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Diffstat (limited to 'Documentation/core-api')
| -rw-r--r-- | Documentation/core-api/flexible-arrays.rst | 130 | ||||
| -rw-r--r-- | Documentation/core-api/genericirq.rst | 440 | ||||
| -rw-r--r-- | Documentation/core-api/index.rst | 3 | ||||
| -rw-r--r-- | Documentation/core-api/kernel-api.rst | 346 |
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diff --git a/Documentation/core-api/flexible-arrays.rst b/Documentation/core-api/flexible-arrays.rst new file mode 100644 index 000000000000..b6b85a1b518e --- /dev/null +++ b/Documentation/core-api/flexible-arrays.rst @@ -0,0 +1,130 @@ + +=================================== +Using flexible arrays in the kernel +=================================== + +Large contiguous memory allocations can be unreliable in the Linux kernel. +Kernel programmers will sometimes respond to this problem by allocating +pages with :c:func:`vmalloc()`. This solution not ideal, though. On 32-bit +systems, memory from vmalloc() must be mapped into a relatively small address +space; it's easy to run out. On SMP systems, the page table changes required +by vmalloc() allocations can require expensive cross-processor interrupts on +all CPUs. And, on all systems, use of space in the vmalloc() range increases +pressure on the translation lookaside buffer (TLB), reducing the performance +of the system. + +In many cases, the need for memory from vmalloc() can be eliminated by piecing +together an array from smaller parts; the flexible array library exists to make +this task easier. + +A flexible array holds an arbitrary (within limits) number of fixed-sized +objects, accessed via an integer index. Sparse arrays are handled +reasonably well. Only single-page allocations are made, so memory +allocation failures should be relatively rare. The down sides are that the +arrays cannot be indexed directly, individual object size cannot exceed the +system page size, and putting data into a flexible array requires a copy +operation. It's also worth noting that flexible arrays do no internal +locking at all; if concurrent access to an array is possible, then the +caller must arrange for appropriate mutual exclusion. + +The creation of a flexible array is done with :c:func:`flex_array_alloc()`:: + + #include <linux/flex_array.h> + + struct flex_array *flex_array_alloc(int element_size, + unsigned int total, + gfp_t flags); + +The individual object size is provided by ``element_size``, while total is the +maximum number of objects which can be stored in the array. The flags +argument is passed directly to the internal memory allocation calls. With +the current code, using flags to ask for high memory is likely to lead to +notably unpleasant side effects. + +It is also possible to define flexible arrays at compile time with:: + + DEFINE_FLEX_ARRAY(name, element_size, total); + +This macro will result in a definition of an array with the given name; the +element size and total will be checked for validity at compile time. + +Storing data into a flexible array is accomplished with a call to +:c:func:`flex_array_put()`:: + + int flex_array_put(struct flex_array *array, unsigned int element_nr, + void *src, gfp_t flags); + +This call will copy the data from src into the array, in the position +indicated by ``element_nr`` (which must be less than the maximum specified when +the array was created). If any memory allocations must be performed, flags +will be used. The return value is zero on success, a negative error code +otherwise. + +There might possibly be a need to store data into a flexible array while +running in some sort of atomic context; in this situation, sleeping in the +memory allocator would be a bad thing. That can be avoided by using +``GFP_ATOMIC`` for the flags value, but, often, there is a better way. The +trick is to ensure that any needed memory allocations are done before +entering atomic context, using :c:func:`flex_array_prealloc()`:: + + int flex_array_prealloc(struct flex_array *array, unsigned int start, + unsigned int nr_elements, gfp_t flags); + +This function will ensure that memory for the elements indexed in the range +defined by ``start`` and ``nr_elements`` has been allocated. Thereafter, a +``flex_array_put()`` call on an element in that range is guaranteed not to +block. + +Getting data back out of the array is done with :c:func:`flex_array_get()`:: + + void *flex_array_get(struct flex_array *fa, unsigned int element_nr); + +The return value is a pointer to the data element, or NULL if that +particular element has never been allocated. + +Note that it is possible to get back a valid pointer for an element which +has never been stored in the array. Memory for array elements is allocated +one page at a time; a single allocation could provide memory for several +adjacent elements. Flexible array elements are normally initialized to the +value ``FLEX_ARRAY_FREE`` (defined as 0x6c in <linux/poison.h>), so errors +involving that number probably result from use of unstored array entries. +Note that, if array elements are allocated with ``__GFP_ZERO``, they will be +initialized to zero and this poisoning will not happen. + +Individual elements in the array can be cleared with +:c:func:`flex_array_clear()`:: + + int flex_array_clear(struct flex_array *array, unsigned int element_nr); + +This function will set the given element to ``FLEX_ARRAY_FREE`` and return +zero. If storage for the indicated element is not allocated for the array, +``flex_array_clear()`` will return ``-EINVAL`` instead. Note that clearing an +element does not release the storage associated with it; to reduce the +allocated size of an array, call :c:func:`flex_array_shrink()`:: + + int flex_array_shrink(struct flex_array *array); + +The return value will be the number of pages of memory actually freed. +This function works by scanning the array for pages containing nothing but +``FLEX_ARRAY_FREE`` bytes, so (1) it can be expensive, and (2) it will not work +if the array's pages are allocated with ``__GFP_ZERO``. + +It is possible to remove all elements of an array with a call to +:c:func:`flex_array_free_parts()`:: + + void flex_array_free_parts(struct flex_array *array); + +This call frees all elements, but leaves the array itself in place. +Freeing the entire array is done with :c:func:`flex_array_free()`:: + + void flex_array_free(struct flex_array *array); + +As of this writing, there are no users of flexible arrays in the mainline +kernel. The functions described here are also not exported to modules; +that will probably be fixed when somebody comes up with a need for it. + + +Flexible array functions +------------------------ + +.. kernel-doc:: include/linux/flex_array.h diff --git a/Documentation/core-api/genericirq.rst b/Documentation/core-api/genericirq.rst new file mode 100644 index 000000000000..0054bd48be84 --- /dev/null +++ b/Documentation/core-api/genericirq.rst @@ -0,0 +1,440 @@ +.. include:: <isonum.txt> + +========================== +Linux generic IRQ handling +========================== + +:Copyright: |copy| 2005-2010: Thomas Gleixner +:Copyright: |copy| 2005-2006: Ingo Molnar + +Introduction +============ + +The generic interrupt handling layer is designed to provide a complete +abstraction of interrupt handling for device drivers. It is able to +handle all the different types of interrupt controller hardware. Device +drivers use generic API functions to request, enable, disable and free +interrupts. The drivers do not have to know anything about interrupt +hardware details, so they can be used on different platforms without +code changes. + +This documentation is provided to developers who want to implement an +interrupt subsystem based for their architecture, with the help of the +generic IRQ handling layer. + +Rationale +========= + +The original implementation of interrupt handling in Linux uses the +:c:func:`__do_IRQ` super-handler, which is able to deal with every type of +interrupt logic. + +Originally, Russell King identified different types of handlers to build +a quite universal set for the ARM interrupt handler implementation in +Linux 2.5/2.6. He distinguished between: + +- Level type + +- Edge type + +- Simple type + +During the implementation we identified another type: + +- Fast EOI type + +In the SMP world of the :c:func:`__do_IRQ` super-handler another type was +identified: + +- Per CPU type + +This split implementation of high-level IRQ handlers allows us to +optimize the flow of the interrupt handling for each specific interrupt +type. This reduces complexity in that particular code path and allows +the optimized handling of a given type. + +The original general IRQ implementation used hw_interrupt_type +structures and their ``->ack``, ``->end`` [etc.] callbacks to differentiate +the flow control in the super-handler. This leads to a mix of flow logic +and low-level hardware logic, and it also leads to unnecessary code +duplication: for example in i386, there is an ``ioapic_level_irq`` and an +``ioapic_edge_irq`` IRQ-type which share many of the low-level details but +have different flow handling. + +A more natural abstraction is the clean separation of the 'irq flow' and +the 'chip details'. + +Analysing a couple of architecture's IRQ subsystem implementations +reveals that most of them can use a generic set of 'irq flow' methods +and only need to add the chip-level specific code. The separation is +also valuable for (sub)architectures which need specific quirks in the +IRQ flow itself but not in the chip details - and thus provides a more +transparent IRQ subsystem design. + +Each interrupt descriptor is assigned its own high-level flow handler, +which is normally one of the generic implementations. (This high-level +flow handler implementation also makes it simple to provide +demultiplexing handlers which can be found in embedded platforms on +various architectures.) + +The separation makes the generic interrupt handling layer more flexible +and extensible. For example, an (sub)architecture can use a generic +IRQ-flow implementation for 'level type' interrupts and add a +(sub)architecture specific 'edge type' implementation. + +To make the transition to the new model easier and prevent the breakage +of existing implementations, the :c:func:`__do_IRQ` super-handler is still +available. This leads to a kind of duality for the time being. Over time +the new model should be used in more and more architectures, as it +enables smaller and cleaner IRQ subsystems. It's deprecated for three +years now and about to be removed. + +Known Bugs And Assumptions +========================== + +None (knock on wood). + +Abstraction layers +================== + +There are three main levels of abstraction in the interrupt code: + +1. High-level driver API + +2. High-level IRQ flow handlers + +3. Chip-level hardware encapsulation + +Interrupt control flow +---------------------- + +Each interrupt is described by an interrupt descriptor structure +irq_desc. The interrupt is referenced by an 'unsigned int' numeric +value which selects the corresponding interrupt description structure in +the descriptor structures array. The descriptor structure contains +status information and pointers to the interrupt flow method and the +interrupt chip structure which are assigned to this interrupt. + +Whenever an interrupt triggers, the low-level architecture code calls +into the generic interrupt code by calling :c:func:`desc->handle_irq`. This +high-level IRQ handling function only uses desc->irq_data.chip +primitives referenced by the assigned chip descriptor structure. + +High-level Driver API +--------------------- + +The high-level Driver API consists of following functions: + +- :c:func:`request_irq` + +- :c:func:`free_irq` + +- :c:func:`disable_irq` + +- :c:func:`enable_irq` + +- :c:func:`disable_irq_nosync` (SMP only) + +- :c:func:`synchronize_irq` (SMP only) + +- :c:func:`irq_set_irq_type` + +- :c:func:`irq_set_irq_wake` + +- :c:func:`irq_set_handler_data` + +- :c:func:`irq_set_chip` + +- :c:func:`irq_set_chip_data` + +See the autogenerated function documentation for details. + +High-level IRQ flow handlers +---------------------------- + +The generic layer provides a set of pre-defined irq-flow methods: + +- :c:func:`handle_level_irq` + +- :c:func:`handle_edge_irq` + +- :c:func:`handle_fasteoi_irq` + +- :c:func:`handle_simple_irq` + +- :c:func:`handle_percpu_irq` + +- :c:func:`handle_edge_eoi_irq` + +- :c:func:`handle_bad_irq` + +The interrupt flow handlers (either pre-defined or architecture +specific) are assigned to specific interrupts by the architecture either +during bootup or during device initialization. + +Default flow implementations +~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Helper functions +^^^^^^^^^^^^^^^^ + +The helper functions call the chip primitives and are used by the +default flow implementations. The following helper functions are +implemented (simplified excerpt):: + + default_enable(struct irq_data *data) + { + desc->irq_data.chip->irq_unmask(data); + } + + default_disable(struct irq_data *data) + { + if (!delay_disable(data)) + desc->irq_data.chip->irq_mask(data); + } + + default_ack(struct irq_data *data) + { + chip->irq_ack(data); + } + + default_mask_ack(struct irq_data *data) + { + if (chip->irq_mask_ack) { + chip->irq_mask_ack(data); + } else { + chip->irq_mask(data); + chip->irq_ack(data); + } + } + + noop(struct irq_data *data)) + { + } + + + +Default flow handler implementations +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Default Level IRQ flow handler +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +handle_level_irq provides a generic implementation for level-triggered +interrupts. + +The following control flow is implemented (simplified excerpt):: + + :c:func:`desc->irq_data.chip->irq_mask_ack`; + handle_irq_event(desc->action); + :c:func:`desc->irq_data.chip->irq_unmask`; + + +Default Fast EOI IRQ flow handler +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +handle_fasteoi_irq provides a generic implementation for interrupts, +which only need an EOI at the end of the handler. + +The following control flow is implemented (simplified excerpt):: + + handle_irq_event(desc->action); + :c:func:`desc->irq_data.chip->irq_eoi`; + + +Default Edge IRQ flow handler +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +handle_edge_irq provides a generic implementation for edge-triggered +interrupts. + +The following control flow is implemented (simplified excerpt):: + + if (desc->status & running) { + :c:func:`desc->irq_data.chip->irq_mask_ack`; + desc->status |= pending | masked; + return; + } + :c:func:`desc->irq_data.chip->irq_ack`; + desc->status |= running; + do { + if (desc->status & masked) + :c:func:`desc->irq_data.chip->irq_unmask`; + desc->status &= ~pending; + handle_irq_event(desc->action); + } while (status & pending); + desc->status &= ~running; + + +Default simple IRQ flow handler +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +handle_simple_irq provides a generic implementation for simple +interrupts. + +.. note:: + + The simple flow handler does not call any handler/chip primitives. + +The following control flow is implemented (simplified excerpt):: + + handle_irq_event(desc->action); + + +Default per CPU flow handler +^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +handle_percpu_irq provides a generic implementation for per CPU +interrupts. + +Per CPU interrupts are only available on SMP and the handler provides a +simplified version without locking. + +The following control flow is implemented (simplified excerpt):: + + if (desc->irq_data.chip->irq_ack) + :c:func:`desc->irq_data.chip->irq_ack`; + handle_irq_event(desc->action); + if (desc->irq_data.chip->irq_eoi) + :c:func:`desc->irq_data.chip->irq_eoi`; + + +EOI Edge IRQ flow handler +^^^^^^^^^^^^^^^^^^^^^^^^^ + +handle_edge_eoi_irq provides an abnomination of the edge handler +which is solely used to tame a badly wreckaged irq controller on +powerpc/cell. + +Bad IRQ flow handler +^^^^^^^^^^^^^^^^^^^^ + +handle_bad_irq is used for spurious interrupts which have no real +handler assigned.. + +Quirks and optimizations +~~~~~~~~~~~~~~~~~~~~~~~~ + +The generic functions are intended for 'clean' architectures and chips, +which have no platform-specific IRQ handling quirks. If an architecture +needs to implement quirks on the 'flow' level then it can do so by +overriding the high-level irq-flow handler. + +Delayed interrupt disable +~~~~~~~~~~~~~~~~~~~~~~~~~ + +This per interrupt selectable feature, which was introduced by Russell +King in the ARM interrupt implementation, does not mask an interrupt at +the hardware level when :c:func:`disable_irq` is called. The interrupt is kept +enabled and is masked in the flow handler when an interrupt event +happens. This prevents losing edge interrupts on hardware which does not +store an edge interrupt event while the interrupt is disabled at the +hardware level. When an interrupt arrives while the IRQ_DISABLED flag +is set, then the interrupt is masked at the hardware level and the +IRQ_PENDING bit is set. When the interrupt is re-enabled by +:c:func:`enable_irq` the pending bit is checked and if it is set, the interrupt +is resent either via hardware or by a software resend mechanism. (It's +necessary to enable CONFIG_HARDIRQS_SW_RESEND when you want to use +the delayed interrupt disable feature and your hardware is not capable +of retriggering an interrupt.) The delayed interrupt disable is not +configurable. + +Chip-level hardware encapsulation +--------------------------------- + +The chip-level hardware descriptor structure :c:type:`irq_chip` contains all +the direct chip relevant functions, which can be utilized by the irq flow +implementations. + +- ``irq_ack`` + +- ``irq_mask_ack`` - Optional, recommended for performance + +- ``irq_mask`` + +- ``irq_unmask`` + +- ``irq_eoi`` - Optional, required for EOI flow handlers + +- ``irq_retrigger`` - Optional + +- ``irq_set_type`` - Optional + +- ``irq_set_wake`` - Optional + +These primitives are strictly intended to mean what they say: ack means +ACK, masking means masking of an IRQ line, etc. It is up to the flow +handler(s) to use these basic units of low-level functionality. + +__do_IRQ entry point +==================== + +The original implementation :c:func:`__do_IRQ` was an alternative entry point +for all types of interrupts. It no longer exists. + +This handler turned out to be not suitable for all interrupt hardware +and was therefore reimplemented with split functionality for +edge/level/simple/percpu interrupts. This is not only a functional +optimization. It also shortens code paths for interrupts. + +Locking on SMP +============== + +The locking of chip registers is up to the architecture that defines the +chip primitives. The per-irq structure is protected via desc->lock, by +the generic layer. + +Generic interrupt chip +====================== + +To avoid copies of identical implementations of IRQ chips the core +provides a configurable generic interrupt chip implementation. +Developers should check carefully whether the generic chip fits their +needs before implementing the same functionality slightly differently +themselves. + +.. kernel-doc:: kernel/irq/generic-chip.c + :export: + +Structures +========== + +This chapter contains the autogenerated documentation of the structures +which are used in the generic IRQ layer. + +.. kernel-doc:: include/linux/irq.h + :internal: + +.. kernel-doc:: include/linux/interrupt.h + :internal: + +Public Functions Provided +========================= + +This chapter contains the autogenerated documentation of the kernel API +functions which are exported. + +.. kernel-doc:: kernel/irq/manage.c + +.. kernel-doc:: kernel/irq/chip.c + +Internal Functions Provided +=========================== + +This chapter contains the autogenerated documentation of the internal +functions. + +.. kernel-doc:: kernel/irq/irqdesc.c + +.. kernel-doc:: kernel/irq/handle.c + +.. kernel-doc:: kernel/irq/chip.c + +Credits +======= + +The following people have contributed to this document: + +1. Thomas Gleixner tglx@linutronix.de + +2. Ingo Molnar mingo@elte.hu diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst index 0d93d8089136..62abd36bfffb 100644 --- a/Documentation/core-api/index.rst +++ b/Documentation/core-api/index.rst @@ -11,11 +11,14 @@ Core utilities .. toctree:: :maxdepth: 1 + kernel-api assoc_array atomic_ops cpu_hotplug local_ops workqueue + genericirq + flexible-arrays Interfaces for kernel debugging =============================== diff --git a/Documentation/core-api/kernel-api.rst b/Documentation/core-api/kernel-api.rst new file mode 100644 index 000000000000..9ec8488319dc --- /dev/null +++ b/Documentation/core-api/kernel-api.rst @@ -0,0 +1,346 @@ +==================== +The Linux Kernel API +==================== + +Data Types +========== + +Doubly Linked Lists +------------------- + +.. kernel-doc:: include/linux/list.h + :internal: + +Basic C Library Functions +========================= + +When writing drivers, you cannot in general use routines which are from +the C Library. Some of the functions have been found generally useful +and they are listed below. The behaviour of these functions may vary +slightly from those defined by ANSI, and these deviations are noted in +the text. + +String Conversions +------------------ + +.. kernel-doc:: lib/vsprintf.c + :export: + +.. kernel-doc:: include/linux/kernel.h + :functions: kstrtol + +.. kernel-doc:: include/linux/kernel.h + :functions: kstrtoul + +.. kernel-doc:: lib/kstrtox.c + :export: + +String Manipulation +------------------- + +.. kernel-doc:: lib/string.c + :export: + +Bit Operations +-------------- + +.. kernel-doc:: arch/x86/include/asm/bitops.h + :internal: + +Basic Kernel Library Functions +============================== + +The Linux kernel provides more basic utility functions. + +Bitmap Operations +----------------- + +.. kernel-doc:: lib/bitmap.c + :export: + +.. kernel-doc:: lib/bitmap.c + :internal: + +Command-line Parsing +-------------------- + +.. kernel-doc:: lib/cmdline.c + :export: + +CRC Functions +------------- + +.. kernel-doc:: lib/crc7.c + :export: + +.. kernel-doc:: lib/crc16.c + :export: + +.. kernel-doc:: lib/crc-itu-t.c + :export: + +.. kernel-doc:: lib/crc32.c + +.. kernel-doc:: lib/crc-ccitt.c + :export: + +idr/ida Functions +----------------- + +.. kernel-doc:: include/linux/idr.h + :doc: idr sync + +.. kernel-doc:: lib/idr.c + :doc: IDA description + +.. kernel-doc:: lib/idr.c + :export: + +Memory Management in Linux +========================== + +The Slab Cache +-------------- + +.. kernel-doc:: include/linux/slab.h + :internal: + +.. kernel-doc:: mm/slab.c + :export: + +.. kernel-doc:: mm/util.c + :export: + +User Space Memory Access +------------------------ + +.. kernel-doc:: arch/x86/include/asm/uaccess_32.h + :internal: + +.. kernel-doc:: arch/x86/lib/usercopy_32.c + :export: + +More Memory Management Functions +-------------------------------- + +.. kernel-doc:: mm/readahead.c + :export: + +.. kernel-doc:: mm/filemap.c + :export: + +.. kernel-doc:: mm/memory.c + :export: + +.. kernel-doc:: mm/vmalloc.c + :export: + +.. kernel-doc:: mm/page_alloc.c + :internal: + +.. kernel-doc:: mm/mempool.c + :export: + +.. kernel-doc:: mm/dmapool.c + :export: + +.. kernel-doc:: mm/page-writeback.c + :export: + +.. kernel-doc:: mm/truncate.c + :export: + +Kernel IPC facilities +===================== + +IPC utilities +------------- + +.. kernel-doc:: ipc/util.c + :internal: + +FIFO Buffer +=========== + +kfifo interface +--------------- + +.. kernel-doc:: include/linux/kfifo.h + :internal: + +relay interface support +======================= + +Relay interface support is designed to provide an efficient mechanism +for tools and facilities to relay large amounts of data from kernel +space to user space. + +relay interface +--------------- + +.. kernel-doc:: kernel/relay.c + :export: + +.. kernel-doc:: kernel/relay.c + :internal: + +Module Support +============== + +Module Loading +-------------- + +.. kernel-doc:: kernel/kmod.c + :export: + +Inter Module support +-------------------- + +Refer to the file kernel/module.c for more information. + +Hardware Interfaces +=================== + +Interrupt Handling +------------------ + +.. kernel-doc:: kernel/irq/manage.c + :export: + +DMA Channels +------------ + +.. kernel-doc:: kernel/dma.c + :export: + +Resources Management +-------------------- + +.. kernel-doc:: kernel/resource.c + :internal: + +.. kernel-doc:: kernel/resource.c + :export: + +MTRR Handling +------------- + +.. kernel-doc:: arch/x86/kernel/cpu/mtrr/main.c + :export: + +Security Framework +================== + +.. kernel-doc:: security/security.c + :internal: + +.. kernel-doc:: security/inode.c + :export: + +Audit Interfaces +================ + +.. kernel-doc:: kernel/audit.c + :export: + +.. kernel-doc:: kernel/auditsc.c + :internal: + +.. kernel-doc:: kernel/auditfilter.c + :internal: + +Accounting Framework +==================== + +.. kernel-doc:: kernel/acct.c + :internal: + +Block Devices +============= + +.. kernel-doc:: block/blk-core.c + :export: + +.. kernel-doc:: block/blk-core.c + :internal: + +.. kernel-doc:: block/blk-map.c + :export: + +.. kernel-doc:: block/blk-sysfs.c + :internal: + +.. kernel-doc:: block/blk-settings.c + :export: + +.. kernel-doc:: block/blk-exec.c + :export: + +.. kernel-doc:: block/blk-flush.c + :export: + +.. kernel-doc:: block/blk-lib.c + :export: + +.. kernel-doc:: block/blk-tag.c + :export: + +.. kernel-doc:: block/blk-tag.c + :internal: + +.. kernel-doc:: block/blk-integrity.c + :export: + +.. kernel-doc:: kernel/trace/blktrace.c + :internal: + +.. kernel-doc:: block/genhd.c + :internal: + +.. kernel-doc:: block/genhd.c + :export: + +Char devices +============ + +.. kernel-doc:: fs/char_dev.c + :export: + +Clock Framework +=============== + +The clock framework defines programming interfaces to support software +management of the system clock tree. This framework is widely used with +System-On-Chip (SOC) platforms to support power management and various +devices which may need custom clock rates. Note that these "clocks" +don't relate to timekeeping or real time clocks (RTCs), each of which +have separate frameworks. These :c:type:`struct clk <clk>` +instances may be used to manage for example a 96 MHz signal that is used +to shift bits into and out of peripherals or busses, or otherwise +trigger synchronous state machine transitions in system hardware. + +Power management is supported by explicit software clock gating: unused +clocks are disabled, so the system doesn't waste power changing the +state of transistors that aren't in active use. On some systems this may +be backed by hardware clock gating, where clocks are gated without being +disabled in software. Sections of chips that are powered but not clocked +may be able to retain their last state. This low power state is often +called a *retention mode*. This mode still incurs leakage currents, +especially with finer circuit geometries, but for CMOS circuits power is +mostly used by clocked state changes. + +Power-aware drivers only enable their clocks when the device they manage +is in active use. Also, system sleep states often differ according to +which clock domains are active: while a "standby" state may allow wakeup +from several active domains, a "mem" (suspend-to-RAM) state may require +a more wholesale shutdown of clocks derived from higher speed PLLs and +oscillators, limiting the number of possible wakeup event sources. A +driver's suspend method may need to be aware of system-specific clock +constraints on the target sleep state. + +Some platforms support programmable clock generators. These can be used +by external chips of various kinds, such as other CPUs, multimedia +codecs, and devices with strict requirements for interface clocking. + +.. kernel-doc:: include/linux/clk.h + :internal: |