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, 819 insertions, 0 deletions

diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
new file mode 100644
index 000000000..e758f4472
--- /dev/null
+++ b/drivers/tty/tty_ldisc.c
@@ -0,0 +1,819 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/types.h>
+#include <linux/errno.h>
+#include <linux/kmod.h>
+#include <linux/sched.h>
+#include <linux/interrupt.h>
+#include <linux/tty.h>
+#include <linux/tty_driver.h>
+#include <linux/file.h>
+#include <linux/mm.h>
+#include <linux/string.h>
+#include <linux/slab.h>
+#include <linux/poll.h>
+#include <linux/proc_fs.h>
+#include <linux/module.h>
+#include <linux/device.h>
+#include <linux/wait.h>
+#include <linux/bitops.h>
+#include <linux/seq_file.h>
+#include <linux/uaccess.h>
+#include <linux/ratelimit.h>
+#include "tty.h"
+
+#undef LDISC_DEBUG_HANGUP
+
+#ifdef LDISC_DEBUG_HANGUP
+#define tty_ldisc_debug(tty, f, args...)	tty_debug(tty, f, ##args)
+#else
+#define tty_ldisc_debug(tty, f, args...)
+#endif
+
+/* lockdep nested classes for tty->ldisc_sem */
+enum {
+	LDISC_SEM_NORMAL,
+	LDISC_SEM_OTHER,
+};
+
+
+/*
+ *	This guards the refcounted line discipline lists. The lock
+ *	must be taken with irqs off because there are hangup path
+ *	callers who will do ldisc lookups and cannot sleep.
+ */
+
+static DEFINE_RAW_SPINLOCK(tty_ldiscs_lock);
+/* Line disc dispatch table */
+static struct tty_ldisc_ops *tty_ldiscs[NR_LDISCS];
+
+/**
+ * tty_register_ldisc	-	install a line discipline
+ * @new_ldisc: pointer to the ldisc object
+ *
+ * Installs a new line discipline into the kernel. The discipline is set up as
+ * unreferenced and then made available to the kernel from this point onwards.
+ *
+ * Locking: takes %tty_ldiscs_lock to guard against ldisc races
+ */
+int tty_register_ldisc(struct tty_ldisc_ops *new_ldisc)
+{
+	unsigned long flags;
+	int ret = 0;
+
+	if (new_ldisc->num < N_TTY || new_ldisc->num >= NR_LDISCS)
+		return -EINVAL;
+
+	raw_spin_lock_irqsave(&tty_ldiscs_lock, flags);
+	tty_ldiscs[new_ldisc->num] = new_ldisc;
+	raw_spin_unlock_irqrestore(&tty_ldiscs_lock, flags);
+
+	return ret;
+}
+EXPORT_SYMBOL(tty_register_ldisc);
+
+/**
+ * tty_unregister_ldisc	-	unload a line discipline
+ * @ldisc: ldisc number
+ *
+ * Remove a line discipline from the kernel providing it is not currently in
+ * use.
+ *
+ * Locking: takes %tty_ldiscs_lock to guard against ldisc races
+ */
+
+void tty_unregister_ldisc(struct tty_ldisc_ops *ldisc)
+{
+	unsigned long flags;
+
+	raw_spin_lock_irqsave(&tty_ldiscs_lock, flags);
+	tty_ldiscs[ldisc->num] = NULL;
+	raw_spin_unlock_irqrestore(&tty_ldiscs_lock, flags);
+}
+EXPORT_SYMBOL(tty_unregister_ldisc);
+
+static struct tty_ldisc_ops *get_ldops(int disc)
+{
+	unsigned long flags;
+	struct tty_ldisc_ops *ldops, *ret;
+
+	raw_spin_lock_irqsave(&tty_ldiscs_lock, flags);
+	ret = ERR_PTR(-EINVAL);
+	ldops = tty_ldiscs[disc];
+	if (ldops) {
+		ret = ERR_PTR(-EAGAIN);
+		if (try_module_get(ldops->owner))
+			ret = ldops;
+	}
+	raw_spin_unlock_irqrestore(&tty_ldiscs_lock, flags);
+	return ret;
+}
+
+static void put_ldops(struct tty_ldisc_ops *ldops)
+{
+	unsigned long flags;
+
+	raw_spin_lock_irqsave(&tty_ldiscs_lock, flags);
+	module_put(ldops->owner);
+	raw_spin_unlock_irqrestore(&tty_ldiscs_lock, flags);
+}
+
+int tty_ldisc_autoload = IS_BUILTIN(CONFIG_LDISC_AUTOLOAD);
+
+/**
+ * tty_ldisc_get	-	take a reference to an ldisc
+ * @tty: tty device
+ * @disc: ldisc number
+ *
+ * Takes a reference to a line discipline. Deals with refcounts and module
+ * locking counts. If the discipline is not available, its module loaded, if
+ * possible.
+ *
+ * Returns:
+ * * -%EINVAL if the discipline index is not [%N_TTY .. %NR_LDISCS] or if the
+ *   discipline is not registered
+ * * -%EAGAIN if request_module() failed to load or register the discipline
+ * * -%ENOMEM if allocation failure
+ * * Otherwise, returns a pointer to the discipline and bumps the ref count
+ *
+ * Locking: takes %tty_ldiscs_lock to guard against ldisc races
+ */
+static struct tty_ldisc *tty_ldisc_get(struct tty_struct *tty, int disc)
+{
+	struct tty_ldisc *ld;
+	struct tty_ldisc_ops *ldops;
+
+	if (disc < N_TTY || disc >= NR_LDISCS)
+		return ERR_PTR(-EINVAL);
+
+	/*
+	 * Get the ldisc ops - we may need to request them to be loaded
+	 * dynamically and try again.
+	 */
+	ldops = get_ldops(disc);
+	if (IS_ERR(ldops)) {
+		if (!capable(CAP_SYS_MODULE) && !tty_ldisc_autoload)
+			return ERR_PTR(-EPERM);
+		request_module("tty-ldisc-%d", disc);
+		ldops = get_ldops(disc);
+		if (IS_ERR(ldops))
+			return ERR_CAST(ldops);
+	}
+
+	/*
+	 * There is no way to handle allocation failure of only 16 bytes.
+	 * Let's simplify error handling and save more memory.
+	 */
+	ld = kmalloc(sizeof(struct tty_ldisc), GFP_KERNEL | __GFP_NOFAIL);
+	ld->ops = ldops;
+	ld->tty = tty;
+
+	return ld;
+}
+
+/**
+ * tty_ldisc_put	-	release the ldisc
+ * @ld: lisdsc to release
+ *
+ * Complement of tty_ldisc_get().
+ */
+static void tty_ldisc_put(struct tty_ldisc *ld)
+{
+	if (WARN_ON_ONCE(!ld))
+		return;
+
+	put_ldops(ld->ops);
+	kfree(ld);
+}
+
+static void *tty_ldiscs_seq_start(struct seq_file *m, loff_t *pos)
+{
+	return (*pos < NR_LDISCS) ? pos : NULL;
+}
+
+static void *tty_ldiscs_seq_next(struct seq_file *m, void *v, loff_t *pos)
+{
+	(*pos)++;
+	return (*pos < NR_LDISCS) ? pos : NULL;
+}
+
+static void tty_ldiscs_seq_stop(struct seq_file *m, void *v)
+{
+}
+
+static int tty_ldiscs_seq_show(struct seq_file *m, void *v)
+{
+	int i = *(loff_t *)v;
+	struct tty_ldisc_ops *ldops;
+
+	ldops = get_ldops(i);
+	if (IS_ERR(ldops))
+		return 0;
+	seq_printf(m, "%-10s %2d\n", ldops->name ? ldops->name : "???", i);
+	put_ldops(ldops);
+	return 0;
+}
+
+const struct seq_operations tty_ldiscs_seq_ops = {
+	.start	= tty_ldiscs_seq_start,
+	.next	= tty_ldiscs_seq_next,
+	.stop	= tty_ldiscs_seq_stop,
+	.show	= tty_ldiscs_seq_show,
+};
+
+/**
+ * tty_ldisc_ref_wait	-	wait for the tty ldisc
+ * @tty: tty device
+ *
+ * Dereference the line discipline for the terminal and take a reference to it.
+ * If the line discipline is in flux then wait patiently until it changes.
+ *
+ * Returns: %NULL if the tty has been hungup and not re-opened with a new file
+ * descriptor, otherwise valid ldisc reference
+ *
+ * Note 1: Must not be called from an IRQ/timer context. The caller must also
+ * be careful not to hold other locks that will deadlock against a discipline
+ * change, such as an existing ldisc reference (which we check for).
+ *
+ * Note 2: a file_operations routine (read/poll/write) should use this function
+ * to wait for any ldisc lifetime events to finish.
+ */
+struct tty_ldisc *tty_ldisc_ref_wait(struct tty_struct *tty)
+{
+	struct tty_ldisc *ld;
+
+	ldsem_down_read(&tty->ldisc_sem, MAX_SCHEDULE_TIMEOUT);
+	ld = tty->ldisc;
+	if (!ld)
+		ldsem_up_read(&tty->ldisc_sem);
+	return ld;
+}
+EXPORT_SYMBOL_GPL(tty_ldisc_ref_wait);
+
+/**
+ * tty_ldisc_ref	-	get the tty ldisc
+ * @tty: tty device
+ *
+ * Dereference the line discipline for the terminal and take a reference to it.
+ * If the line discipline is in flux then return %NULL. Can be called from IRQ
+ * and timer functions.
+ */
+struct tty_ldisc *tty_ldisc_ref(struct tty_struct *tty)
+{
+	struct tty_ldisc *ld = NULL;
+
+	if (ldsem_down_read_trylock(&tty->ldisc_sem)) {
+		ld = tty->ldisc;
+		if (!ld)
+			ldsem_up_read(&tty->ldisc_sem);
+	}
+	return ld;
+}
+EXPORT_SYMBOL_GPL(tty_ldisc_ref);
+
+/**
+ * tty_ldisc_deref	-	free a tty ldisc reference
+ * @ld: reference to free up
+ *
+ * Undoes the effect of tty_ldisc_ref() or tty_ldisc_ref_wait(). May be called
+ * in IRQ context.
+ */
+void tty_ldisc_deref(struct tty_ldisc *ld)
+{
+	ldsem_up_read(&ld->tty->ldisc_sem);
+}
+EXPORT_SYMBOL_GPL(tty_ldisc_deref);
+
+
+static inline int
+__tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout)
+{
+	return ldsem_down_write(&tty->ldisc_sem, timeout);
+}
+
+static inline int
+__tty_ldisc_lock_nested(struct tty_struct *tty, unsigned long timeout)
+{
+	return ldsem_down_write_nested(&tty->ldisc_sem,
+				       LDISC_SEM_OTHER, timeout);
+}
+
+static inline void __tty_ldisc_unlock(struct tty_struct *tty)
+{
+	ldsem_up_write(&tty->ldisc_sem);
+}
+
+int tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout)
+{
+	int ret;
+
+	/* Kindly asking blocked readers to release the read side */
+	set_bit(TTY_LDISC_CHANGING, &tty->flags);
+	wake_up_interruptible_all(&tty->read_wait);
+	wake_up_interruptible_all(&tty->write_wait);
+
+	ret = __tty_ldisc_lock(tty, timeout);
+	if (!ret)
+		return -EBUSY;
+	set_bit(TTY_LDISC_HALTED, &tty->flags);
+	return 0;
+}
+
+void tty_ldisc_unlock(struct tty_struct *tty)
+{
+	clear_bit(TTY_LDISC_HALTED, &tty->flags);
+	/* Can be cleared here - ldisc_unlock will wake up writers firstly */
+	clear_bit(TTY_LDISC_CHANGING, &tty->flags);
+	__tty_ldisc_unlock(tty);
+}
+
+static int
+tty_ldisc_lock_pair_timeout(struct tty_struct *tty, struct tty_struct *tty2,
+			    unsigned long timeout)
+{
+	int ret;
+
+	if (tty < tty2) {
+		ret = __tty_ldisc_lock(tty, timeout);
+		if (ret) {
+			ret = __tty_ldisc_lock_nested(tty2, timeout);
+			if (!ret)
+				__tty_ldisc_unlock(tty);
+		}
+	} else {
+		/* if this is possible, it has lots of implications */
+		WARN_ON_ONCE(tty == tty2);
+		if (tty2 && tty != tty2) {
+			ret = __tty_ldisc_lock(tty2, timeout);
+			if (ret) {
+				ret = __tty_ldisc_lock_nested(tty, timeout);
+				if (!ret)
+					__tty_ldisc_unlock(tty2);
+			}
+		} else
+			ret = __tty_ldisc_lock(tty, timeout);
+	}
+
+	if (!ret)
+		return -EBUSY;
+
+	set_bit(TTY_LDISC_HALTED, &tty->flags);
+	if (tty2)
+		set_bit(TTY_LDISC_HALTED, &tty2->flags);
+	return 0;
+}
+
+static void tty_ldisc_lock_pair(struct tty_struct *tty, struct tty_struct *tty2)
+{
+	tty_ldisc_lock_pair_timeout(tty, tty2, MAX_SCHEDULE_TIMEOUT);
+}
+
+static void tty_ldisc_unlock_pair(struct tty_struct *tty,
+				  struct tty_struct *tty2)
+{
+	__tty_ldisc_unlock(tty);
+	if (tty2)
+		__tty_ldisc_unlock(tty2);
+}
+
+/**
+ * tty_ldisc_flush		-	flush line discipline queue
+ * @tty: tty to flush ldisc for
+ *
+ * Flush the line discipline queue (if any) and the tty flip buffers for this
+ * @tty.
+ */
+void tty_ldisc_flush(struct tty_struct *tty)
+{
+	struct tty_ldisc *ld = tty_ldisc_ref(tty);
+
+	tty_buffer_flush(tty, ld);
+	if (ld)
+		tty_ldisc_deref(ld);
+}
+EXPORT_SYMBOL_GPL(tty_ldisc_flush);
+
+/**
+ * tty_set_termios_ldisc	-	set ldisc field
+ * @tty: tty structure
+ * @disc: line discipline number
+ *
+ * This is probably overkill for real world processors but they are not on hot
+ * paths so a little discipline won't do any harm.
+ *
+ * The line discipline-related tty_struct fields are reset to prevent the ldisc
+ * driver from re-using stale information for the new ldisc instance.
+ *
+ * Locking: takes termios_rwsem
+ */
+static void tty_set_termios_ldisc(struct tty_struct *tty, int disc)
+{
+	down_write(&tty->termios_rwsem);
+	tty->termios.c_line = disc;
+	up_write(&tty->termios_rwsem);
+
+	tty->disc_data = NULL;
+	tty->receive_room = 0;
+}
+
+/**
+ * tty_ldisc_open		-	open a line discipline
+ * @tty: tty we are opening the ldisc on
+ * @ld: discipline to open
+ *
+ * A helper opening method. Also a convenient debugging and check point.
+ *
+ * Locking: always called with BTM already held.
+ */
+static int tty_ldisc_open(struct tty_struct *tty, struct tty_ldisc *ld)
+{
+	WARN_ON(test_and_set_bit(TTY_LDISC_OPEN, &tty->flags));
+	if (ld->ops->open) {
+		int ret;
+		/* BTM here locks versus a hangup event */
+		ret = ld->ops->open(tty);
+		if (ret)
+			clear_bit(TTY_LDISC_OPEN, &tty->flags);
+
+		tty_ldisc_debug(tty, "%p: opened\n", ld);
+		return ret;
+	}
+	return 0;
+}
+
+/**
+ * tty_ldisc_close		-	close a line discipline
+ * @tty: tty we are opening the ldisc on
+ * @ld: discipline to close
+ *
+ * A helper close method. Also a convenient debugging and check point.
+ */
+static void tty_ldisc_close(struct tty_struct *tty, struct tty_ldisc *ld)
+{
+	lockdep_assert_held_write(&tty->ldisc_sem);
+	WARN_ON(!test_bit(TTY_LDISC_OPEN, &tty->flags));
+	clear_bit(TTY_LDISC_OPEN, &tty->flags);
+	if (ld->ops->close)
+		ld->ops->close(tty);
+	tty_ldisc_debug(tty, "%p: closed\n", ld);
+}
+
+/**
+ * tty_ldisc_failto	-	helper for ldisc failback
+ * @tty: tty to open the ldisc on
+ * @ld: ldisc we are trying to fail back to
+ *
+ * Helper to try and recover a tty when switching back to the old ldisc fails
+ * and we need something attached.
+ */
+static int tty_ldisc_failto(struct tty_struct *tty, int ld)
+{
+	struct tty_ldisc *disc = tty_ldisc_get(tty, ld);
+	int r;
+
+	lockdep_assert_held_write(&tty->ldisc_sem);
+	if (IS_ERR(disc))
+		return PTR_ERR(disc);
+	tty->ldisc = disc;
+	tty_set_termios_ldisc(tty, ld);
+	r = tty_ldisc_open(tty, disc);
+	if (r < 0)
+		tty_ldisc_put(disc);
+	return r;
+}
+
+/**
+ * tty_ldisc_restore	-	helper for tty ldisc change
+ * @tty: tty to recover
+ * @old: previous ldisc
+ *
+ * Restore the previous line discipline or %N_TTY when a line discipline change
+ * fails due to an open error
+ */
+static void tty_ldisc_restore(struct tty_struct *tty, struct tty_ldisc *old)
+{
+	/* There is an outstanding reference here so this is safe */
+	if (tty_ldisc_failto(tty, old->ops->num) < 0) {
+		const char *name = tty_name(tty);
+
+		pr_warn("Falling back ldisc for %s.\n", name);
+		/*
+		 * The traditional behaviour is to fall back to N_TTY, we
+		 * want to avoid falling back to N_NULL unless we have no
+		 * choice to avoid the risk of breaking anything
+		 */
+		if (tty_ldisc_failto(tty, N_TTY) < 0 &&
+		    tty_ldisc_failto(tty, N_NULL) < 0)
+			panic("Couldn't open N_NULL ldisc for %s.", name);
+	}
+}
+
+/**
+ * tty_set_ldisc		-	set line discipline
+ * @tty: the terminal to set
+ * @disc: the line discipline number
+ *
+ * Set the discipline of a tty line. Must be called from a process context. The
+ * ldisc change logic has to protect itself against any overlapping ldisc
+ * change (including on the other end of pty pairs), the close of one side of a
+ * tty/pty pair, and eventually hangup.
+ */
+int tty_set_ldisc(struct tty_struct *tty, int disc)
+{
+	int retval;
+	struct tty_ldisc *old_ldisc, *new_ldisc;
+
+	new_ldisc = tty_ldisc_get(tty, disc);
+	if (IS_ERR(new_ldisc))
+		return PTR_ERR(new_ldisc);
+
+	tty_lock(tty);
+	retval = tty_ldisc_lock(tty, 5 * HZ);
+	if (retval)
+		goto err;
+
+	if (!tty->ldisc) {
+		retval = -EIO;
+		goto out;
+	}
+
+	/* Check the no-op case */
+	if (tty->ldisc->ops->num == disc)
+		goto out;
+
+	if (test_bit(TTY_HUPPED, &tty->flags)) {
+		/* We were raced by hangup */
+		retval = -EIO;
+		goto out;
+	}
+
+	old_ldisc = tty->ldisc;
+
+	/* Shutdown the old discipline. */
+	tty_ldisc_close(tty, old_ldisc);
+
+	/* Now set up the new line discipline. */
+	tty->ldisc = new_ldisc;
+	tty_set_termios_ldisc(tty, disc);
+
+	retval = tty_ldisc_open(tty, new_ldisc);
+	if (retval < 0) {
+		/* Back to the old one or N_TTY if we can't */
+		tty_ldisc_put(new_ldisc);
+		tty_ldisc_restore(tty, old_ldisc);
+	}
+
+	if (tty->ldisc->ops->num != old_ldisc->ops->num && tty->ops->set_ldisc) {
+		down_read(&tty->termios_rwsem);
+		tty->ops->set_ldisc(tty);
+		up_read(&tty->termios_rwsem);
+	}
+
+	/*
+	 * At this point we hold a reference to the new ldisc and a
+	 * reference to the old ldisc, or we hold two references to
+	 * the old ldisc (if it was restored as part of error cleanup
+	 * above). In either case, releasing a single reference from
+	 * the old ldisc is correct.
+	 */
+	new_ldisc = old_ldisc;
+out:
+	tty_ldisc_unlock(tty);
+
+	/*
+	 * Restart the work queue in case no characters kick it off. Safe if
+	 * already running
+	 */
+	tty_buffer_restart_work(tty->port);
+err:
+	tty_ldisc_put(new_ldisc);	/* drop the extra reference */
+	tty_unlock(tty);
+	return retval;
+}
+EXPORT_SYMBOL_GPL(tty_set_ldisc);
+
+/**
+ * tty_ldisc_kill	-	teardown ldisc
+ * @tty: tty being released
+ *
+ * Perform final close of the ldisc and reset @tty->ldisc
+ */
+static void tty_ldisc_kill(struct tty_struct *tty)
+{
+	lockdep_assert_held_write(&tty->ldisc_sem);
+	if (!tty->ldisc)
+		return;
+	/*
+	 * Now kill off the ldisc
+	 */
+	tty_ldisc_close(tty, tty->ldisc);
+	tty_ldisc_put(tty->ldisc);
+	/* Force an oops if we mess this up */
+	tty->ldisc = NULL;
+}
+
+/**
+ * tty_reset_termios	-	reset terminal state
+ * @tty: tty to reset
+ *
+ * Restore a terminal to the driver default state.
+ */
+static void tty_reset_termios(struct tty_struct *tty)
+{
+	down_write(&tty->termios_rwsem);
+	tty->termios = tty->driver->init_termios;
+	tty->termios.c_ispeed = tty_termios_input_baud_rate(&tty->termios);
+	tty->termios.c_ospeed = tty_termios_baud_rate(&tty->termios);
+	up_write(&tty->termios_rwsem);
+}
+
+
+/**
+ * tty_ldisc_reinit	-	reinitialise the tty ldisc
+ * @tty: tty to reinit
+ * @disc: line discipline to reinitialize
+ *
+ * Completely reinitialize the line discipline state, by closing the current
+ * instance, if there is one, and opening a new instance. If an error occurs
+ * opening the new non-%N_TTY instance, the instance is dropped and @tty->ldisc
+ * reset to %NULL. The caller can then retry with %N_TTY instead.
+ *
+ * Returns: 0 if successful, otherwise error code < 0
+ */
+int tty_ldisc_reinit(struct tty_struct *tty, int disc)
+{
+	struct tty_ldisc *ld;
+	int retval;
+
+	lockdep_assert_held_write(&tty->ldisc_sem);
+	ld = tty_ldisc_get(tty, disc);
+	if (IS_ERR(ld)) {
+		BUG_ON(disc == N_TTY);
+		return PTR_ERR(ld);
+	}
+
+	if (tty->ldisc) {
+		tty_ldisc_close(tty, tty->ldisc);
+		tty_ldisc_put(tty->ldisc);
+	}
+
+	/* switch the line discipline */
+	tty->ldisc = ld;
+	tty_set_termios_ldisc(tty, disc);
+	retval = tty_ldisc_open(tty, tty->ldisc);
+	if (retval) {
+		tty_ldisc_put(tty->ldisc);
+		tty->ldisc = NULL;
+	}
+	return retval;
+}
+
+/**
+ * tty_ldisc_hangup	-	hangup ldisc reset
+ * @tty: tty being hung up
+ * @reinit: whether to re-initialise the tty
+ *
+ * Some tty devices reset their termios when they receive a hangup event. In
+ * that situation we must also switch back to %N_TTY properly before we reset
+ * the termios data.
+ *
+ * Locking: We can take the ldisc mutex as the rest of the code is careful to
+ * allow for this.
+ *
+ * In the pty pair case this occurs in the close() path of the tty itself so we
+ * must be careful about locking rules.
+ */
+void tty_ldisc_hangup(struct tty_struct *tty, bool reinit)
+{
+	struct tty_ldisc *ld;
+
+	tty_ldisc_debug(tty, "%p: hangup\n", tty->ldisc);
+
+	ld = tty_ldisc_ref(tty);
+	if (ld != NULL) {
+		if (ld->ops->flush_buffer)
+			ld->ops->flush_buffer(tty);
+		tty_driver_flush_buffer(tty);
+		if ((test_bit(TTY_DO_WRITE_WAKEUP, &tty->flags)) &&
+		    ld->ops->write_wakeup)
+			ld->ops->write_wakeup(tty);
+		if (ld->ops->hangup)
+			ld->ops->hangup(tty);
+		tty_ldisc_deref(ld);
+	}
+
+	wake_up_interruptible_poll(&tty->write_wait, EPOLLOUT);
+	wake_up_interruptible_poll(&tty->read_wait, EPOLLIN);
+
+	/*
+	 * Shutdown the current line discipline, and reset it to
+	 * N_TTY if need be.
+	 *
+	 * Avoid racing set_ldisc or tty_ldisc_release
+	 */
+	tty_ldisc_lock(tty, MAX_SCHEDULE_TIMEOUT);
+
+	if (tty->driver->flags & TTY_DRIVER_RESET_TERMIOS)
+		tty_reset_termios(tty);
+
+	if (tty->ldisc) {
+		if (reinit) {
+			if (tty_ldisc_reinit(tty, tty->termios.c_line) < 0 &&
+			    tty_ldisc_reinit(tty, N_TTY) < 0)
+				WARN_ON(tty_ldisc_reinit(tty, N_NULL) < 0);
+		} else
+			tty_ldisc_kill(tty);
+	}
+	tty_ldisc_unlock(tty);
+}
+
+/**
+ * tty_ldisc_setup	-	open line discipline
+ * @tty: tty being shut down
+ * @o_tty: pair tty for pty/tty pairs
+ *
+ * Called during the initial open of a tty/pty pair in order to set up the line
+ * disciplines and bind them to the @tty. This has no locking issues as the
+ * device isn't yet active.
+ */
+int tty_ldisc_setup(struct tty_struct *tty, struct tty_struct *o_tty)
+{
+	int retval = tty_ldisc_open(tty, tty->ldisc);
+
+	if (retval)
+		return retval;
+
+	if (o_tty) {
+		/*
+		 * Called without o_tty->ldisc_sem held, as o_tty has been
+		 * just allocated and no one has a reference to it.
+		 */
+		retval = tty_ldisc_open(o_tty, o_tty->ldisc);
+		if (retval) {
+			tty_ldisc_close(tty, tty->ldisc);
+			return retval;
+		}
+	}
+	return 0;
+}
+
+/**
+ * tty_ldisc_release	-	release line discipline
+ * @tty: tty being shut down (or one end of pty pair)
+ *
+ * Called during the final close of a tty or a pty pair in order to shut down
+ * the line discpline layer. On exit, each tty's ldisc is %NULL.
+ */
+void tty_ldisc_release(struct tty_struct *tty)
+{
+	struct tty_struct *o_tty = tty->link;
+
+	/*
+	 * Shutdown this line discipline. As this is the final close,
+	 * it does not race with the set_ldisc code path.
+	 */
+
+	tty_ldisc_lock_pair(tty, o_tty);
+	tty_ldisc_kill(tty);
+	if (o_tty)
+		tty_ldisc_kill(o_tty);
+	tty_ldisc_unlock_pair(tty, o_tty);
+
+	/*
+	 * And the memory resources remaining (buffers, termios) will be
+	 * disposed of when the kref hits zero
+	 */
+
+	tty_ldisc_debug(tty, "released\n");
+}
+
+/**
+ * tty_ldisc_init	-	ldisc setup for new tty
+ * @tty: tty being allocated
+ *
+ * Set up the line discipline objects for a newly allocated tty. Note that the
+ * tty structure is not completely set up when this call is made.
+ */
+int tty_ldisc_init(struct tty_struct *tty)
+{
+	struct tty_ldisc *ld = tty_ldisc_get(tty, N_TTY);
+
+	if (IS_ERR(ld))
+		return PTR_ERR(ld);
+	tty->ldisc = ld;
+	return 0;
+}
+
+/**
+ * tty_ldisc_deinit	-	ldisc cleanup for new tty
+ * @tty: tty that was allocated recently
+ *
+ * The tty structure must not be completely set up (tty_ldisc_setup()) when
+ * this call is made.
+ */
+void tty_ldisc_deinit(struct tty_struct *tty)
+{
+	/* no ldisc_sem, tty is being destroyed */
+	if (tty->ldisc)
+		tty_ldisc_put(tty->ldisc);
+	tty->ldisc = NULL;
+}