Merge tag 'v3.5-rc4' into regulator-drivers

Linux 3.5-rc4 contains patches which conflict with some of the
development work.

7 files changed, 458 insertions, 21 deletions

diff --git a/Documentation/ABI/testing/sysfs-bus-iio b/Documentation/ABI/testing/sysfs-bus-iio
index 5bc8a47..cfedf63 100644
--- a/Documentation/ABI/testing/sysfs-bus-iio
+++ b/Documentation/ABI/testing/sysfs-bus-iio
@@ -219,6 +219,7 @@ What:		/sys/bus/iio/devices/iio:deviceX/in_voltageY_scale
 What:		/sys/bus/iio/devices/iio:deviceX/in_voltageY_supply_scale
 What:		/sys/bus/iio/devices/iio:deviceX/in_voltage_scale
 What:		/sys/bus/iio/devices/iio:deviceX/out_voltageY_scale
+What:		/sys/bus/iio/devices/iio:deviceX/out_altvoltageY_scale
 What:		/sys/bus/iio/devices/iio:deviceX/in_accel_scale
 What:		/sys/bus/iio/devices/iio:deviceX/in_accel_peak_scale
 What:		/sys/bus/iio/devices/iio:deviceX/in_anglvel_scale
@@ -273,6 +274,7 @@ What:		/sys/bus/iio/devices/iio:deviceX/in_accel_scale_available
 What:		/sys/.../iio:deviceX/in_voltageX_scale_available
 What:		/sys/.../iio:deviceX/in_voltage-voltage_scale_available
 What:		/sys/.../iio:deviceX/out_voltageX_scale_available
+What:		/sys/.../iio:deviceX/out_altvoltageX_scale_available
 What:		/sys/.../iio:deviceX/in_capacitance_scale_available
 KernelVersion:	2.635
 Contact:	linux-iio@vger.kernel.org
@@ -298,14 +300,19 @@ Description:
 		gives the 3dB frequency of the filter in Hz.
 
 What:		/sys/bus/iio/devices/iio:deviceX/out_voltageY_raw
+What:		/sys/bus/iio/devices/iio:deviceX/out_altvoltageY_raw
 KernelVersion:	2.6.37
 Contact:	linux-iio@vger.kernel.org
 Description:
 		Raw (unscaled, no bias etc.) output voltage for
 		channel Y.  The number must always be specified and
 		unique if the output corresponds to a single channel.
+		While DAC like devices typically use out_voltage,
+		a continuous frequency generating device, such as
+		a DDS or PLL should use out_altvoltage.
 
 What:		/sys/bus/iio/devices/iio:deviceX/out_voltageY&Z_raw
+What:		/sys/bus/iio/devices/iio:deviceX/out_altvoltageY&Z_raw
 KernelVersion:	2.6.37
 Contact:	linux-iio@vger.kernel.org
 Description:
@@ -316,6 +323,8 @@ Description:
 
 What:		/sys/bus/iio/devices/iio:deviceX/out_voltageY_powerdown_mode
 What:		/sys/bus/iio/devices/iio:deviceX/out_voltage_powerdown_mode
+What:		/sys/bus/iio/devices/iio:deviceX/out_altvoltageY_powerdown_mode
+What:		/sys/bus/iio/devices/iio:deviceX/out_altvoltage_powerdown_mode
 KernelVersion:	2.6.38
 Contact:	linux-iio@vger.kernel.org
 Description:
@@ -330,6 +339,8 @@ Description:
 
 What:		/sys/.../iio:deviceX/out_votlageY_powerdown_mode_available
 What:		/sys/.../iio:deviceX/out_voltage_powerdown_mode_available
+What:		/sys/.../iio:deviceX/out_altvotlageY_powerdown_mode_available
+What:		/sys/.../iio:deviceX/out_altvoltage_powerdown_mode_available
 KernelVersion:	2.6.38
 Contact:	linux-iio@vger.kernel.org
 Description:
@@ -338,6 +349,8 @@ Description:
 
 What:		/sys/bus/iio/devices/iio:deviceX/out_voltageY_powerdown
 What:		/sys/bus/iio/devices/iio:deviceX/out_voltage_powerdown
+What:		/sys/bus/iio/devices/iio:deviceX/out_altvoltageY_powerdown
+What:		/sys/bus/iio/devices/iio:deviceX/out_altvoltage_powerdown
 KernelVersion:	2.6.38
 Contact:	linux-iio@vger.kernel.org
 Description:
@@ -346,6 +359,24 @@ Description:
 		normal operation. Y may be suppressed if all outputs are
 		controlled together.
 
+What:		/sys/bus/iio/devices/iio:deviceX/out_altvoltageY_frequency
+KernelVersion:	3.4.0
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Output frequency for channel Y in Hz. The number must always be
+		specified and unique if the output corresponds to a single
+		channel.
+
+What:		/sys/bus/iio/devices/iio:deviceX/out_altvoltageY_phase
+KernelVersion:	3.4.0
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Phase in radians of one frequency/clock output Y
+		(out_altvoltageY) relative to another frequency/clock output
+		(out_altvoltageZ) of the device X. The number must always be
+		specified and unique if the output corresponds to a single
+		channel.
+
 What:		/sys/bus/iio/devices/iio:deviceX/events
 KernelVersion:	2.6.35
 Contact:	linux-iio@vger.kernel.org
diff --git a/Documentation/arm/SPEAr/overview.txt b/Documentation/arm/SPEAr/overview.txt
index 57aae77..65610bf 100644
--- a/Documentation/arm/SPEAr/overview.txt
+++ b/Documentation/arm/SPEAr/overview.txt
@@ -60,4 +60,4 @@ Introduction
   Document Author
   ---------------
 
-  Viresh Kumar <viresh.kumar@st.com>, (c) 2010-2012 ST Microelectronics
+  Viresh Kumar <viresh.linux@gmail.com>, (c) 2010-2012 ST Microelectronics
diff --git a/Documentation/devicetree/bindings/i2c/i2c-mux-pinctrl.txt b/Documentation/devicetree/bindings/i2c/i2c-mux-pinctrl.txt
new file mode 100644
index 0000000..ae8af16
--- /dev/null
+++ b/Documentation/devicetree/bindings/i2c/i2c-mux-pinctrl.txt
@@ -0,0 +1,93 @@
+Pinctrl-based I2C Bus Mux
+
+This binding describes an I2C bus multiplexer that uses pin multiplexing to
+route the I2C signals, and represents the pin multiplexing configuration
+using the pinctrl device tree bindings.
+
+                                 +-----+  +-----+
+                                 | dev |  | dev |
+    +------------------------+   +-----+  +-----+
+    | SoC                    |      |        |
+    |                   /----|------+--------+
+    |   +---+   +------+     | child bus A, on first set of pins
+    |   |I2C|---|Pinmux|     |
+    |   +---+   +------+     | child bus B, on second set of pins
+    |                   \----|------+--------+--------+
+    |                        |      |        |        |
+    +------------------------+  +-----+  +-----+  +-----+
+                                | dev |  | dev |  | dev |
+                                +-----+  +-----+  +-----+
+
+Required properties:
+- compatible: i2c-mux-pinctrl
+- i2c-parent: The phandle of the I2C bus that this multiplexer's master-side
+  port is connected to.
+
+Also required are:
+
+* Standard pinctrl properties that specify the pin mux state for each child
+  bus. See ../pinctrl/pinctrl-bindings.txt.
+
+* Standard I2C mux properties. See mux.txt in this directory.
+
+* I2C child bus nodes. See mux.txt in this directory.
+
+For each named state defined in the pinctrl-names property, an I2C child bus
+will be created. I2C child bus numbers are assigned based on the index into
+the pinctrl-names property.
+
+The only exception is that no bus will be created for a state named "idle". If
+such a state is defined, it must be the last entry in pinctrl-names. For
+example:
+
+	pinctrl-names = "ddc", "pta", "idle"  ->  ddc = bus 0, pta = bus 1
+	pinctrl-names = "ddc", "idle", "pta"  ->  Invalid ("idle" not last)
+	pinctrl-names = "idle", "ddc", "pta"  ->  Invalid ("idle" not last)
+
+Whenever an access is made to a device on a child bus, the relevant pinctrl
+state will be programmed into hardware.
+
+If an idle state is defined, whenever an access is not being made to a device
+on a child bus, the idle pinctrl state will be programmed into hardware.
+
+If an idle state is not defined, the most recently used pinctrl state will be
+left programmed into hardware whenever no access is being made of a device on
+a child bus.
+
+Example:
+
+	i2cmux {
+		compatible = "i2c-mux-pinctrl";
+		#address-cells = <1>;
+		#size-cells = <0>;
+
+		i2c-parent = <&i2c1>;
+
+		pinctrl-names = "ddc", "pta", "idle";
+		pinctrl-0 = <&state_i2cmux_ddc>;
+		pinctrl-1 = <&state_i2cmux_pta>;
+		pinctrl-2 = <&state_i2cmux_idle>;
+
+		i2c@0 {
+			reg = <0>;
+			#address-cells = <1>;
+			#size-cells = <0>;
+
+			eeprom {
+				compatible = "eeprom";
+				reg = <0x50>;
+			};
+		};
+
+		i2c@1 {
+			reg = <1>;
+			#address-cells = <1>;
+			#size-cells = <0>;
+
+			eeprom {
+				compatible = "eeprom";
+				reg = <0x50>;
+			};
+		};
+	};
+
diff --git a/Documentation/hwmon/coretemp b/Documentation/hwmon/coretemp
index 84d46c0..c86b50c 100644
--- a/Documentation/hwmon/coretemp
+++ b/Documentation/hwmon/coretemp
@@ -6,7 +6,9 @@ Supported chips:
     Prefix: 'coretemp'
     CPUID: family 0x6, models 0xe (Pentium M DC), 0xf (Core 2 DC 65nm),
                               0x16 (Core 2 SC 65nm), 0x17 (Penryn 45nm),
-                              0x1a (Nehalem), 0x1c (Atom), 0x1e (Lynnfield)
+                              0x1a (Nehalem), 0x1c (Atom), 0x1e (Lynnfield),
+                              0x26 (Tunnel Creek Atom), 0x27 (Medfield Atom),
+                              0x36 (Cedar Trail Atom)
     Datasheet: Intel 64 and IA-32 Architectures Software Developer's Manual
                Volume 3A: System Programming Guide
                http://softwarecommunity.intel.com/Wiki/Mobility/720.htm
@@ -52,6 +54,17 @@ Some information comes from ark.intel.com
 
 Process		Processor					TjMax(C)
 
+22nm		Core i5/i7 Processors
+		i7 3920XM, 3820QM, 3720QM, 3667U, 3520M		105
+		i5 3427U, 3360M/3320M				105
+		i7 3770/3770K					105
+		i5 3570/3570K, 3550, 3470/3450			105
+		i7 3770S					103
+		i5 3570S/3550S, 3475S/3470S/3450S		103
+		i7 3770T					94
+		i5 3570T					94
+		i5 3470T					91
+
 32nm		Core i3/i5/i7 Processors
 		i7 660UM/640/620, 640LM/620, 620M, 610E		105
 		i5 540UM/520/430, 540M/520/450/430		105
@@ -65,6 +78,11 @@ Process		Processor					TjMax(C)
 		U3400						105
 		P4505/P4500 					90
 
+32nm		Atom Processors
+		Z2460						90
+		D2700/2550/2500					100
+		N2850/2800/2650/2600				100
+
 45nm		Xeon Processors 5400 Quad-Core
 		X5492, X5482, X5472, X5470, X5460, X5450	85
 		E5472, E5462, E5450/40/30/20/10/05		85
@@ -85,6 +103,8 @@ Process		Processor					TjMax(C)
 		N475/470/455/450				100
 		N280/270					90
 		330/230						125
+		E680/660/640/620				90
+		E680T/660T/640T/620T				110
 
 45nm		Core2 Processors
 		Solo ULV SU3500/3300				100
diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index c45513d..a92c5eb 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2543,6 +2543,15 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 
 	sched_debug	[KNL] Enables verbose scheduler debug messages.
 
+	skew_tick=	[KNL] Offset the periodic timer tick per cpu to mitigate
+			xtime_lock contention on larger systems, and/or RCU lock
+			contention on all systems with CONFIG_MAXSMP set.
+			Format: { "0" | "1" }
+			0 -- disable. (may be 1 via CONFIG_CMDLINE="skew_tick=1"
+			1 -- enable.
+			Note: increases power consumption, thus should only be
+			enabled if running jitter sensitive (HPC/RT) workloads.
+
 	security=	[SECURITY] Choose a security module to enable at boot.
 			If this boot parameter is not specified, only the first
 			security module asking for security registration will be
diff --git a/Documentation/networking/stmmac.txt b/Documentation/networking/stmmac.txt
index ab1e8d7..5cb9a19 100644
--- a/Documentation/networking/stmmac.txt
+++ b/Documentation/networking/stmmac.txt
@@ -10,8 +10,8 @@ Currently this network device driver is for all STM embedded MAC/GMAC
 (i.e. 7xxx/5xxx SoCs), SPEAr (arm), Loongson1B (mips) and XLINX XC2V3000
 FF1152AMT0221 D1215994A VIRTEX FPGA board.
 
-DWC Ether MAC 10/100/1000 Universal version 3.60a (and older) and DWC Ether MAC 10/100
-Universal version 4.0 have been used for developing this driver.
+DWC Ether MAC 10/100/1000 Universal version 3.60a (and older) and DWC Ether
+MAC 10/100 Universal version 4.0 have been used for developing this driver.
 
 This driver supports both the platform bus and PCI.
 
@@ -54,27 +54,27 @@ net_device structure enabling the scatter/gather feature.
 When one or more packets are received, an interrupt happens. The interrupts
 are not queued so the driver has to scan all the descriptors in the ring during
 the receive process.
-This is based on NAPI so the interrupt handler signals only if there is work to be
-done, and it exits.
+This is based on NAPI so the interrupt handler signals only if there is work
+to be done, and it exits.
 Then the poll method will be scheduled at some future point.
 The incoming packets are stored, by the DMA, in a list of pre-allocated socket
 buffers in order to avoid the memcpy (Zero-copy).
 
 4.3) Timer-Driver Interrupt
-Instead of having the device that asynchronously notifies the frame receptions, the
-driver configures a timer to generate an interrupt at regular intervals.
-Based on the granularity of the timer, the frames that are received by the device
-will experience different levels of latency. Some NICs have dedicated timer
-device to perform this task. STMMAC can use either the RTC device or the TMU
-channel 2  on STLinux platforms.
+Instead of having the device that asynchronously notifies the frame receptions,
+the driver configures a timer to generate an interrupt at regular intervals.
+Based on the granularity of the timer, the frames that are received by the
+device will experience different levels of latency. Some NICs have dedicated
+timer device to perform this task. STMMAC can use either the RTC device or the
+TMU channel 2  on STLinux platforms.
 The timers frequency can be passed to the driver as parameter; when change it,
 take care of both hardware capability and network stability/performance impact.
-Several performance tests on STM platforms showed this optimisation allows to spare
-the CPU while having the maximum throughput.
+Several performance tests on STM platforms showed this optimisation allows to
+spare the CPU while having the maximum throughput.
 
 4.4) WOL
-Wake up on Lan feature through Magic and Unicast frames are supported for the GMAC
-core.
+Wake up on Lan feature through Magic and Unicast frames are supported for the
+GMAC core.
 
 4.5) DMA descriptors
 Driver handles both normal and enhanced descriptors. The latter has been only
@@ -106,7 +106,8 @@ Several driver's information can be passed through the platform
 These are included in the include/linux/stmmac.h header file
 and detailed below as well:
 
- struct plat_stmmacenet_data {
+struct plat_stmmacenet_data {
+	char *phy_bus_name;
 	int bus_id;
 	int phy_addr;
 	int interface;
@@ -124,19 +125,24 @@ and detailed below as well:
 	void (*bus_setup)(void __iomem *ioaddr);
 	int (*init)(struct platform_device *pdev);
 	void (*exit)(struct platform_device *pdev);
+	void *custom_cfg;
+	void *custom_data;
 	void *bsp_priv;
  };
 
 Where:
+ o phy_bus_name: phy bus name to attach to the stmmac.
  o bus_id: bus identifier.
  o phy_addr: the physical address can be passed from the platform.
 	    If it is set to -1 the driver will automatically
 	    detect it at run-time by probing all the 32 addresses.
  o interface: PHY device's interface.
  o mdio_bus_data: specific platform fields for the MDIO bus.
- o pbl: the Programmable Burst Length is maximum number of beats to
+ o dma_cfg: internal DMA parameters
+   o pbl: the Programmable Burst Length is maximum number of beats to
        be transferred in one DMA transaction.
        GMAC also enables the 4xPBL by default.
+   o fixed_burst/mixed_burst/burst_len
  o clk_csr: fixed CSR Clock range selection.
  o has_gmac: uses the GMAC core.
  o enh_desc: if sets the MAC will use the enhanced descriptor structure.
@@ -160,8 +166,9 @@ Where:
 	     this is sometime necessary on some platforms (e.g. ST boxes)
 	     where the HW needs to have set some PIO lines or system cfg
 	     registers.
- o custom_cfg: this is a custom configuration that can be passed while
-	      initialising the resources.
+ o custom_cfg/custom_data: this is a custom configuration that can be passed
+			   while initialising the resources.
+ o bsp_priv: another private poiter.
 
 For MDIO bus The we have:
 
@@ -180,7 +187,6 @@ Where:
  o irqs: list of IRQs, one per PHY.
  o probed_phy_irq: if irqs is NULL, use this for probed PHY.
 
-
 For DMA engine we have the following internal fields that should be
 tuned according to the HW capabilities.
 
diff --git a/Documentation/vm/frontswap.txt b/Documentation/vm/frontswap.txt
new file mode 100644
index 0000000..37067cf
--- /dev/null
+++ b/Documentation/vm/frontswap.txt
@@ -0,0 +1,278 @@
+Frontswap provides a "transcendent memory" interface for swap pages.
+In some environments, dramatic performance savings may be obtained because
+swapped pages are saved in RAM (or a RAM-like device) instead of a swap disk.
+
+(Note, frontswap -- and cleancache (merged at 3.0) -- are the "frontends"
+and the only necessary changes to the core kernel for transcendent memory;
+all other supporting code -- the "backends" -- is implemented as drivers.
+See the LWN.net article "Transcendent memory in a nutshell" for a detailed
+overview of frontswap and related kernel parts:
+https://lwn.net/Articles/454795/ )
+
+Frontswap is so named because it can be thought of as the opposite of
+a "backing" store for a swap device.  The storage is assumed to be
+a synchronous concurrency-safe page-oriented "pseudo-RAM device" conforming
+to the requirements of transcendent memory (such as Xen's "tmem", or
+in-kernel compressed memory, aka "zcache", or future RAM-like devices);
+this pseudo-RAM device is not directly accessible or addressable by the
+kernel and is of unknown and possibly time-varying size.  The driver
+links itself to frontswap by calling frontswap_register_ops to set the
+frontswap_ops funcs appropriately and the functions it provides must
+conform to certain policies as follows:
+
+An "init" prepares the device to receive frontswap pages associated
+with the specified swap device number (aka "type").  A "store" will
+copy the page to transcendent memory and associate it with the type and
+offset associated with the page. A "load" will copy the page, if found,
+from transcendent memory into kernel memory, but will NOT remove the page
+from from transcendent memory.  An "invalidate_page" will remove the page
+from transcendent memory and an "invalidate_area" will remove ALL pages
+associated with the swap type (e.g., like swapoff) and notify the "device"
+to refuse further stores with that swap type.
+
+Once a page is successfully stored, a matching load on the page will normally
+succeed.  So when the kernel finds itself in a situation where it needs
+to swap out a page, it first attempts to use frontswap.  If the store returns
+success, the data has been successfully saved to transcendent memory and
+a disk write and, if the data is later read back, a disk read are avoided.
+If a store returns failure, transcendent memory has rejected the data, and the
+page can be written to swap as usual.
+
+If a backend chooses, frontswap can be configured as a "writethrough
+cache" by calling frontswap_writethrough().  In this mode, the reduction
+in swap device writes is lost (and also a non-trivial performance advantage)
+in order to allow the backend to arbitrarily "reclaim" space used to
+store frontswap pages to more completely manage its memory usage.
+
+Note that if a page is stored and the page already exists in transcendent memory
+(a "duplicate" store), either the store succeeds and the data is overwritten,
+or the store fails AND the page is invalidated.  This ensures stale data may
+never be obtained from frontswap.
+
+If properly configured, monitoring of frontswap is done via debugfs in
+the /sys/kernel/debug/frontswap directory.  The effectiveness of
+frontswap can be measured (across all swap devices) with:
+
+failed_stores	- how many store attempts have failed
+loads		- how many loads were attempted (all should succeed)
+succ_stores	- how many store attempts have succeeded
+invalidates	- how many invalidates were attempted
+
+A backend implementation may provide additional metrics.
+
+FAQ
+
+1) Where's the value?
+
+When a workload starts swapping, performance falls through the floor.
+Frontswap significantly increases performance in many such workloads by
+providing a clean, dynamic interface to read and write swap pages to
+"transcendent memory" that is otherwise not directly addressable to the kernel.
+This interface is ideal when data is transformed to a different form
+and size (such as with compression) or secretly moved (as might be
+useful for write-balancing for some RAM-like devices).  Swap pages (and
+evicted page-cache pages) are a great use for this kind of slower-than-RAM-
+but-much-faster-than-disk "pseudo-RAM device" and the frontswap (and
+cleancache) interface to transcendent memory provides a nice way to read
+and write -- and indirectly "name" -- the pages.
+
+Frontswap -- and cleancache -- with a fairly small impact on the kernel,
+provides a huge amount of flexibility for more dynamic, flexible RAM
+utilization in various system configurations:
+
+In the single kernel case, aka "zcache", pages are compressed and
+stored in local memory, thus increasing the total anonymous pages
+that can be safely kept in RAM.  Zcache essentially trades off CPU
+cycles used in compression/decompression for better memory utilization.
+Benchmarks have shown little or no impact when memory pressure is
+low while providing a significant performance improvement (25%+)
+on some workloads under high memory pressure.
+
+"RAMster" builds on zcache by adding "peer-to-peer" transcendent memory
+support for clustered systems.  Frontswap pages are locally compressed
+as in zcache, but then "remotified" to another system's RAM.  This
+allows RAM to be dynamically load-balanced back-and-forth as needed,
+i.e. when system A is overcommitted, it can swap to system B, and
+vice versa.  RAMster can also be configured as a memory server so
+many servers in a cluster can swap, dynamically as needed, to a single
+server configured with a large amount of RAM... without pre-configuring
+how much of the RAM is available for each of the clients!
+
+In the virtual case, the whole point of virtualization is to statistically
+multiplex physical resources acrosst the varying demands of multiple
+virtual machines.  This is really hard to do with RAM and efforts to do
+it well with no kernel changes have essentially failed (except in some
+well-publicized special-case workloads).
+Specifically, the Xen Transcendent Memory backend allows otherwise
+"fallow" hypervisor-owned RAM to not only be "time-shared" between multiple
+virtual machines, but the pages can be compressed and deduplicated to
+optimize RAM utilization.  And when guest OS's are induced to surrender
+underutilized RAM (e.g. with "selfballooning"), sudden unexpected
+memory pressure may result in swapping; frontswap allows those pages
+to be swapped to and from hypervisor RAM (if overall host system memory
+conditions allow), thus mitigating the potentially awful performance impact
+of unplanned swapping.
+
+A KVM implementation is underway and has been RFC'ed to lkml.  And,
+using frontswap, investigation is also underway on the use of NVM as
+a memory extension technology.
+
+2) Sure there may be performance advantages in some situations, but
+   what's the space/time overhead of frontswap?
+
+If CONFIG_FRONTSWAP is disabled, every frontswap hook compiles into
+nothingness and the only overhead is a few extra bytes per swapon'ed
+swap device.  If CONFIG_FRONTSWAP is enabled but no frontswap "backend"
+registers, there is one extra global variable compared to zero for
+every swap page read or written.  If CONFIG_FRONTSWAP is enabled
+AND a frontswap backend registers AND the backend fails every "store"
+request (i.e. provides no memory despite claiming it might),
+CPU overhead is still negligible -- and since every frontswap fail
+precedes a swap page write-to-disk, the system is highly likely
+to be I/O bound and using a small fraction of a percent of a CPU
+will be irrelevant anyway.
+
+As for space, if CONFIG_FRONTSWAP is enabled AND a frontswap backend
+registers, one bit is allocated for every swap page for every swap
+device that is swapon'd.  This is added to the EIGHT bits (which
+was sixteen until about 2.6.34) that the kernel already allocates
+for every swap page for every swap device that is swapon'd.  (Hugh
+Dickins has observed that frontswap could probably steal one of
+the existing eight bits, but let's worry about that minor optimization
+later.)  For very large swap disks (which are rare) on a standard
+4K pagesize, this is 1MB per 32GB swap.
+
+When swap pages are stored in transcendent memory instead of written
+out to disk, there is a side effect that this may create more memory
+pressure that can potentially outweigh the other advantages.  A
+backend, such as zcache, must implement policies to carefully (but
+dynamically) manage memory limits to ensure this doesn't happen.
+
+3) OK, how about a quick overview of what this frontswap patch does
+   in terms that a kernel hacker can grok?
+
+Let's assume that a frontswap "backend" has registered during
+kernel initialization; this registration indicates that this
+frontswap backend has access to some "memory" that is not directly
+accessible by the kernel.  Exactly how much memory it provides is
+entirely dynamic and random.
+
+Whenever a swap-device is swapon'd frontswap_init() is called,
+passing the swap device number (aka "type") as a parameter.
+This notifies frontswap to expect attempts to "store" swap pages
+associated with that number.
+
+Whenever the swap subsystem is readying a page to write to a swap
+device (c.f swap_writepage()), frontswap_store is called.  Frontswap
+consults with the frontswap backend and if the backend says it does NOT
+have room, frontswap_store returns -1 and the kernel swaps the page
+to the swap device as normal.  Note that the response from the frontswap
+backend is unpredictable to the kernel; it may choose to never accept a
+page, it could accept every ninth page, or it might accept every
+page.  But if the backend does accept a page, the data from the page
+has already been copied and associated with the type and offset,
+and the backend guarantees the persistence of the data.  In this case,
+frontswap sets a bit in the "frontswap_map" for the swap device
+corresponding to the page offset on the swap device to which it would
+otherwise have written the data.
+
+When the swap subsystem needs to swap-in a page (swap_readpage()),
+it first calls frontswap_load() which checks the frontswap_map to
+see if the page was earlier accepted by the frontswap backend.  If
+it was, the page of data is filled from the frontswap backend and
+the swap-in is complete.  If not, the normal swap-in code is
+executed to obtain the page of data from the real swap device.
+
+So every time the frontswap backend accepts a page, a swap device read
+and (potentially) a swap device write are replaced by a "frontswap backend
+store" and (possibly) a "frontswap backend loads", which are presumably much
+faster.
+
+4) Can't frontswap be configured as a "special" swap device that is
+   just higher priority than any real swap device (e.g. like zswap,
+   or maybe swap-over-nbd/NFS)?
+
+No.  First, the existing swap subsystem doesn't allow for any kind of
+swap hierarchy.  Perhaps it could be rewritten to accomodate a hierarchy,
+but this would require fairly drastic changes.  Even if it were
+rewritten, the existing swap subsystem uses the block I/O layer which
+assumes a swap device is fixed size and any page in it is linearly
+addressable.  Frontswap barely touches the existing swap subsystem,
+and works around the constraints of the block I/O subsystem to provide
+a great deal of flexibility and dynamicity.
+
+For example, the acceptance of any swap page by the frontswap backend is
+entirely unpredictable. This is critical to the definition of frontswap
+backends because it grants completely dynamic discretion to the
+backend.  In zcache, one cannot know a priori how compressible a page is.
+"Poorly" compressible pages can be rejected, and "poorly" can itself be
+defined dynamically depending on current memory constraints.
+
+Further, frontswap is entirely synchronous whereas a real swap
+device is, by definition, asynchronous and uses block I/O.  The
+block I/O layer is not only unnecessary, but may perform "optimizations"
+that are inappropriate for a RAM-oriented device including delaying
+the write of some pages for a significant amount of time.  Synchrony is
+required to ensure the dynamicity of the backend and to avoid thorny race
+conditions that would unnecessarily and greatly complicate frontswap
+and/or the block I/O subsystem.  That said, only the initial "store"
+and "load" operations need be synchronous.  A separate asynchronous thread
+is free to manipulate the pages stored by frontswap.  For example,
+the "remotification" thread in RAMster uses standard asynchronous
+kernel sockets to move compressed frontswap pages to a remote machine.
+Similarly, a KVM guest-side implementation could do in-guest compression
+and use "batched" hypercalls.
+
+In a virtualized environment, the dynamicity allows the hypervisor
+(or host OS) to do "intelligent overcommit".  For example, it can
+choose to accept pages only until host-swapping might be imminent,
+then force guests to do their own swapping.
+
+There is a downside to the transcendent memory specifications for
+frontswap:  Since any "store" might fail, there must always be a real
+slot on a real swap device to swap the page.  Thus frontswap must be
+implemented as a "shadow" to every swapon'd device with the potential
+capability of holding every page that the swap device might have held
+and the possibility that it might hold no pages at all.  This means
+that frontswap cannot contain more pages than the total of swapon'd
+swap devices.  For example, if NO swap device is configured on some
+installation, frontswap is useless.  Swapless portable devices
+can still use frontswap but a backend for such devices must configure
+some kind of "ghost" swap device and ensure that it is never used.
+
+5) Why this weird definition about "duplicate stores"?  If a page
+   has been previously successfully stored, can't it always be
+   successfully overwritten?
+
+Nearly always it can, but no, sometimes it cannot.  Consider an example
+where data is compressed and the original 4K page has been compressed
+to 1K.  Now an attempt is made to overwrite the page with data that
+is non-compressible and so would take the entire 4K.  But the backend
+has no more space.  In this case, the store must be rejected.  Whenever
+frontswap rejects a store that would overwrite, it also must invalidate
+the old data and ensure that it is no longer accessible.  Since the
+swap subsystem then writes the new data to the read swap device,
+this is the correct course of action to ensure coherency.
+
+6) What is frontswap_shrink for?
+
+When the (non-frontswap) swap subsystem swaps out a page to a real
+swap device, that page is only taking up low-value pre-allocated disk
+space.  But if frontswap has placed a page in transcendent memory, that
+page may be taking up valuable real estate.  The frontswap_shrink
+routine allows code outside of the swap subsystem to force pages out
+of the memory managed by frontswap and back into kernel-addressable memory.
+For example, in RAMster, a "suction driver" thread will attempt
+to "repatriate" pages sent to a remote machine back to the local machine;
+this is driven using the frontswap_shrink mechanism when memory pressure
+subsides.
+
+7) Why does the frontswap patch create the new include file swapfile.h?
+
+The frontswap code depends on some swap-subsystem-internal data
+structures that have, over the years, moved back and forth between
+static and global.  This seemed a reasonable compromise:  Define
+them as global but declare them in a new include file that isn't
+included by the large number of source files that include swap.h.
+
+Dan Magenheimer, last updated April 9, 2012