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awilliam
GitHub Repository: awilliam/linux-vfio
 Path: blob/master/Documentation/blackfin/bfin-gpio-notes.txt
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/*

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 * File:         Documentation/blackfin/bfin-gpio-note.txt

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 * Based on:

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 * Author:

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 *

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 * Created:      $Id: bfin-gpio-note.txt 2008-11-24 16:42 grafyang $

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 * Description:  This file contains the notes in developing/using bfin-gpio.

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 *

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 *

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 * Rev:

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 *

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 * Modified:

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 *               Copyright 2004-2008 Analog Devices Inc.

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 *

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 * Bugs:         Enter bugs at http://blackfin.uclinux.org/

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 *

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 */

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1. Blackfin GPIO introduction

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    There are many GPIO pins on Blackfin. Most of these pins are muxed to

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    multi-functions. They can be configured as peripheral, or just as GPIO,

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    configured to input with interrupt enabled, or output.

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    For detailed information, please see "arch/blackfin/kernel/bfin_gpio.c",

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    or the relevant HRM.

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2. Avoiding resource conflict

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    Followed function groups are used to avoiding resource conflict,

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    - Use the pin as peripheral,

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	int peripheral_request(unsigned short per, const char *label);

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	int peripheral_request_list(const unsigned short per[], const char *label);

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	void peripheral_free(unsigned short per);

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	void peripheral_free_list(const unsigned short per[]);

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    - Use the pin as GPIO,

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	int bfin_gpio_request(unsigned gpio, const char *label);

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	void bfin_gpio_free(unsigned gpio);

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    - Use the pin as GPIO interrupt,

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	int bfin_gpio_irq_request(unsigned gpio, const char *label);

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	void bfin_gpio_irq_free(unsigned gpio);

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    The request functions will record the function state for a certain pin,

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    the free functions will clear its function state.

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    Once a pin is requested, it can't be requested again before it is freed by

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    previous caller, otherwise kernel will dump stacks, and the request

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    function fail.

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    These functions are wrapped by other functions, most of the users need not

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    care.

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3. But there are some exceptions

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    - Kernel permit the identical GPIO be requested both as GPIO and GPIO

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    interrut.

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    Some drivers, like gpio-keys, need this behavior. Kernel only print out

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    warning messages like,

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	bfin-gpio: GPIO 24 is already reserved by gpio-keys: BTN0, and you are

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configuring it as IRQ!

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        Note: Consider the case that, if there are two drivers need the

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	identical GPIO, one of them use it as GPIO, the other use it as

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	GPIO interrupt. This will really cause resource conflict. So if

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	there is any abnormal driver behavior, please check the bfin-gpio

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	warning messages.

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    - Kernel permit the identical GPIO be requested from the same driver twice.

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