+
+		endif
+
+	Here the "Kbuild" file is included from the makefile. This
+	allows an older version of kbuild, which only knows of
+	makefiles, to be used when the "make" and kbuild parts are
+	split into separate files.
+
+3.3 Binary Blobs
+----------------
+
+	Some external modules need to include an object file as a blob.
+	kbuild has support for this, but requires the blob file to be
+	named <filename>_shipped. When the kbuild rules kick in, a copy
+	of <filename>_shipped is created with _shipped stripped off,
+	giving us <filename>. This shortened filename can be used in
+	the assignment to the module.
+
+	Throughout this section, 8123_bin.o_shipped has been used to
+	build the kernel module 8123.ko; it has been included as
+	8123_bin.o::
+
+		8123-y := 8123_if.o 8123_pci.o 8123_bin.o
+
+	Although there is no distinction between the ordinary source
+	files and the binary file, kbuild will pick up different rules
+	when creating the object file for the module.
+
+3.4 Building Multiple Modules
+=============================
+
+	kbuild supports building multiple modules with a single build
+	file. For example, if you wanted to build two modules, foo.ko
+	and bar.ko, the kbuild lines would be::
+
+		obj-m := foo.o bar.o
+		foo-y := <foo_srcs>
+		bar-y := <bar_srcs>
+
+	It is that simple!
+
+
+4. Include Files
+================
+
+Within the kernel, header files are kept in standard locations
+according to the following rule:
+
+	* If the header file only describes the internal interface of a
+	  module, then the file is placed in the same directory as the
+	  source files.
+	* If the header file describes an interface used by other parts
+	  of the kernel that are located in different directories, then
+	  the file is placed in include/linux/.
+
+	  NOTE:
+	      There are two notable exceptions to this rule: larger
+	      subsystems have their own directory under include/, such as
+	      include/scsi; and architecture specific headers are located
+	      under arch/$(SRCARCH)/include/.
+
+4.1 Kernel Includes
+-------------------
+
+	To include a header file located under include/linux/, simply
+	use::
+
+		#include <linux/module.h>
+
+	kbuild will add options to "gcc" so the relevant directories
+	are searched.
+
+4.2 Single Subdirectory
+-----------------------
+
+	External modules tend to place header files in a separate
+	include/ directory where their source is located, although this
+	is not the usual kernel style. To inform kbuild of the
+	directory, use either ccflags-y or CFLAGS_<filename>.o.
+
+	Using the example from section 3, if we moved 8123_if.h to a
+	subdirectory named include, the resulting kbuild file would
+	look like::
+
+		--> filename: Kbuild
+		obj-m := 8123.o
+
+		ccflags-y := -Iinclude
+		8123-y := 8123_if.o 8123_pci.o 8123_bin.o
+
+	Note that in the assignment there is no space between -I and
+	the path. This is a limitation of kbuild: there must be no
+	space present.
+
+4.3 Several Subdirectories
+--------------------------
+
+	kbuild can handle files that are spread over several directories.
+	Consider the following example::
+
+		.
+		|__ src
+		|   |__ complex_main.c
+		|   |__ hal
+		|	|__ hardwareif.c
+		|	|__ include
+		|	    |__ hardwareif.h
+		|__ include
+		|__ complex.h
+
+	To build the module complex.ko, we then need the following
+	kbuild file::
+
+		--> filename: Kbuild
+		obj-m := complex.o
+		complex-y := src/complex_main.o
+		complex-y += src/hal/hardwareif.o
+
+		ccflags-y := -I$(src)/include
+		ccflags-y += -I$(src)/src/hal/include
+
+	As you can see, kbuild knows how to handle object files located
+	in other directories. The trick is to specify the directory
+	relative to the kbuild file's location. That being said, this
+	is NOT recommended practice.
+
+	For the header files, kbuild must be explicitly told where to
+	look. When kbuild executes, the current directory is always the
+	root of the kernel tree (the argument to "-C") and therefore an
+	absolute path is needed. $(src) provides the absolute path by
+	pointing to the directory where the currently executing kbuild
+	file is located.
+
+
+5. Module Installation
+======================
+
+Modules which are included in the kernel are installed in the
+directory:
+
+	/lib/modules/$(KERNELRELEASE)/kernel/
+
+And external modules are installed in:
+
+	/lib/modules/$(KERNELRELEASE)/extra/
+
+5.1 INSTALL_MOD_PATH
+--------------------
+
+	Above are the default directories but as always some level of
+	customization is possible. A prefix can be added to the
+	installation path using the variable INSTALL_MOD_PATH::
+
+		$ make INSTALL_MOD_PATH=/frodo modules_install
+		=> Install dir: /frodo/lib/modules/$(KERNELRELEASE)/kernel/
+
+	INSTALL_MOD_PATH may be set as an ordinary shell variable or,
+	as shown above, can be specified on the command line when
+	calling "make." This has effect when installing both in-tree
+	and out-of-tree modules.
+
+5.2 INSTALL_MOD_DIR
+-------------------
+
+	External modules are by default installed to a directory under
+	/lib/modules/$(KERNELRELEASE)/extra/, but you may wish to
+	locate modules for a specific functionality in a separate
+	directory. For this purpose, use INSTALL_MOD_DIR to specify an
+	alternative name to "extra."::
+
+		$ make INSTALL_MOD_DIR=gandalf -C $KDIR \
+		       M=$PWD modules_install
+		=> Install dir: /lib/modules/$(KERNELRELEASE)/gandalf/
+
+
+6. Module Versioning
+====================
+
+Module versioning is enabled by the CONFIG_MODVERSIONS tag, and is used
+as a simple ABI consistency check. A CRC value of the full prototype
+for an exported symbol is created. When a module is loaded/used, the
+CRC values contained in the kernel are compared with similar values in
+the module; if they are not equal, the kernel refuses to load the
+module.
+
+Module.symvers contains a list of all exported symbols from a kernel
+build.
+
+6.1 Symbols From the Kernel (vmlinux + modules)
+-----------------------------------------------
+
+	During a kernel build, a file named Module.symvers will be
+	generated. Module.symvers contains all exported symbols from
+	the kernel and compiled modules. For each symbol, the
+	corresponding CRC value is also stored.
+
+	The syntax of the Module.symvers file is::
+
+		<CRC>       <Symbol>         <Module>                         <Export Type>     <Namespace>
+
+		0xe1cc2a05  usb_stor_suspend drivers/usb/storage/usb-storage  EXPORT_SYMBOL_GPL USB_STORAGE
+
+	The fields are separated by tabs and values may be empty (e.g.
+	if no namespace is defined for an exported symbol).
+
+	For a kernel build without CONFIG_MODVERSIONS enabled, the CRC
+	would read 0x00000000.
+
+	Module.symvers serves two purposes:
+
+	1) It lists all exported symbols from vmlinux and all modules.
+	2) It lists the CRC if CONFIG_MODVERSIONS is enabled.
+
+6.2 Symbols and External Modules
+--------------------------------
+
+	When building an external module, the build system needs access
+	to the symbols from the kernel to check if all external symbols
+	are defined. This is done in the MODPOST step. modpost obtains
+	the symbols by reading Module.symvers from the kernel source
+	tree. During the MODPOST step, a new Module.symvers file will be
+	written containing all exported symbols from that external module.
+
+6.3 Symbols From Another External Module
+----------------------------------------
+
+	Sometimes, an external module uses exported symbols from
+	another external module. Kbuild needs to have full knowledge of
+	all symbols to avoid spitting out warnings about undefined
+	symbols. Two solutions exist for this situation.
+
+	NOTE: The method with a top-level kbuild file is recommended
+	but may be impractical in certain situations.
+
+	Use a top-level kbuild file
+		If you have two modules, foo.ko and bar.ko, where
+		foo.ko needs symbols from bar.ko, you can use a
+		common top-level kbuild file so both modules are
+		compiled in the same build. Consider the following
+		directory layout::
+
+			./foo/ <= contains foo.ko
+			./bar/ <= contains bar.ko
+
+		The top-level kbuild file would then look like::
+
+			#./Kbuild (or ./Makefile):
+				obj-m := foo/ bar/
+
+		And executing::
+
+			$ make -C $KDIR M=$PWD
+
+		will then do the expected and compile both modules with
+		full knowledge of symbols from either module.
+
+	Use "make" variable KBUILD_EXTRA_SYMBOLS
+		If it is impractical to add a top-level kbuild file,
+		you can assign a space separated list
+		of files to KBUILD_EXTRA_SYMBOLS in your build file.
+		These files will be loaded by modpost during the
+		initialization of its symbol tables.
+
+
+7. Tips & Tricks
+================
+
+7.1 Testing for CONFIG_FOO_BAR
+------------------------------
+
+	Modules often need to check for certain `CONFIG_` options to
+	decide if a specific feature is included in the module. In
+	kbuild this is done by referencing the `CONFIG_` variable
+	directly::
+
+		#fs/ext2/Makefile
+		obj-$(CONFIG_EXT2_FS) += ext2.o
+
+		ext2-y := balloc.o bitmap.o dir.o
+		ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o
+
+	External modules have traditionally used "grep" to check for
+	specific `CONFIG_` settings directly in .config. This usage is
+	broken. As introduced before, external modules should use
+	kbuild for building and can therefore use the same methods as
+	in-tree modules when testing for `CONFIG_` definitions.