[patch 2.6.24-rc6-mm 3/9] gpiolib: update Documentation/gpio.txt

[David Brownell <david-b@pacbell.net>]

From: David Brownell <dbrownell@users.sourceforge.net>

Update Documentation/gpio.txt, primarily to include the new
optional "gpiolib" implementation infrastructure.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
---
 Documentation/gpio.txt |  133 ++++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 121 insertions(+), 12 deletions(-)

--- at91.orig/Documentation/gpio.txt
+++ at91/Documentation/gpio.txt
@@ -32,7 +32,7 @@ The exact capabilities of GPIOs vary bet
   - Input values are likewise readable (1, 0).  Some chips support readback
     of pins configured as "output", which is very useful in such "wire-OR"
     cases (to support bidirectional signaling).  GPIO controllers may have
-    input de-glitch logic, sometimes with software controls.
+    input de-glitch/debounce logic, sometimes with software controls.
 
   - Inputs can often be used as IRQ signals, often edge triggered but
     sometimes level triggered.  Such IRQs may be configurable as system
@@ -60,10 +60,13 @@ used on a board that's wired differently
 functionality can be very portable.  Other features are platform-specific,
 and that can be critical for glue logic.
 
-Plus, this doesn't define an implementation framework, just an interface.
+Plus, this doesn't require any implementation framework, just an interface.
 One platform might implement it as simple inline functions accessing chip
 registers; another might implement it by delegating through abstractions
-used for several very different kinds of GPIO controller.
+used for several very different kinds of GPIO controller.  (There is some
+optional code supporting such an implementation strategy, described later
+in this document, but drivers acting as clients to the GPIO interface must
+not care how it's implemented.)
 
 That said, if the convention is supported on their platform, drivers should
 use it when possible.  Platforms should declare GENERIC_GPIO support in
@@ -121,6 +124,11 @@ before tasking is enabled, as part of ea
 For output GPIOs, the value provided becomes the initial output value.
 This helps avoid signal glitching during system startup.
 
+For compatibility with legacy interfaces to GPIOs, setting the direction
+of a GPIO implicitly requests that GPIO (see below) if it has not been
+requested already.  That compatibility may be removed in the future;
+explicitly requesting GPIOs is strongly preferred.
+
 Setting the direction can fail if the GPIO number is invalid, or when
 that particular GPIO can't be used in that mode.  It's generally a bad
 idea to rely on boot firmware to have set the direction correctly, since
@@ -133,6 +141,7 @@ Spinlock-Safe GPIO access
 -------------------------
 Most GPIO controllers can be accessed with memory read/write instructions.
 That doesn't need to sleep, and can safely be done from inside IRQ handlers.
+(That includes hardirq contexts on RT kernels.)
 
 Use these calls to access such GPIOs:
 
@@ -145,7 +154,7 @@ Use these calls to access such GPIOs:
 The values are boolean, zero for low, nonzero for high.  When reading the
 value of an output pin, the value returned should be what's seen on the
 pin ... that won't always match the specified output value, because of
-issues including wire-OR and output latencies.
+issues including open-drain signaling and output latencies.
 
 The get/set calls have no error returns because "invalid GPIO" should have
 been reported earlier from gpio_direction_*().  However, note that not all
@@ -170,7 +179,8 @@ get to the head of a queue to transmit a
 This requires sleeping, which can't be done from inside IRQ handlers.
 
 Platforms that support this type of GPIO distinguish them from other GPIOs
-by returning nonzero from this call:
+by returning nonzero from this call (which requires a valid GPIO number,
+either explicitly or implicitly requested):
 
 	int gpio_cansleep(unsigned gpio);
 
@@ -209,8 +219,11 @@ before tasking is enabled, as part of ea
 These calls serve two basic purposes.  One is marking the signals which
 are actually in use as GPIOs, for better diagnostics; systems may have
 several hundred potential GPIOs, but often only a dozen are used on any
-given board.  Another is to catch conflicts between drivers, reporting
-errors when drivers wrongly think they have exclusive use of that signal.
+given board.  Another is to catch conflicts, identifying errors when
+(a) two or more drivers wrongly think they have exclusive use of that
+signal, or (b) something wrongly believes it's safe to remove drivers
+needed to manage a signal that's in active use.  That is, requesting a
+GPIO can serve as a kind of lock.
 
 These two calls are optional because not not all current Linux platforms
 offer such functionality in their GPIO support; a valid implementation
@@ -223,6 +236,9 @@ Note that requesting a GPIO does NOT cau
 way; it just marks that GPIO as in use.  Separate code must handle any
 pin setup (e.g. controlling which pin the GPIO uses, pullup/pulldown).
 
+Also note that it's your responsibility to have stopped using a GPIO
+before you free it.
+
 
 GPIOs mapped to IRQs
 --------------------
@@ -238,7 +254,7 @@ map between them using calls like:
 
 Those return either the corresponding number in the other namespace, or
 else a negative errno code if the mapping can't be done.  (For example,
-some GPIOs can't used as IRQs.)  It is an unchecked error to use a GPIO
+some GPIOs can't be used as IRQs.)  It is an unchecked error to use a GPIO
 number that wasn't set up as an input using gpio_direction_input(), or
 to use an IRQ number that didn't originally come from gpio_to_irq().
 
@@ -299,17 +315,110 @@ Related to multiplexing is configuration
 pulldowns integrated on some platforms.  Not all platforms support them,
 or support them in the same way; and any given board might use external
 pullups (or pulldowns) so that the on-chip ones should not be used.
+(When a circuit needs 5 kOhm, on-chip 100 kOhm resistors won't do.)
 
 There are other system-specific mechanisms that are not specified here,
 like the aforementioned options for input de-glitching and wire-OR output.
 Hardware may support reading or writing GPIOs in gangs, but that's usually
 configuration dependent:  for GPIOs sharing the same bank.  (GPIOs are
 commonly grouped in banks of 16 or 32, with a given SOC having several such
-banks.)  Some systems can trigger IRQs from output GPIOs.  Code relying on
-such mechanisms will necessarily be nonportable.
+banks.)  Some systems can trigger IRQs from output GPIOs, or read values
+from pins not managed as GPIOs.  Code relying on such mechanisms will
+necessarily be nonportable.
 
-Dynamic definition of GPIOs is not currently supported; for example, as
+Dynamic definition of GPIOs is not currently standard; for example, as
 a side effect of configuring an add-on board with some GPIO expanders.
 
 These calls are purely for kernel space, but a userspace API could be built
-on top of it.
+on top of them.
+
+
+GPIO implementor's framework (OPTIONAL)
+=======================================
+As noted earlier, there is an optional implementation framework making it
+easier for platforms to support different kinds of GPIO controller using
+the same programming interface.
+
+As a debugging aid, if debugfs is available a /sys/kernel/debug/gpio file
+will be found there.  That will list all the controllers registered through
+this framework, and the state of the GPIOs currently in use.
+
+
+Controller Drivers: gpio_chip
+-----------------------------
+In this framework each GPIO controller is packaged as a "struct gpio_chip"
+with information common to each controller of that type:
+
+ - methods to establish GPIO direction
+ - methods used to access GPIO values
+ - flag saying whether calls to its methods may sleep
+ - optional debugfs dump method (showing extra state like pullup config)
+ - label for diagnostics
+
+There is also per-instance data, which may come from device.platform_data:
+the number of its first GPIO, and how many GPIOs it exposes.
+
+The code implementing a gpio_chip should support multiple instances of the
+controller, possibly using the driver model.  That code will configure each
+gpio_chip and issue gpiochip_add().  Removing a GPIO controller should be
+rare; use gpiochip_remove() when it is unavoidable.
+
+Most often a gpio_chip is part of an instance-specific structure with state
+not exposed by the GPIO interfaces, such as addressing, power management,
+and more.  Chips such as codecs will have complex non-GPIO state,
+
+Any debugfs dump method should normally ignore signals which haven't been
+requested as GPIOs.  They can use gpiochip_is_requested(), which returns
+either NULL or the label associated with that GPIO when it was requested.
+
+
+Platform Support
+----------------
+To support this framework, a platform's Kconfig will "select GPIO_LIB" and
+arrange that its <asm/gpio.h> then include <asm-generic/gpio.h> and define
+three functions: gpio_get_value(), gpio_set_value(), and gpio_cansleep().
+They may also want to provide a custom value for ARCH_NR_GPIOS.
+
+Trivial implementations of those functions can directly use framework
+code, which always dispatches through the gpio_chip:
+
+  #define gpio_get_value	__gpio_get_value
+  #define gpio_set_value	__gpio_set_value
+  #define gpio_cansleep		__gpio_cansleep
+
+Fancier implementations could instead define those as inline functions with
+logic optimizing access to specific SOC-based GPIOs.  For example, if the
+referenced GPIO is the constant "12", getting or setting its value could
+cost as little as two or three instructions, never sleeping.  When such an
+optimization is not possible those calls must delegate to the framework
+code, costing at least a few dozen instructions.  For bitbanged I/O, such
+instruction savings can be significant.
+
+For SOCs, platform-specific code defines and registers gpio_chip instances
+for each bank of on-chip GPIOs.  Those GPIOs should be numbered/labeled to
+match chip vendor documentation, and directly match board schematics.  They
+may well start at zero and go up to a platform-specific limit.  Such GPIOs
+are normally integrated into platform initialization to make them always be
+available, from arch_initcall() or earlier; they can often serve as IRQs.
+
+
+Board Support
+-------------
+For external GPIO controllers -- such as I2C or SPI expanders, ASICs, multi
+function devices, FPGAs or CPLDs -- most often board-specific code handles
+registering controller devices and ensures that their drivers know what GPIO
+numbers to use with gpiochip_add().  Their numbers often start right after
+platform-specific GPIOs.
+
+For example, board setup code could create structures identifying the range
+of GPIOs that chip will expose, and passes them to each GPIO expander chip
+using platform_data.  Then the chip driver's probe() routine could pass that
+data to gpiochip_add().
+
+Initialization order can be important.  For example, when a device relies on
+an I2C-based GPIO, its probe() routine should only be called after that GPIO
+becomes available.  That may mean the device should not be registered until
+calls for that GPIO can work.  One way to address such dependencies is for
+such gpio_chip controllers to provide setup() and teardown() callbacks to
+board specific code; those board specific callbacks would register devices
+once all the necessary resources are available.
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