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