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, 458 insertions, 0 deletions

diff --git a/Documentation/scsi/libsas.rst b/Documentation/scsi/libsas.rst
new file mode 100644
index 000000000..305a253d5
--- /dev/null
+++ b/Documentation/scsi/libsas.rst
@@ -0,0 +1,458 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=========
+SAS Layer
+=========
+
+The SAS Layer is a management infrastructure which manages
+SAS LLDDs.  It sits between SCSI Core and SAS LLDDs.  The
+layout is as follows: while SCSI Core is concerned with
+SAM/SPC issues, and a SAS LLDD+sequencer is concerned with
+phy/OOB/link management, the SAS layer is concerned with:
+
+      * SAS Phy/Port/HA event management (LLDD generates,
+        SAS Layer processes),
+      * SAS Port management (creation/destruction),
+      * SAS Domain discovery and revalidation,
+      * SAS Domain device management,
+      * SCSI Host registration/unregistration,
+      * Device registration with SCSI Core (SAS) or libata
+        (SATA), and
+      * Expander management and exporting expander control
+        to user space.
+
+A SAS LLDD is a PCI device driver.  It is concerned with
+phy/OOB management, and vendor specific tasks and generates
+events to the SAS layer.
+
+The SAS Layer does most SAS tasks as outlined in the SAS 1.1
+spec.
+
+The sas_ha_struct describes the SAS LLDD to the SAS layer.
+Most of it is used by the SAS Layer but a few fields need to
+be initialized by the LLDDs.
+
+After initializing your hardware, from the probe() function
+you call sas_register_ha(). It will register your LLDD with
+the SCSI subsystem, creating a SCSI host and it will
+register your SAS driver with the sysfs SAS tree it creates.
+It will then return.  Then you enable your phys to actually
+start OOB (at which point your driver will start calling the
+notify_* event callbacks).
+
+Structure descriptions
+======================
+
+``struct sas_phy``
+------------------
+
+Normally this is statically embedded to your driver's
+phy structure::
+
+    struct my_phy {
+	    blah;
+	    struct sas_phy sas_phy;
+	    bleh;
+    };
+
+And then all the phys are an array of my_phy in your HA
+struct (shown below).
+
+Then as you go along and initialize your phys you also
+initialize the sas_phy struct, along with your own
+phy structure.
+
+In general, the phys are managed by the LLDD and the ports
+are managed by the SAS layer.  So the phys are initialized
+and updated by the LLDD and the ports are initialized and
+updated by the SAS layer.
+
+There is a scheme where the LLDD can RW certain fields,
+and the SAS layer can only read such ones, and vice versa.
+The idea is to avoid unnecessary locking.
+
+enabled
+    - must be set (0/1)
+
+id
+    - must be set [0,MAX_PHYS)]
+
+class, proto, type, role, oob_mode, linkrate
+    - must be set
+
+oob_mode
+    - you set this when OOB has finished and then notify
+      the SAS Layer.
+
+sas_addr
+    - this normally points to an array holding the sas
+      address of the phy, possibly somewhere in your my_phy
+      struct.
+
+attached_sas_addr
+    - set this when you (LLDD) receive an
+      IDENTIFY frame or a FIS frame, _before_ notifying the SAS
+      layer.  The idea is that sometimes the LLDD may want to fake
+      or provide a different SAS address on that phy/port and this
+      allows it to do this.  At best you should copy the sas
+      address from the IDENTIFY frame or maybe generate a SAS
+      address for SATA directly attached devices.  The Discover
+      process may later change this.
+
+frame_rcvd
+    - this is where you copy the IDENTIFY/FIS frame
+      when you get it; you lock, copy, set frame_rcvd_size and
+      unlock the lock, and then call the event.  It is a pointer
+      since there's no way to know your hw frame size _exactly_,
+      so you define the actual array in your phy struct and let
+      this pointer point to it.  You copy the frame from your
+      DMAable memory to that area holding the lock.
+
+sas_prim
+    - this is where primitives go when they're
+      received.  See sas.h. Grab the lock, set the primitive,
+      release the lock, notify.
+
+port
+    - this points to the sas_port if the phy belongs
+      to a port -- the LLDD only reads this. It points to the
+      sas_port this phy is part of.  Set by the SAS Layer.
+
+ha
+    - may be set; the SAS layer sets it anyway.
+
+lldd_phy
+    - you should set this to point to your phy so you
+      can find your way around faster when the SAS layer calls one
+      of your callbacks and passes you a phy.  If the sas_phy is
+      embedded you can also use container_of -- whatever you
+      prefer.
+
+
+``struct sas_port``
+-------------------
+
+The LLDD doesn't set any fields of this struct -- it only
+reads them.  They should be self explanatory.
+
+phy_mask is 32 bit, this should be enough for now, as I
+haven't heard of a HA having more than 8 phys.
+
+lldd_port
+    - I haven't found use for that -- maybe other
+      LLDD who wish to have internal port representation can make
+      use of this.
+
+``struct sas_ha_struct``
+------------------------
+
+It normally is statically declared in your own LLDD
+structure describing your adapter::
+
+    struct my_sas_ha {
+	blah;
+	struct sas_ha_struct sas_ha;
+	struct my_phy phys[MAX_PHYS];
+	struct sas_port sas_ports[MAX_PHYS]; /* (1) */
+	bleh;
+    };
+
+    (1) If your LLDD doesn't have its own port representation.
+
+What needs to be initialized (sample function given below).
+
+pcidev
+^^^^^^
+
+sas_addr
+       - since the SAS layer doesn't want to mess with
+	 memory allocation, etc, this points to statically
+	 allocated array somewhere (say in your host adapter
+	 structure) and holds the SAS address of the host
+	 adapter as given by you or the manufacturer, etc.
+
+sas_port
+^^^^^^^^
+
+sas_phy
+      - an array of pointers to structures. (see
+	note above on sas_addr).
+	These must be set.  See more notes below.
+
+num_phys
+       - the number of phys present in the sas_phy array,
+	 and the number of ports present in the sas_port
+	 array.  There can be a maximum num_phys ports (one per
+	 port) so we drop the num_ports, and only use
+	 num_phys.
+
+The event interface::
+
+	/* LLDD calls these to notify the class of an event. */
+	void sas_notify_port_event(struct sas_phy *, enum port_event, gfp_t);
+	void sas_notify_phy_event(struct sas_phy *, enum phy_event, gfp_t);
+
+The port notification::
+
+	/* The class calls these to notify the LLDD of an event. */
+	void (*lldd_port_formed)(struct sas_phy *);
+	void (*lldd_port_deformed)(struct sas_phy *);
+
+If the LLDD wants notification when a port has been formed
+or deformed it sets those to a function satisfying the type.
+
+A SAS LLDD should also implement at least one of the Task
+Management Functions (TMFs) described in SAM::
+
+	/* Task Management Functions. Must be called from process context. */
+	int (*lldd_abort_task)(struct sas_task *);
+	int (*lldd_abort_task_set)(struct domain_device *, u8 *lun);
+	int (*lldd_clear_task_set)(struct domain_device *, u8 *lun);
+	int (*lldd_I_T_nexus_reset)(struct domain_device *);
+	int (*lldd_lu_reset)(struct domain_device *, u8 *lun);
+	int (*lldd_query_task)(struct sas_task *);
+
+For more information please read SAM from T10.org.
+
+Port and Adapter management::
+
+	/* Port and Adapter management */
+	int (*lldd_clear_nexus_port)(struct sas_port *);
+	int (*lldd_clear_nexus_ha)(struct sas_ha_struct *);
+
+A SAS LLDD should implement at least one of those.
+
+Phy management::
+
+	/* Phy management */
+	int (*lldd_control_phy)(struct sas_phy *, enum phy_func);
+
+lldd_ha
+    - set this to point to your HA struct. You can also
+      use container_of if you embedded it as shown above.
+
+A sample initialization and registration function
+can look like this (called last thing from probe())
+*but* before you enable the phys to do OOB::
+
+    static int register_sas_ha(struct my_sas_ha *my_ha)
+    {
+	    int i;
+	    static struct sas_phy   *sas_phys[MAX_PHYS];
+	    static struct sas_port  *sas_ports[MAX_PHYS];
+
+	    my_ha->sas_ha.sas_addr = &my_ha->sas_addr[0];
+
+	    for (i = 0; i < MAX_PHYS; i++) {
+		    sas_phys[i] = &my_ha->phys[i].sas_phy;
+		    sas_ports[i] = &my_ha->sas_ports[i];
+	    }
+
+	    my_ha->sas_ha.sas_phy  = sas_phys;
+	    my_ha->sas_ha.sas_port = sas_ports;
+	    my_ha->sas_ha.num_phys = MAX_PHYS;
+
+	    my_ha->sas_ha.lldd_port_formed = my_port_formed;
+
+	    my_ha->sas_ha.lldd_dev_found = my_dev_found;
+	    my_ha->sas_ha.lldd_dev_gone = my_dev_gone;
+
+	    my_ha->sas_ha.lldd_execute_task = my_execute_task;
+
+	    my_ha->sas_ha.lldd_abort_task     = my_abort_task;
+	    my_ha->sas_ha.lldd_abort_task_set = my_abort_task_set;
+	    my_ha->sas_ha.lldd_clear_task_set = my_clear_task_set;
+	    my_ha->sas_ha.lldd_I_T_nexus_reset= NULL; (2)
+	    my_ha->sas_ha.lldd_lu_reset       = my_lu_reset;
+	    my_ha->sas_ha.lldd_query_task     = my_query_task;
+
+	    my_ha->sas_ha.lldd_clear_nexus_port = my_clear_nexus_port;
+	    my_ha->sas_ha.lldd_clear_nexus_ha = my_clear_nexus_ha;
+
+	    my_ha->sas_ha.lldd_control_phy = my_control_phy;
+
+	    return sas_register_ha(&my_ha->sas_ha);
+    }
+
+(2) SAS 1.1 does not define I_T Nexus Reset TMF.
+
+Events
+======
+
+Events are **the only way** a SAS LLDD notifies the SAS layer
+of anything.  There is no other method or way a LLDD to tell
+the SAS layer of anything happening internally or in the SAS
+domain.
+
+Phy events::
+
+	PHYE_LOSS_OF_SIGNAL, (C)
+	PHYE_OOB_DONE,
+	PHYE_OOB_ERROR,      (C)
+	PHYE_SPINUP_HOLD.
+
+Port events, passed on a _phy_::
+
+	PORTE_BYTES_DMAED,      (M)
+	PORTE_BROADCAST_RCVD,   (E)
+	PORTE_LINK_RESET_ERR,   (C)
+	PORTE_TIMER_EVENT,      (C)
+	PORTE_HARD_RESET.
+
+Host Adapter event:
+	HAE_RESET
+
+A SAS LLDD should be able to generate
+
+	- at least one event from group C (choice),
+	- events marked M (mandatory) are mandatory (only one),
+	- events marked E (expander) if it wants the SAS layer
+	  to handle domain revalidation (only one such).
+	- Unmarked events are optional.
+
+Meaning:
+
+HAE_RESET
+    - when your HA got internal error and was reset.
+
+PORTE_BYTES_DMAED
+    - on receiving an IDENTIFY/FIS frame
+
+PORTE_BROADCAST_RCVD
+    - on receiving a primitive
+
+PORTE_LINK_RESET_ERR
+    - timer expired, loss of signal, loss of DWS, etc. [1]_
+
+PORTE_TIMER_EVENT
+    - DWS reset timeout timer expired [1]_
+
+PORTE_HARD_RESET
+    - Hard Reset primitive received.
+
+PHYE_LOSS_OF_SIGNAL
+    - the device is gone [1]_
+
+PHYE_OOB_DONE
+    - OOB went fine and oob_mode is valid
+
+PHYE_OOB_ERROR
+    - Error while doing OOB, the device probably
+      got disconnected. [1]_
+
+PHYE_SPINUP_HOLD
+    - SATA is present, COMWAKE not sent.
+
+.. [1] should set/clear the appropriate fields in the phy,
+       or alternatively call the inlined sas_phy_disconnected()
+       which is just a helper, from their tasklet.
+
+The Execute Command SCSI RPC::
+
+	int (*lldd_execute_task)(struct sas_task *, gfp_t gfp_flags);
+
+Used to queue a task to the SAS LLDD.  @task is the task to be executed.
+@gfp_mask is the gfp_mask defining the context of the caller.
+
+This function should implement the Execute Command SCSI RPC,
+
+That is, when lldd_execute_task() is called, the command
+go out on the transport *immediately*.  There is *no*
+queuing of any sort and at any level in a SAS LLDD.
+
+Returns:
+
+   * -SAS_QUEUE_FULL, -ENOMEM, nothing was queued;
+   * 0, the task(s) were queued.
+
+::
+
+    struct sas_task {
+	    dev -- the device this task is destined to
+	    task_proto -- _one_ of enum sas_proto
+	    scatter -- pointer to scatter gather list array
+	    num_scatter -- number of elements in scatter
+	    total_xfer_len -- total number of bytes expected to be transferred
+	    data_dir -- PCI_DMA_...
+	    task_done -- callback when the task has finished execution
+    };
+
+Discovery
+=========
+
+The sysfs tree has the following purposes:
+
+    a) It shows you the physical layout of the SAS domain at
+       the current time, i.e. how the domain looks in the
+       physical world right now.
+    b) Shows some device parameters _at_discovery_time_.
+
+This is a link to the tree(1) program, very useful in
+viewing the SAS domain:
+ftp://mama.indstate.edu/linux/tree/
+
+I expect user space applications to actually create a
+graphical interface of this.
+
+That is, the sysfs domain tree doesn't show or keep state if
+you e.g., change the meaning of the READY LED MEANING
+setting, but it does show you the current connection status
+of the domain device.
+
+Keeping internal device state changes is responsibility of
+upper layers (Command set drivers) and user space.
+
+When a device or devices are unplugged from the domain, this
+is reflected in the sysfs tree immediately, and the device(s)
+removed from the system.
+
+The structure domain_device describes any device in the SAS
+domain.  It is completely managed by the SAS layer.  A task
+points to a domain device, this is how the SAS LLDD knows
+where to send the task(s) to.  A SAS LLDD only reads the
+contents of the domain_device structure, but it never creates
+or destroys one.
+
+Expander management from User Space
+===================================
+
+In each expander directory in sysfs, there is a file called
+"smp_portal".  It is a binary sysfs attribute file, which
+implements an SMP portal (Note: this is *NOT* an SMP port),
+to which user space applications can send SMP requests and
+receive SMP responses.
+
+Functionality is deceptively simple:
+
+1. Build the SMP frame you want to send. The format and layout
+   is described in the SAS spec.  Leave the CRC field equal 0.
+
+open(2)
+
+2. Open the expander's SMP portal sysfs file in RW mode.
+
+write(2)
+
+3. Write the frame you built in 1.
+
+read(2)
+
+4. Read the amount of data you expect to receive for the frame you built.
+   If you receive different amount of data you expected to receive,
+   then there was some kind of error.
+
+close(2)
+
+All this process is shown in detail in the function do_smp_func()
+and its callers, in the file "expander_conf.c".
+
+The kernel functionality is implemented in the file
+"sas_expander.c".
+
+The program "expander_conf.c" implements this. It takes one
+argument, the sysfs file name of the SMP portal to the
+expander, and gives expander information, including routing
+tables.
+
+The SMP portal gives you complete control of the expander,
+so please be careful.