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+The U-Boot Driver Model Project
+===============================
+GPIO analysis
+=============
+Viktor Krivak <viktor.krivak@gmail.com>
+2012-02-24
+
+I) Overview
+-----------
+
+ At this moment U-Boot provides standard API that consists of 7 functions.
+
+ int gpio_request(unsigned gpio, const char *label)
+ int gpio_free(unsigned gpio)
+ int gpio_direction_input(unsigned gpio)
+ int gpio_direction_output(unsigned gpio, int value)
+ int gpio_get_value(unsigned gpio)
+ void gpio_set_value(unsigned gpio, int value)
+
+ Methods "gpio_request()" and "gpio_free()" are used for claiming and releasing
+ GPIOs. First one should check if the desired pin exists and if the pin wasn't
+ requested already elsewhere. The method also has a label argument that can be
+ used for debug purposes. The label argument should be copied into the internal
+ memory, but only if the DEBUG macro is set. The "gpio_free()" is the exact
+ opposite. It releases the particular pin. Other methods are used for setting
+ input or output direction and obtaining or setting values of the pins.
+
+II) Approach
+------------
+
+ 1) Request and free GPIO
+ ------------------------
+
+ The "gpio_request()" implementation is basically the same for all boards.
+ The function checks if the particular GPIO is correct and checks if the
+ GPIO pin is still free. If the conditions are met, the method marks the
+ GPIO claimed in it's internal structure. If macro DEBUG is defined, the
+ function also copies the label argument to the structure. If the pin is
+ already locked, the function returns -1 and if DEBUG is defined, certain
+ debug output is generated, including the contents of the label argument.
+ The "gpio_free()" function releases the lock and eventually deallocates
+ data used by the copied label argument.
+
+ 2) Internal data
+ ----------------
+
+ Internal data are driver specific. They have to contain some mechanism to
+ realise the locking though. This can be done for example using a bit field.
+
+ 3) Operations provided by the driver
+ ------------------------------------
+
+ The driver operations basically meet API that is already defined and used.
+ Except for "gpio_request()" and "gpio_free()", all methods can be converted in
+ a simple manner. The driver provides the following structure:
+
+ struct gpio_driver_ops {
+ int (*gpio_request)(struct instance *i, unsigned gpio,
+ const char *label);
+ int (*gpio_free)(struct instance *i, unsigned gpio);
+ int (*gpio_direction_input)(struct instance *i, unsigned gpio);
+ int (*gpio_direction_output)(struct instance *i, unsigned gpio,
+ int value);
+ int (*gpio_get_value)(struct instance *i, unsigned gpio);
+ void (*gpio_set_value)(struct instance *i, unsigned gpio, int value);
+ }
+
+III) Analysis of in-tree drivers
+--------------------------------
+
+ 1) altera_pio.c
+ ---------------
+ Meets standard API. Implements gpio_request() properly. Simple conversion
+ possible.
+
+ 2) at91_gpio.c
+ --------------
+ Don't meet standard API. Need some other methods to implement.
+
+ 3) da8xx_gpio.c
+ ---------------
+ Meets standard API. Implements gpio_request() properly. Simple conversion
+ possible.
+
+ 4) kw_gpio.c
+ ------------
+ Doesn't meet standard API. Needs some other methods to implement and move some
+ methods to another file.
+
+ 5) mpc83xx_gpio.c
+ -----------------
+ Meets standard API. Doesn't implement gpio_request() properly (only checks
+ if the pin is valid). Simple conversion possible.
+
+ 6) mvgpio.c
+ -----------
+ Meets standard API. Doesn't implement gpio_request() properly (only checks
+ if the pin is valid). Simple conversion possible.
+
+ 7) mvgpio.h
+ -----------
+ Wrong placement. Will be moved to another location.
+
+ 8) mvmfp.c
+ ----------
+ Wrong placement. Will be moved to another location.