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diff --git a/Documentation/arm/msm/gpiomux.txt b/Documentation/arm/msm/gpiomux.txt deleted file mode 100644 index 67a81620adf6..000000000000 --- a/Documentation/arm/msm/gpiomux.txt +++ /dev/null @@ -1,176 +0,0 @@ -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. |
