4.2 Merge window part 1

Some of the more interesting user-visible changes include:

• The core file-name lookup code has been reworked to avoid using recursion. The visible effects are that stack pressure will be reduced (making things more robust when complex storage systems are in use) and the limit on the depth of nested symbolic links has been lifted.

• The performance-monitoring subsystem and associated perf tool have seen another long list of improvements; see this commit message for details.

• The jitter entropy random number generator has been merged.

• The KVM hypervisor has gained support for multiple address spaces and for system management mode, both of which are used to support secure boot in guests.

• New hardware support includes:
  • Processors and systems: ARM Ltd. Version 3 memory-management units.

  • GPIO: NXP LPC18XX/43XX GPIO controllers, Netlogic XLP GPIO controllers, Broadco BRCMSTB GPIO controllers, and Axis ETRAX FS GPIO controllers.

  • Input: Logitech M560 wireless mice and Sony motion controllers.

  • Miscellaneous: NXP LPC32xx/18xx/43xx timers, Marvell cryptographic engines, MediaTek SD/MMC card interfaces, Microchip TC74 single-input temperature sensor chips, Broadcom iProc PCIe BCMA buses, AppliedMicro X-Gene v1 PCIe MSI controllers, Cisco PCI-Express SCSI HBAs, Mikrotik RB4XX SPI controllers, Xilinx ZynqMP GQSPI controllers, Dialog Semiconductor DA9062 regulators, Qualcomm SPMI regulators, PowerNV flash controllers, STMicro LPC-based watchdogs and realtime clocks, and Broadcom STB NAND controllers.

  • Power supply: TI BQ24257 and BQ25890 battery chargers, X-Power AXP288 PMIC integrated chargers, and Richtek RT9455 battery chargers.

Changes visible to kernel developers include:

• The prototypes for the follow_link() and put_link() methods in struct inode_operations have changed to:

      const char *(*follow_link) (struct dentry *, void **);
      void (*put_link) (struct dentry *, void *);
  

  In-tree filesystems have been changed accordingly.

• The bulk of the read-copy-update (RCU) configuration options have been hidden behind an "expert" option; for the most part, RCU configuration is completely automatic now.

• Queued spinlocks are now used on the x86 architecture; they improve performance but should not bring any other visible changes. A queued reader/writer lock implementation has also been merged for x86.

• The x86 floating-point unit code has been massively rewritten. Ingo Molnar warns (in the commit message) that "these changes have a substantial regression risk", but none are known at the moment.

• Write-through mappings can now be created with ioremap_wt() or pgprot_writethrough(); this feature is currently supported on the x86 architecture.

The 4.2 merge window should remain open through July 5. If the usual schedule holds — and Linus doesn't take any ill-timed vacations this time — the final 4.2 release can be expected on August 23.