From 11c869eaf1a9c97ef273f824a697fac017d68286 Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Thu, 29 Jun 2006 02:24:47 -0700 Subject: [PATCH] genirq: doc: add design documentation Add docbook file - includes API documentation. Signed-off-by: Thomas Gleixner Signed-off-by: Ingo Molnar Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- Documentation/DocBook/Makefile | 3 +- Documentation/DocBook/genericirq.tmpl | 474 ++++++++++++++++++++++++++++++++++ 2 files changed, 476 insertions(+), 1 deletion(-) create mode 100644 Documentation/DocBook/genericirq.tmpl diff --git a/Documentation/DocBook/Makefile b/Documentation/DocBook/Makefile index 5a2882d275b..66e1cf73357 100644 --- a/Documentation/DocBook/Makefile +++ b/Documentation/DocBook/Makefile @@ -10,7 +10,8 @@ DOCBOOKS := wanbook.xml z8530book.xml mcabook.xml videobook.xml \ kernel-hacking.xml kernel-locking.xml deviceiobook.xml \ procfs-guide.xml writing_usb_driver.xml \ kernel-api.xml journal-api.xml lsm.xml usb.xml \ - gadget.xml libata.xml mtdnand.xml librs.xml rapidio.xml + gadget.xml libata.xml mtdnand.xml librs.xml rapidio.xml \ + genericirq.xml ### # The build process is as follows (targets): diff --git a/Documentation/DocBook/genericirq.tmpl b/Documentation/DocBook/genericirq.tmpl new file mode 100644 index 00000000000..0f4a4b6321e --- /dev/null +++ b/Documentation/DocBook/genericirq.tmpl @@ -0,0 +1,474 @@ + + + + + + Linux generic IRQ handling + + + + Thomas + Gleixner + +
+ tglx@linutronix.de +
+ + + + Ingo + Molnar + +
+ mingo@elte.hu +
+ + + + + + 2005-2006 + Thomas Gleixner + + + 2005-2006 + Ingo Molnar + + + + + This documentation is free software; you can redistribute + it and/or modify it under the terms of the GNU General Public + License version 2 as published by the Free Software Foundation. + + + + This program is distributed in the hope that it will be + useful, but WITHOUT ANY WARRANTY; without even the implied + warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. + See the GNU General Public License for more details. + + + + You should have received a copy of the GNU General Public + License along with this program; if not, write to the Free + Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, + MA 02111-1307 USA + + + + For more details see the file COPYING in the source + distribution of Linux. + + + + + + + + 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 is using + the __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 + + In the SMP world of the __do_IRQ() super-handler another type + was identified: + + Per CPU type + + + + This split implementation of highlevel IRQ handlers allows us to + optimize the flow of the interrupt handling for each specific + interrupt type. This reduces complexity in that particular codepath + 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 lowlevel hardware logic, and it also leads + to unnecessary code duplication: for example in i386, there is a + ioapic_level_irq and a ioapic_edge_irq irq-type which share many + of the lowlevel 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 highlevel flow + handler, which is normally one of the generic + implementations. (This highlevel 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 __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. + + + + Known Bugs And Assumptions + + None (knock on wood). + + + + + Abstraction layers + + There are three main levels of abstraction in the interrupt code: + + Highlevel driver API + Highlevel IRQ flow handlers + Chiplevel 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 decription 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 lowlevel arch code calls into + the generic interrupt code by calling desc->handle_irq(). + This highlevel IRQ handling function only uses desc->chip primitives + referenced by the assigned chip descriptor structure. + + + + Highlevel Driver API + + The highlevel Driver API consists of following functions: + + request_irq() + free_irq() + disable_irq() + enable_irq() + disable_irq_nosync() (SMP only) + synchronize_irq() (SMP only) + set_irq_type() + set_irq_wake() + set_irq_data() + set_irq_chip() + set_irq_chip_data() + + See the autogenerated function documentation for details. + + + + Highlevel IRQ flow handlers + + The generic layer provides a set of pre-defined irq-flow methods: + + handle_level_irq + handle_edge_irq + handle_simple_irq + handle_percpu_irq + + The interrupt flow handlers (either predefined 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(irq) +{ + desc->chip->unmask(irq); +} + +default_disable(irq) +{ + if (!delay_disable(irq)) + desc->chip->mask(irq); +} + +default_ack(irq) +{ + chip->ack(irq); +} + +default_mask_ack(irq) +{ + if (chip->mask_ack) { + chip->mask_ack(irq); + } else { + chip->mask(irq); + chip->ack(irq); + } +} + +noop(irq) +{ +} + + + + + + + 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): + +desc->chip->start(); +handle_IRQ_event(desc->action); +desc->chip->end(); + + + + + 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) { + desc->chip->hold(); + desc->status |= pending | masked; + return; +} +desc->chip->start(); +desc->status |= running; +do { + if (desc->status & masked) + desc->chip->enable(); + desc-status &= ~pending; + handle_IRQ_event(desc->action); +} while (status & pending); +desc-status &= ~running; +desc->chip->end(); + + + + + 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): + +desc->chip->start(); +handle_IRQ_event(desc->action); +desc->chip->end(); + + + + + + 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 highlevel 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 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 + 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 can be runtime enabled, per interrupt, + by setting the IRQ_DELAYED_DISABLE flag in the irq_desc status field. + + + + + Chiplevel hardware encapsulation + + The chip level hardware descriptor structure irq_chip + contains all the direct chip relevant functions, which + can be utilized by the irq flow implementations. + + ack() + mask_ack() - Optional, recommended for performance + mask() + unmask() + retrigger() - Optional + set_type() - Optional + 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 lowlevel functionality. + + + + + + __do_IRQ entry point + + The original implementation __do_IRQ() is an alternative entry + point for all types of interrupts. + + + This handler turned out to be not suitable for all + interrupt hardware and was therefore reimplemented with split + functionality for egde/level/simple/percpu interrupts. This is not + only a functional optimization. It also shortens code paths for + interrupts. + + + To make use of the split implementation, replace the call to + __do_IRQ by a call to desc->chip->handle_irq() and associate + the appropriate handler function to desc->chip->handle_irq(). + In most cases the generic handler implementations should + be sufficient. + + + + + Locking on SMP + + The locking of chip registers is up to the architecture that + defines the chip primitives. There is a chip->lock field that can be used + for serialization, but the generic layer does not touch it. The per-irq + structure is protected via desc->lock, by the generic layer. + + + + Structures + + This chapter contains the autogenerated documentation of the structures which are + used in the generic IRQ layer. + +!Iinclude/linux/irq.h + + + + Public Functions Provided + + This chapter contains the autogenerated documentation of the kernel API functions + which are exported. + +!Ekernel/irq/manage.c +!Ekernel/irq/chip.c + + + + Internal Functions Provided + + This chapter contains the autogenerated documentation of the internal functions. + +!Ikernel/irq/handle.c +!Ikernel/irq/chip.c + + + + Credits + + The following people have contributed to this document: + + Thomas Gleixnertglx@linutronix.de + Ingo Molnarmingo@elte.hu + + + + -- cgit v1.2.3-70-g09d2