1/*
2 * GPIO-based I2C Arbitration Using a Challenge & Response Mechanism
3 *
4 * Copyright (C) 2012 Google, Inc
5 *
6 * This software is licensed under the terms of the GNU General Public
7 * License version 2, as published by the Free Software Foundation, and
8 * may be copied, distributed, and modified under those terms.
9 *
10 * This program is distributed in the hope that it will be useful,
11 * but WITHOUT ANY WARRANTY; without even the implied warranty of
12 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
13 * GNU General Public License for more details.
14 *
15 */
16
17#include <linux/delay.h>
18#include <linux/gpio.h>
19#include <linux/kernel.h>
20#include <linux/i2c.h>
21#include <linux/i2c-mux.h>
22#include <linux/module.h>
23#include <linux/of_gpio.h>
24#include <linux/platform_device.h>
25#include <linux/slab.h>
26
27
28/**
29 * struct i2c_arbitrator_data - Driver data for I2C arbitrator
30 *
31 * @parent: Parent adapter
32 * @child: Child bus
33 * @our_gpio: GPIO we'll use to claim.
34 * @our_gpio_release: 0 if active high; 1 if active low; AKA if the GPIO ==
35 *   this then consider it released.
36 * @their_gpio: GPIO that the other side will use to claim.
37 * @their_gpio_release: 0 if active high; 1 if active low; AKA if the GPIO ==
38 *   this then consider it released.
39 * @slew_delay_us: microseconds to wait for a GPIO to go high.
40 * @wait_retry_us: we'll attempt another claim after this many microseconds.
41 * @wait_free_us: we'll give up after this many microseconds.
42 */
43
44struct i2c_arbitrator_data {
45	struct i2c_adapter *parent;
46	struct i2c_adapter *child;
47	int our_gpio;
48	int our_gpio_release;
49	int their_gpio;
50	int their_gpio_release;
51	unsigned int slew_delay_us;
52	unsigned int wait_retry_us;
53	unsigned int wait_free_us;
54};
55
56
57/**
58 * i2c_arbitrator_select - claim the I2C bus
59 *
60 * Use the GPIO-based signalling protocol; return -EBUSY if we fail.
61 */
62static int i2c_arbitrator_select(struct i2c_adapter *adap, void *data, u32 chan)
63{
64	const struct i2c_arbitrator_data *arb = data;
65	unsigned long stop_retry, stop_time;
66
67	/* Start a round of trying to claim the bus */
68	stop_time = jiffies + usecs_to_jiffies(arb->wait_free_us) + 1;
69	do {
70		/* Indicate that we want to claim the bus */
71		gpio_set_value(arb->our_gpio, !arb->our_gpio_release);
72		udelay(arb->slew_delay_us);
73
74		/* Wait for the other master to release it */
75		stop_retry = jiffies + usecs_to_jiffies(arb->wait_retry_us) + 1;
76		while (time_before(jiffies, stop_retry)) {
77			int gpio_val = !!gpio_get_value(arb->their_gpio);
78
79			if (gpio_val == arb->their_gpio_release) {
80				/* We got it, so return */
81				return 0;
82			}
83
84			usleep_range(50, 200);
85		}
86
87		/* It didn't release, so give up, wait, and try again */
88		gpio_set_value(arb->our_gpio, arb->our_gpio_release);
89
90		usleep_range(arb->wait_retry_us, arb->wait_retry_us * 2);
91	} while (time_before(jiffies, stop_time));
92
93	/* Give up, release our claim */
94	gpio_set_value(arb->our_gpio, arb->our_gpio_release);
95	udelay(arb->slew_delay_us);
96	dev_err(&adap->dev, "Could not claim bus, timeout\n");
97	return -EBUSY;
98}
99
100/**
101 * i2c_arbitrator_deselect - release the I2C bus
102 *
103 * Release the I2C bus using the GPIO-based signalling protocol.
104 */
105static int i2c_arbitrator_deselect(struct i2c_adapter *adap, void *data,
106				   u32 chan)
107{
108	const struct i2c_arbitrator_data *arb = data;
109
110	/* Release the bus and wait for the other master to notice */
111	gpio_set_value(arb->our_gpio, arb->our_gpio_release);
112	udelay(arb->slew_delay_us);
113
114	return 0;
115}
116
117static int i2c_arbitrator_probe(struct platform_device *pdev)
118{
119	struct device *dev = &pdev->dev;
120	struct device_node *np = dev->of_node;
121	struct device_node *parent_np;
122	struct i2c_arbitrator_data *arb;
123	enum of_gpio_flags gpio_flags;
124	unsigned long out_init;
125	int ret;
126
127	/* We only support probing from device tree; no platform_data */
128	if (!np) {
129		dev_err(dev, "Cannot find device tree node\n");
130		return -ENODEV;
131	}
132	if (dev_get_platdata(dev)) {
133		dev_err(dev, "Platform data is not supported\n");
134		return -EINVAL;
135	}
136
137	arb = devm_kzalloc(dev, sizeof(*arb), GFP_KERNEL);
138	if (!arb) {
139		dev_err(dev, "Cannot allocate i2c_arbitrator_data\n");
140		return -ENOMEM;
141	}
142	platform_set_drvdata(pdev, arb);
143
144	/* Request GPIOs */
145	ret = of_get_named_gpio_flags(np, "our-claim-gpio", 0, &gpio_flags);
146	if (!gpio_is_valid(ret)) {
147		if (ret != -EPROBE_DEFER)
148			dev_err(dev, "Error getting our-claim-gpio\n");
149		return ret;
150	}
151	arb->our_gpio = ret;
152	arb->our_gpio_release = !!(gpio_flags & OF_GPIO_ACTIVE_LOW);
153	out_init = (gpio_flags & OF_GPIO_ACTIVE_LOW) ?
154		GPIOF_OUT_INIT_HIGH : GPIOF_OUT_INIT_LOW;
155	ret = devm_gpio_request_one(dev, arb->our_gpio, out_init,
156				    "our-claim-gpio");
157	if (ret) {
158		if (ret != -EPROBE_DEFER)
159			dev_err(dev, "Error requesting our-claim-gpio\n");
160		return ret;
161	}
162
163	ret = of_get_named_gpio_flags(np, "their-claim-gpios", 0, &gpio_flags);
164	if (!gpio_is_valid(ret)) {
165		if (ret != -EPROBE_DEFER)
166			dev_err(dev, "Error getting their-claim-gpio\n");
167		return ret;
168	}
169	arb->their_gpio = ret;
170	arb->their_gpio_release = !!(gpio_flags & OF_GPIO_ACTIVE_LOW);
171	ret = devm_gpio_request_one(dev, arb->their_gpio, GPIOF_IN,
172				    "their-claim-gpio");
173	if (ret) {
174		if (ret != -EPROBE_DEFER)
175			dev_err(dev, "Error requesting their-claim-gpio\n");
176		return ret;
177	}
178
179	/* At the moment we only support a single two master (us + 1 other) */
180	if (gpio_is_valid(of_get_named_gpio(np, "their-claim-gpios", 1))) {
181		dev_err(dev, "Only one other master is supported\n");
182		return -EINVAL;
183	}
184
185	/* Arbitration parameters */
186	if (of_property_read_u32(np, "slew-delay-us", &arb->slew_delay_us))
187		arb->slew_delay_us = 10;
188	if (of_property_read_u32(np, "wait-retry-us", &arb->wait_retry_us))
189		arb->wait_retry_us = 3000;
190	if (of_property_read_u32(np, "wait-free-us", &arb->wait_free_us))
191		arb->wait_free_us = 50000;
192
193	/* Find our parent */
194	parent_np = of_parse_phandle(np, "i2c-parent", 0);
195	if (!parent_np) {
196		dev_err(dev, "Cannot parse i2c-parent\n");
197		return -EINVAL;
198	}
199	arb->parent = of_find_i2c_adapter_by_node(parent_np);
200	if (!arb->parent) {
201		dev_err(dev, "Cannot find parent bus\n");
202		return -EPROBE_DEFER;
203	}
204
205	/* Actually add the mux adapter */
206	arb->child = i2c_add_mux_adapter(arb->parent, dev, arb, 0, 0, 0,
207					 i2c_arbitrator_select,
208					 i2c_arbitrator_deselect);
209	if (!arb->child) {
210		dev_err(dev, "Failed to add adapter\n");
211		ret = -ENODEV;
212		i2c_put_adapter(arb->parent);
213	}
214
215	return ret;
216}
217
218static int i2c_arbitrator_remove(struct platform_device *pdev)
219{
220	struct i2c_arbitrator_data *arb = platform_get_drvdata(pdev);
221
222	i2c_del_mux_adapter(arb->child);
223	i2c_put_adapter(arb->parent);
224
225	return 0;
226}
227
228static const struct of_device_id i2c_arbitrator_of_match[] = {
229	{ .compatible = "i2c-arb-gpio-challenge", },
230	{},
231};
232MODULE_DEVICE_TABLE(of, i2c_arbitrator_of_match);
233
234static struct platform_driver i2c_arbitrator_driver = {
235	.probe	= i2c_arbitrator_probe,
236	.remove	= i2c_arbitrator_remove,
237	.driver	= {
238		.name	= "i2c-arb-gpio-challenge",
239		.of_match_table = i2c_arbitrator_of_match,
240	},
241};
242
243module_platform_driver(i2c_arbitrator_driver);
244
245MODULE_DESCRIPTION("GPIO-based I2C Arbitration");
246MODULE_AUTHOR("Doug Anderson <dianders@chromium.org>");
247MODULE_LICENSE("GPL v2");
248MODULE_ALIAS("platform:i2c-arb-gpio-challenge");
249