最近接触到的一个代码，这个代码看起来很简单，但是却蕴藏了人类的智慧与结晶。正是这些不断产生的智慧与结晶，让我们的电子产品越来越稳定，越来越智能。

周五了，评论文章，选两个同学赠送书籍《Linux内核完全剖析》基于0.12。

#功能

通过ADC值的不同来判断是哪个按键按下，这种方案应该是很常见了，而且这种方案可以节省GPIO口。实现起来的难度也不是特别大。

#硬件连接

#原来的旧代码

static void adc_key_poll(struct work_struct *work)
{struct rk_keys_drvdata *ddata;int i, result = -1;ddata = container_of(work, struct rk_keys_drvdata, adc_poll_work.work);if (!ddata->in_suspend) {result = rk_key_adc_iio_read(ddata);/**读取SARADC值*/if (result > INVALID_ADVALUE &&┊   result < (EMPTY_DEFAULT_ADVALUE - ddata->drift_advalue))ddata->result = result;for (i = 0; i < ddata->nbuttons; i++) {struct rk_keys_button *button = &ddata->button[i];if (!button->adc_value)continue;if (result < button->adc_value + ddata->drift_advalue &&┊   result > button->adc_value - ddata->drift_advalue)button->adc_state = 1;elsebutton->adc_state = 0;if (button->state != button->adc_state)mod_timer(&button->timer,┊ jiffies + DEBOUNCE_JIFFIES);}}schedule_delayed_work(&ddata->adc_poll_work, ADC_SAMPLE_JIFFIES);
}

#升级代码

static void adc_keys_poll(struct input_polled_dev *dev)
{struct adc_keys_state *st = dev->private;int i, value, ret;u32 diff, closest = 0xffffffff;int keycode = 0;ret = iio_read_channel_processed(st->channel, &value);if (unlikely(ret < 0)) {/* Forcibly release key if any was pressed */value = st->keyup_voltage;} else {/*先把电压值与所有按键列表的值比较，找出最接近的一个，以及找出最接近的差值。*/for (i = 0; i < st->num_keys; i++) {diff = abs(st->map[i].voltage - value);if (diff < closest) {closest = diff;keycode = st->map[i].keycode;}}}//printk("adc_keys_poll value:%d keycode:%d\n",value,keycode);/*然后把如果发现当前电压值与按键弹起的电压值很接近，（标准是小于上面找出的最小差值），那么认为没有按键按下，所以设置keycode为0，不上报。*/if (abs(st->keyup_voltage - value) < closest)keycode = 0;if (st->last_key && st->last_key != keycode)input_report_key(dev->input, st->last_key, 0);if (keycode){input_report_key(dev->input, keycode, 1);printk("adc_key_poll keycode:%d\n",keycode);}/*他这么做有个好处，就是只要硬件设计合理的情况下，电阻不同批料有误差的情况下，也不至于需要驱动工程师去修改电压值或者误差范围才能识别按键。*//*这个做法肯定是在量产出现了很多问题后作出的改善。*/input_sync(dev->input);st->last_key = keycode;
}

完整的驱动代码可以看下面这段，我觉得这小段代码的意义非常大。很有意思，通过一个for循环找到一个最接近的数值，判断为按下的这个键值。而且如果按下和 抬起来的数值接近，就判断为没有按键按下。

这就不需要再去考虑一个问题，那就误差范围的问题了。之前的那套旧代码使用的是误差范围，如果ADC值设定为 2000 ，误差范围设定为100，那么读取的ADC值在1900~2100范围内都可以认为是这个按键按下的。

但是存在一个情况，就是不同批次，不同物料的硬件，误差总是不能令人满意，软件需要不断的调整这个误差范围。在不断的调整过程中，发现调整误差范围已经不能解决当下的问题了，所以就产生了新的算法。而我们现在看到的这个新的算法就这样应运而生了。当然，实际情况可能还要复杂化，可能需要加上一些滤波算法先过滤等等。

总之，各位可以保存下来这段代码，在需要的时候拿来使用。展现自己的码农魅力。

#完整升级代码

/** Input driver for resistor ladder connected on ADC** Copyright (c) 2016 Alexandre Belloni** This program is free software; you can redistribute it and/or modify it* under the terms of the GNU General Public License version 2 as published by* the Free Software Foundation.*/
#include <linux/err.h>
#include <linux/iio/consumer.h>
#include <linux/iio/types.h>
#include <linux/input.h>
#include <linux/input-polldev.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/of.h>
#include <linux/platform_device.h>
#include <linux/property.h>
#include <linux/slab.h>
struct adc_keys_button {u32 voltage;u32 keycode;
};
struct adc_keys_state {struct iio_channel *channel;u32 num_keys;u32 last_key;u32 keyup_voltage;const struct adc_keys_button *map;
};
static void adc_keys_poll(struct input_polled_dev *dev)
{struct adc_keys_state *st = dev->private;int i, value, ret;u32 diff, closest = 0xffffffff;int keycode = 0;ret = iio_read_channel_processed(st->channel, &value);if (unlikely(ret < 0)) {/* Forcibly release key if any was pressed */value = st->keyup_voltage;} else {/*先把电压值与所有按键列表的值比较，找出最接近的一个，以及找出最接近的差值。*/for (i = 0; i < st->num_keys; i++) {diff = abs(st->map[i].voltage - value);if (diff < closest) {closest = diff;keycode = st->map[i].keycode;}}}//printk("adc_keys_poll value:%d keycode:%d\n",value,keycode);/*然后把如果发现当前电压值与按键弹起的电压值很接近，（标准是小于上面找出的最小差值），那么认为没有按键按下，所以设置keycode为0，不上报。*/if (abs(st->keyup_voltage - value) < closest)keycode = 0;if (st->last_key && st->last_key != keycode)input_report_key(dev->input, st->last_key, 0);if (keycode){input_report_key(dev->input, keycode, 1);printk("adc_key_poll keycode:%d\n",keycode);}/*他这么做有个好处，就是只要硬件设计合理的情况下，电阻不同批料有误差的情况下，也不至于需要驱动工程师去修改电压值或者误差范围才能识别按键。*//*这个做法肯定是在量产出现了很多问题后作出的改善。*/input_sync(dev->input);st->last_key = keycode;
}
static int adc_keys_load_keymap(struct device *dev, struct adc_keys_state *st)
{struct adc_keys_button *map;struct fwnode_handle *child;int i;st->num_keys = device_get_child_node_count(dev);if (st->num_keys == 0) {dev_err(dev, "keymap is missing\n");return -EINVAL;}map = devm_kmalloc_array(dev, st->num_keys, sizeof(*map), GFP_KERNEL);if (!map)return -ENOMEM;i = 0;device_for_each_child_node(dev, child) {if (fwnode_property_read_u32(child, "press-threshold-microvolt",&map[i].voltage)) {dev_err(dev, "Key with invalid or missing voltage\n");fwnode_handle_put(child);return -EINVAL;}map[i].voltage /= 1000;if (fwnode_property_read_u32(child, "linux,code",&map[i].keycode)) {dev_err(dev, "Key with invalid or missing linux,code\n");fwnode_handle_put(child);return -EINVAL;}i++;}st->map = map;return 0;
}
static int adc_keys_probe(struct platform_device *pdev)
{struct device *dev = &pdev->dev;struct adc_keys_state *st;struct input_polled_dev *poll_dev;struct input_dev *input;enum iio_chan_type type;int i, value;int error;st = devm_kzalloc(dev, sizeof(*st), GFP_KERNEL);if (!st)return -ENOMEM;st->channel = devm_iio_channel_get(dev, "buttons");if (IS_ERR(st->channel))return PTR_ERR(st->channel);if (!st->channel->indio_dev)return -ENXIO;error = iio_get_channel_type(st->channel, &type);if (error < 0)return error;if (type != IIO_VOLTAGE) {dev_err(dev, "Incompatible channel type %d\n", type);return -EINVAL;}if (device_property_read_u32(dev, "keyup-threshold-microvolt",&st->keyup_voltage)) {dev_err(dev, "Invalid or missing keyup voltage\n");return -EINVAL;}st->keyup_voltage /= 1000;error = adc_keys_load_keymap(dev, st);if (error)return error;platform_set_drvdata(pdev, st);poll_dev = devm_input_allocate_polled_device(dev);if (!poll_dev) {dev_err(dev, "failed to allocate input device\n");return -ENOMEM;}if (!device_property_read_u32(dev, "poll-interval", &value))poll_dev->poll_interval = value;poll_dev->poll = adc_keys_poll;poll_dev->private = st;input = poll_dev->input;input->name = pdev->name;input->phys = "adc-keys/input0";input->id.bustype = BUS_HOST;input->id.vendor = 0x0001;input->id.product = 0x0001;input->id.version = 0x0100;__set_bit(EV_KEY, input->evbit);for (i = 0; i < st->num_keys; i++)__set_bit(st->map[i].keycode, input->keybit);if (device_property_read_bool(dev, "autorepeat"))__set_bit(EV_REP, input->evbit);error = input_register_polled_device(poll_dev);if (error) {dev_err(dev, "Unable to register input device: %d\n", error);return error;}return 0;
}
#ifdef CONFIG_OF
static const struct of_device_id adc_keys_of_match[] = {{ .compatible = "adc-keys", },{ }
};
MODULE_DEVICE_TABLE(of, adc_keys_of_match);
#endif
static struct platform_driver __refdata adc_keys_driver = {.driver = {.name = "adc_keys",.of_match_table = of_match_ptr(adc_keys_of_match),},.probe = adc_keys_probe,
};
module_platform_driver(adc_keys_driver);
MODULE_AUTHOR("Alexandre Belloni <alexandre.belloni@free-electrons.com>");
MODULE_DESCRIPTION("Input driver for resistor ladder connected on ADC");
MODULE_LICENSE("GPL v2");

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