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().
  ...

5 files changed, 483 insertions, 0 deletions

diff --git a/Documentation/admin-guide/gpio/gpio-aggregator.rst b/Documentation/admin-guide/gpio/gpio-aggregator.rst
new file mode 100644
index 000000000..5cd1e7221
--- /dev/null
+++ b/Documentation/admin-guide/gpio/gpio-aggregator.rst
@@ -0,0 +1,111 @@
+.. SPDX-License-Identifier: GPL-2.0-only
+
+GPIO Aggregator
+===============
+
+The GPIO Aggregator provides a mechanism to aggregate GPIOs, and expose them as
+a new gpio_chip.  This supports the following use cases.
+
+
+Aggregating GPIOs using Sysfs
+-----------------------------
+
+GPIO controllers are exported to userspace using /dev/gpiochip* character
+devices.  Access control to these devices is provided by standard UNIX file
+system permissions, on an all-or-nothing basis: either a GPIO controller is
+accessible for a user, or it is not.
+
+The GPIO Aggregator provides access control for a set of one or more GPIOs, by
+aggregating them into a new gpio_chip, which can be assigned to a group or user
+using standard UNIX file ownership and permissions.  Furthermore, this
+simplifies and hardens exporting GPIOs to a virtual machine, as the VM can just
+grab the full GPIO controller, and no longer needs to care about which GPIOs to
+grab and which not, reducing the attack surface.
+
+Aggregated GPIO controllers are instantiated and destroyed by writing to
+write-only attribute files in sysfs.
+
+    /sys/bus/platform/drivers/gpio-aggregator/
+
+	"new_device" ...
+		Userspace may ask the kernel to instantiate an aggregated GPIO
+		controller by writing a string describing the GPIOs to
+		aggregate to the "new_device" file, using the format
+
+		.. code-block:: none
+
+		    [<gpioA>] [<gpiochipB> <offsets>] ...
+
+		Where:
+
+		    "<gpioA>" ...
+			    is a GPIO line name,
+
+		    "<gpiochipB>" ...
+			    is a GPIO chip label, and
+
+		    "<offsets>" ...
+			    is a comma-separated list of GPIO offsets and/or
+			    GPIO offset ranges denoted by dashes.
+
+		Example: Instantiate a new GPIO aggregator by aggregating GPIO
+		line 19 of "e6052000.gpio" and GPIO lines 20-21 of
+		"e6050000.gpio" into a new gpio_chip:
+
+		.. code-block:: sh
+
+		    $ echo 'e6052000.gpio 19 e6050000.gpio 20-21' > new_device
+
+	"delete_device" ...
+		Userspace may ask the kernel to destroy an aggregated GPIO
+		controller after use by writing its device name to the
+		"delete_device" file.
+
+		Example: Destroy the previously-created aggregated GPIO
+		controller, assumed to be "gpio-aggregator.0":
+
+		.. code-block:: sh
+
+		    $ echo gpio-aggregator.0 > delete_device
+
+
+Generic GPIO Driver
+-------------------
+
+The GPIO Aggregator can also be used as a generic driver for a simple
+GPIO-operated device described in DT, without a dedicated in-kernel driver.
+This is useful in industrial control, and is not unlike e.g. spidev, which
+allows the user to communicate with an SPI device from userspace.
+
+Binding a device to the GPIO Aggregator is performed either by modifying the
+gpio-aggregator driver, or by writing to the "driver_override" file in Sysfs.
+
+Example: If "door" is a GPIO-operated device described in DT, using its own
+compatible value::
+
+	door {
+		compatible = "myvendor,mydoor";
+
+		gpios = <&gpio2 19 GPIO_ACTIVE_HIGH>,
+			<&gpio2 20 GPIO_ACTIVE_LOW>;
+		gpio-line-names = "open", "lock";
+	};
+
+it can be bound to the GPIO Aggregator by either:
+
+1. Adding its compatible value to ``gpio_aggregator_dt_ids[]``,
+2. Binding manually using "driver_override":
+
+.. code-block:: sh
+
+    $ echo gpio-aggregator > /sys/bus/platform/devices/door/driver_override
+    $ echo door > /sys/bus/platform/drivers/gpio-aggregator/bind
+
+After that, a new gpiochip "door" has been created:
+
+.. code-block:: sh
+
+    $ gpioinfo door
+    gpiochip12 - 2 lines:
+	    line   0:       "open"       unused   input  active-high
+	    line   1:       "lock"       unused   input  active-high
diff --git a/Documentation/admin-guide/gpio/gpio-mockup.rst b/Documentation/admin-guide/gpio/gpio-mockup.rst
new file mode 100644
index 000000000..493071da1
--- /dev/null
+++ b/Documentation/admin-guide/gpio/gpio-mockup.rst
@@ -0,0 +1,51 @@
+.. SPDX-License-Identifier: GPL-2.0-only
+
+GPIO Testing Driver
+===================
+
+The GPIO Testing Driver (gpio-mockup) provides a way to create simulated GPIO
+chips for testing purposes. The lines exposed by these chips can be accessed
+using the standard GPIO character device interface as well as manipulated
+using the dedicated debugfs directory structure.
+
+Creating simulated chips using module params
+--------------------------------------------
+
+When loading the gpio-mockup driver a number of parameters can be passed to the
+module.
+
+    gpio_mockup_ranges
+
+        This parameter takes an argument in the form of an array of integer
+        pairs. Each pair defines the base GPIO number (non-negative integer)
+        and the first number after the last of this chip. If the base GPIO
+        is -1, the gpiolib will assign it automatically. while the following
+        parameter is the number of lines exposed by the chip.
+
+        Example: gpio_mockup_ranges=-1,8,-1,16,405,409
+
+        The line above creates three chips. The first one will expose 8 lines,
+        the second 16 and the third 4. The base GPIO for the third chip is set
+        to 405 while for two first chips it will be assigned automatically.
+
+    gpio_mockup_named_lines
+
+        This parameter doesn't take any arguments. It lets the driver know that
+        GPIO lines exposed by it should be named.
+
+        The name format is: gpio-mockup-X-Y where X is mockup chip's ID
+        and Y is the line offset.
+
+Manipulating simulated lines
+----------------------------
+
+Each mockup chip creates its own subdirectory in /sys/kernel/debug/gpio-mockup/.
+The directory is named after the chip's label. A symlink is also created, named
+after the chip's name, which points to the label directory.
+
+Inside each subdirectory, there's a separate attribute for each GPIO line. The
+name of the attribute represents the line's offset in the chip.
+
+Reading from a line attribute returns the current value. Writing to it (0 or 1)
+changes the configuration of the simulated pull-up/pull-down resistor
+(1 - pull-up, 0 - pull-down).
diff --git a/Documentation/admin-guide/gpio/gpio-sim.rst b/Documentation/admin-guide/gpio/gpio-sim.rst
new file mode 100644
index 000000000..d8a90c81b
--- /dev/null
+++ b/Documentation/admin-guide/gpio/gpio-sim.rst
@@ -0,0 +1,134 @@
+.. SPDX-License-Identifier: GPL-2.0-or-later
+
+Configfs GPIO Simulator
+=======================
+
+The configfs GPIO Simulator (gpio-sim) provides a way to create simulated GPIO
+chips for testing purposes. The lines exposed by these chips can be accessed
+using the standard GPIO character device interface as well as manipulated
+using sysfs attributes.
+
+Creating simulated chips
+------------------------
+
+The gpio-sim module registers a configfs subsystem called ``'gpio-sim'``. For
+details of the configfs filesystem, please refer to the configfs documentation.
+
+The user can create a hierarchy of configfs groups and items as well as modify
+values of exposed attributes. Once the chip is instantiated, this hierarchy
+will be translated to appropriate device properties. The general structure is:
+
+**Group:** ``/config/gpio-sim``
+
+This is the top directory of the gpio-sim configfs tree.
+
+**Group:** ``/config/gpio-sim/gpio-device``
+
+**Attribute:** ``/config/gpio-sim/gpio-device/dev_name``
+
+**Attribute:** ``/config/gpio-sim/gpio-device/live``
+
+This is a directory representing a GPIO platform device. The ``'dev_name'``
+attribute is read-only and allows the user-space to read the platform device
+name (e.g. ``'gpio-sim.0'``). The ``'live'`` attribute allows to trigger the
+actual creation of the device once it's fully configured. The accepted values
+are: ``'1'`` to enable the simulated device and ``'0'`` to disable and tear
+it down.
+
+**Group:** ``/config/gpio-sim/gpio-device/gpio-bankX``
+
+**Attribute:** ``/config/gpio-sim/gpio-device/gpio-bankX/chip_name``
+
+**Attribute:** ``/config/gpio-sim/gpio-device/gpio-bankX/num_lines``
+
+This group represents a bank of GPIOs under the top platform device. The
+``'chip_name'`` attribute is read-only and allows the user-space to read the
+device name of the bank device. The ``'num_lines'`` attribute allows to specify
+the number of lines exposed by this bank.
+
+**Group:** ``/config/gpio-sim/gpio-device/gpio-bankX/lineY``
+
+**Attribute:** ``/config/gpio-sim/gpio-device/gpio-bankX/lineY/name``
+
+This group represents a single line at the offset Y. The 'name' attribute
+allows to set the line name as represented by the 'gpio-line-names' property.
+
+**Item:** ``/config/gpio-sim/gpio-device/gpio-bankX/lineY/hog``
+
+**Attribute:** ``/config/gpio-sim/gpio-device/gpio-bankX/lineY/hog/name``
+
+**Attribute:** ``/config/gpio-sim/gpio-device/gpio-bankX/lineY/hog/direction``
+
+This item makes the gpio-sim module hog the associated line. The ``'name'``
+attribute specifies the in-kernel consumer name to use. The ``'direction'``
+attribute specifies the hog direction and must be one of: ``'input'``,
+``'output-high'`` and ``'output-low'``.
+
+Inside each bank directory, there's a set of attributes that can be used to
+configure the new chip. Additionally the user can ``mkdir()`` subdirectories
+inside the chip's directory that allow to pass additional configuration for
+specific lines. The name of those subdirectories must take the form of:
+``'line<offset>'`` (e.g. ``'line0'``, ``'line20'``, etc.) as the name will be
+used by the module to assign the config to the specific line at given offset.
+
+Once the confiuration is complete, the ``'live'`` attribute must be set to 1 in
+order to instantiate the chip. It can be set back to 0 to destroy the simulated
+chip. The module will synchronously wait for the new simulated device to be
+successfully probed and if this doesn't happen, writing to ``'live'`` will
+result in an error.
+
+Simulated GPIO chips can also be defined in device-tree. The compatible string
+must be: ``"gpio-simulator"``. Supported properties are:
+
+  ``"gpio-sim,label"`` - chip label
+
+Other standard GPIO properties (like ``"gpio-line-names"``, ``"ngpios"`` or
+``"gpio-hog"``) are also supported. Please refer to the GPIO documentation for
+details.
+
+An example device-tree code defining a GPIO simulator:
+
+.. code-block :: none
+
+    gpio-sim {
+        compatible = "gpio-simulator";
+
+        bank0 {
+            gpio-controller;
+            #gpio-cells = <2>;
+            ngpios = <16>;
+            gpio-sim,label = "dt-bank0";
+            gpio-line-names = "", "sim-foo", "", "sim-bar";
+        };
+
+        bank1 {
+            gpio-controller;
+            #gpio-cells = <2>;
+            ngpios = <8>;
+            gpio-sim,label = "dt-bank1";
+
+            line3 {
+                gpio-hog;
+                gpios = <3 0>;
+                output-high;
+                line-name = "sim-hog-from-dt";
+            };
+        };
+    };
+
+Manipulating simulated lines
+----------------------------
+
+Each simulated GPIO chip creates a separate sysfs group under its device
+directory for each exposed line
+(e.g. ``/sys/devices/platform/gpio-sim.X/gpiochipY/``). The name of each group
+is of the form: ``'sim_gpioX'`` where X is the offset of the line. Inside each
+group there are two attibutes:
+
+    ``pull`` - allows to read and set the current simulated pull setting for
+               every line, when writing the value must be one of: ``'pull-up'``,
+               ``'pull-down'``
+
+    ``value`` - allows to read the current value of the line which may be
+                different from the pull if the line is being driven from
+                user-space
diff --git a/Documentation/admin-guide/gpio/index.rst b/Documentation/admin-guide/gpio/index.rst
new file mode 100644
index 000000000..f6861ca16
--- /dev/null
+++ b/Documentation/admin-guide/gpio/index.rst
@@ -0,0 +1,20 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+====
+gpio
+====
+
+.. toctree::
+    :maxdepth: 1
+
+    gpio-aggregator
+    sysfs
+    gpio-mockup
+    gpio-sim
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`
diff --git a/Documentation/admin-guide/gpio/sysfs.rst b/Documentation/admin-guide/gpio/sysfs.rst
new file mode 100644
index 000000000..ec09ffd98
--- /dev/null
+++ b/Documentation/admin-guide/gpio/sysfs.rst
@@ -0,0 +1,167 @@
+GPIO Sysfs Interface for Userspace
+==================================
+
+.. warning::
+
+  THIS ABI IS DEPRECATED, THE ABI DOCUMENTATION HAS BEEN MOVED TO
+  Documentation/ABI/obsolete/sysfs-gpio AND NEW USERSPACE CONSUMERS
+  ARE SUPPOSED TO USE THE CHARACTER DEVICE ABI. THIS OLD SYSFS ABI WILL
+  NOT BE DEVELOPED (NO NEW FEATURES), IT WILL JUST BE MAINTAINED.
+
+Refer to the examples in tools/gpio/* for an introduction to the new
+character device ABI. Also see the userspace header in
+include/uapi/linux/gpio.h
+
+The deprecated sysfs ABI
+------------------------
+Platforms which use the "gpiolib" implementors framework may choose to
+configure a sysfs user interface to GPIOs. This is different from the
+debugfs interface, since it provides control over GPIO direction and
+value instead of just showing a gpio state summary. Plus, it could be
+present on production systems without debugging support.
+
+Given appropriate hardware documentation for the system, userspace could
+know for example that GPIO #23 controls the write protect line used to
+protect boot loader segments in flash memory. System upgrade procedures
+may need to temporarily remove that protection, first importing a GPIO,
+then changing its output state, then updating the code before re-enabling
+the write protection. In normal use, GPIO #23 would never be touched,
+and the kernel would have no need to know about it.
+
+Again depending on appropriate hardware documentation, on some systems
+userspace GPIO can be used to determine system configuration data that
+standard kernels won't know about. And for some tasks, simple userspace
+GPIO drivers could be all that the system really needs.
+
+DO NOT ABUSE SYSFS TO CONTROL HARDWARE THAT HAS PROPER KERNEL DRIVERS.
+PLEASE READ THE DOCUMENT AT Documentation/driver-api/gpio/drivers-on-gpio.rst
+TO AVOID REINVENTING KERNEL WHEELS IN USERSPACE. I MEAN IT. REALLY.
+
+Paths in Sysfs
+--------------
+There are three kinds of entries in /sys/class/gpio:
+
+   -	Control interfaces used to get userspace control over GPIOs;
+
+   -	GPIOs themselves; and
+
+   -	GPIO controllers ("gpio_chip" instances).
+
+That's in addition to standard files including the "device" symlink.
+
+The control interfaces are write-only:
+
+    /sys/class/gpio/
+
+	"export" ...
+		Userspace may ask the kernel to export control of
+		a GPIO to userspace by writing its number to this file.
+
+		Example:  "echo 19 > export" will create a "gpio19" node
+		for GPIO #19, if that's not requested by kernel code.
+
+	"unexport" ...
+		Reverses the effect of exporting to userspace.
+
+		Example:  "echo 19 > unexport" will remove a "gpio19"
+		node exported using the "export" file.
+
+GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42)
+and have the following read/write attributes:
+
+    /sys/class/gpio/gpioN/
+
+	"direction" ...
+		reads as either "in" or "out". This value may
+		normally be written. Writing as "out" defaults to
+		initializing the value as low. To ensure glitch free
+		operation, values "low" and "high" may be written to
+		configure the GPIO as an output with that initial value.
+
+		Note that this attribute *will not exist* if the kernel
+		doesn't support changing the direction of a GPIO, or
+		it was exported by kernel code that didn't explicitly
+		allow userspace to reconfigure this GPIO's direction.
+
+	"value" ...
+		reads as either 0 (low) or 1 (high). If the GPIO
+		is configured as an output, this value may be written;
+		any nonzero value is treated as high.
+
+		If the pin can be configured as interrupt-generating interrupt
+		and if it has been configured to generate interrupts (see the
+		description of "edge"), you can poll(2) on that file and
+		poll(2) will return whenever the interrupt was triggered. If
+		you use poll(2), set the events POLLPRI and POLLERR. If you
+		use select(2), set the file descriptor in exceptfds. After
+		poll(2) returns, either lseek(2) to the beginning of the sysfs
+		file and read the new value or close the file and re-open it
+		to read the value.
+
+	"edge" ...
+		reads as either "none", "rising", "falling", or
+		"both". Write these strings to select the signal edge(s)
+		that will make poll(2) on the "value" file return.
+
+		This file exists only if the pin can be configured as an
+		interrupt generating input pin.
+
+	"active_low" ...
+		reads as either 0 (false) or 1 (true). Write
+		any nonzero value to invert the value attribute both
+		for reading and writing. Existing and subsequent
+		poll(2) support configuration via the edge attribute
+		for "rising" and "falling" edges will follow this
+		setting.
+
+GPIO controllers have paths like /sys/class/gpio/gpiochip42/ (for the
+controller implementing GPIOs starting at #42) and have the following
+read-only attributes:
+
+    /sys/class/gpio/gpiochipN/
+
+	"base" ...
+		same as N, the first GPIO managed by this chip
+
+	"label" ...
+		provided for diagnostics (not always unique)
+
+	"ngpio" ...
+		how many GPIOs this manages (N to N + ngpio - 1)
+
+Board documentation should in most cases cover what GPIOs are used for
+what purposes. However, those numbers are not always stable; GPIOs on
+a daughtercard might be different depending on the base board being used,
+or other cards in the stack. In such cases, you may need to use the
+gpiochip nodes (possibly in conjunction with schematics) to determine
+the correct GPIO number to use for a given signal.
+
+
+Exporting from Kernel code
+--------------------------
+Kernel code can explicitly manage exports of GPIOs which have already been
+requested using gpio_request()::
+
+	/* export the GPIO to userspace */
+	int gpiod_export(struct gpio_desc *desc, bool direction_may_change);
+
+	/* reverse gpio_export() */
+	void gpiod_unexport(struct gpio_desc *desc);
+
+	/* create a sysfs link to an exported GPIO node */
+	int gpiod_export_link(struct device *dev, const char *name,
+		      struct gpio_desc *desc);
+
+After a kernel driver requests a GPIO, it may only be made available in
+the sysfs interface by gpiod_export(). The driver can control whether the
+signal direction may change. This helps drivers prevent userspace code
+from accidentally clobbering important system state.
+
+This explicit exporting can help with debugging (by making some kinds
+of experiments easier), or can provide an always-there interface that's
+suitable for documenting as part of a board support package.
+
+After the GPIO has been exported, gpiod_export_link() allows creating
+symlinks from elsewhere in sysfs to the GPIO sysfs node. Drivers can
+use this to provide the interface under their own device in sysfs with
+a descriptive name.