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-This document provides an overview of the msm_gpiomux interface, which
-is used to provide gpio pin multiplexing and configuration on mach-msm
-targets.
-
-History
-=======
-
-The first-generation API for gpio configuration & multiplexing on msm
-is the function gpio_tlmm_config(). This function has a few notable
-shortcomings, which led to its deprecation and replacement by gpiomux:
-
-The 'disable' parameter: Setting the second parameter to
-gpio_tlmm_config to GPIO_CFG_DISABLE tells the peripheral
-processor in charge of the subsystem to perform a look-up into a
-low-power table and apply the low-power/sleep setting for the pin.
-As the msm family evolved this became problematic. Not all pins
-have sleep settings, not all peripheral processors will accept requests
-to apply said sleep settings, and not all msm targets have their gpio
-subsystems managed by a peripheral processor. In order to get consistent
-behavior on all targets, drivers are forced to ignore this parameter,
-rendering it useless.
-
-The 'direction' flag: for all mux-settings other than raw-gpio (0),
-the output-enable bit of a gpio is hard-wired to a known
-input (usually VDD or ground). For those settings, the direction flag
-is meaningless at best, and deceptive at worst. In addition, using the
-direction flag to change output-enable (OE) directly can cause trouble in
-gpiolib, which has no visibility into gpio direction changes made
-in this way. Direction control in gpio mode should be made through gpiolib.
-
-Key Features of gpiomux
-=======================
-
-- A consistent interface across all generations of msm. Drivers can expect
-the same results on every target.
-- gpiomux plays nicely with gpiolib. Functions that should belong to gpiolib
-are left to gpiolib and not duplicated here. gpiomux is written with the
-intent that gpio_chips will call gpiomux reference-counting methods
-from their request() and free() hooks, providing full integration.
-- Tabular configuration. Instead of having to call gpio_tlmm_config
-hundreds of times, gpio configuration is placed in a single table.
-- Per-gpio sleep. Each gpio is individually reference counted, allowing only
-those lines which are in use to be put in high-power states.
-- 0 means 'do nothing': all flags are designed so that the default memset-zero
-equates to a sensible default of 'no configuration', preventing users
-from having to provide hundreds of 'no-op' configs for unused or
-unwanted lines.
-
-Usage
-=====
-
-To use gpiomux, provide configuration information for relevant gpio lines
-in the msm_gpiomux_configs table. Since a 0 equates to "unconfigured",
-only those lines to be managed by gpiomux need to be specified. Here
-is a completely fictional example:
-
-struct msm_gpiomux_config msm_gpiomux_configs[GPIOMUX_NGPIOS] = {
- [12] = {
- .active = GPIOMUX_VALID | GPIOMUX_DRV_8MA | GPIOMUX_FUNC_1,
- .suspended = GPIOMUX_VALID | GPIOMUX_PULL_DOWN,
- },
- [34] = {
- .suspended = GPIOMUX_VALID | GPIOMUX_PULL_DOWN,
- },
-};
-
-To indicate that a gpio is in use, call msm_gpiomux_get() to increase
-its reference count. To decrease the reference count, call msm_gpiomux_put().
-
-The effect of this configuration is as follows:
-
-When the system boots, gpios 12 and 34 will be initialized with their
-'suspended' configurations. All other gpios, which were left unconfigured,
-will not be touched.
-
-When msm_gpiomux_get() is called on gpio 12 to raise its reference count
-above 0, its active configuration will be applied. Since no other gpio
-line has a valid active configuration, msm_gpiomux_get() will have no
-effect on any other line.
-
-When msm_gpiomux_put() is called on gpio 12 or 34 to drop their reference
-count to 0, their suspended configurations will be applied.
-Since no other gpio line has a valid suspended configuration, no other
-gpio line will be effected by msm_gpiomux_put(). Since gpio 34 has no valid
-active configuration, this is effectively a no-op for gpio 34 as well,
-with one small caveat, see the section "About Output-Enable Settings".
-
-All of the GPIOMUX_VALID flags may seem like unnecessary overhead, but
-they address some important issues. As unused entries (all those
-except 12 and 34) are zero-filled, gpiomux needs a way to distinguish
-the used fields from the unused. In addition, the all-zero pattern
-is a valid configuration! Therefore, gpiomux defines an additional bit
-which is used to indicate when a field is used. This has the pleasant
-side-effect of allowing calls to msm_gpiomux_write to use '0' to indicate
-that a value should not be changed:
-
- msm_gpiomux_write(0, GPIOMUX_VALID, 0);
-
-replaces the active configuration of gpio 0 with an all-zero configuration,
-but leaves the suspended configuration as it was.
-
-Static Configurations
-=====================
-
-To install a static configuration, which is applied at boot and does
-not change after that, install a configuration with a suspended component
-but no active component, as in the previous example:
-
- [34] = {
- .suspended = GPIOMUX_VALID | GPIOMUX_PULL_DOWN,
- },
-
-The suspended setting is applied during boot, and the lack of any valid
-active setting prevents any other setting from being applied at runtime.
-If other subsystems attempting to access the line is a concern, one could
-*really* anchor the configuration down by calling msm_gpiomux_get on the
-line at initialization to move the line into active mode. With the line
-held, it will never be re-suspended, and with no valid active configuration,
-no new configurations will be applied.
-
-But then, if having other subsystems grabbing for the line is truly a concern,
-it should be reserved with gpio_request instead, which carries an implicit
-msm_gpiomux_get.
-
-gpiomux and gpiolib
-===================
-
-It is expected that msm gpio_chips will call msm_gpiomux_get() and
-msm_gpiomux_put() from their request and free hooks, like this fictional
-example:
-
-static int request(struct gpio_chip *chip, unsigned offset)
-{
- return msm_gpiomux_get(chip->base + offset);
-}
-
-static void free(struct gpio_chip *chip, unsigned offset)
-{
- msm_gpiomux_put(chip->base + offset);
-}
-
- ...somewhere in a gpio_chip declaration...
- .request = request,
- .free = free,
-
-This provides important functionality:
-- It guarantees that a gpio line will have its 'active' config applied
- when the line is requested, and will not be suspended while the line
- remains requested; and
-- It guarantees that gpio-direction settings from gpiolib behave sensibly.
- See "About Output-Enable Settings."
-
-This mechanism allows for "auto-request" of gpiomux lines via gpiolib
-when it is suitable. Drivers wishing more exact control are, of course,
-free to also use msm_gpiomux_set and msm_gpiomux_get.
-
-About Output-Enable Settings
-============================
-
-Some msm targets do not have the ability to query the current gpio
-configuration setting. This means that changes made to the output-enable
-(OE) bit by gpiolib cannot be consistently detected and preserved by gpiomux.
-Therefore, when gpiomux applies a configuration setting, any direction
-settings which may have been applied by gpiolib are lost and the default
-input settings are re-applied.
-
-For this reason, drivers should not assume that gpio direction settings
-continue to hold if they free and then re-request a gpio. This seems like
-common sense - after all, anybody could have obtained the line in the
-meantime - but it needs saying.
-
-This also means that calls to msm_gpiomux_write will reset the OE bit,
-which means that if the gpio line is held by a client of gpiolib and
-msm_gpiomux_write is called, the direction setting has been lost and
-gpiolib's internal state has been broken.
-Release gpio lines before reconfiguring them.