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  * @our_gpio: GPIO we'll use to claim.
32  * @our_gpio_release: 0 if active high; 1 if active low; AKA if the GPIO ==
33  *   this then consider it released.
34  * @their_gpio: GPIO that the other side will use to claim.
35  * @their_gpio_release: 0 if active high; 1 if active low; AKA if the GPIO ==
36  *   this then consider it released.
37  * @slew_delay_us: microseconds to wait for a GPIO to go high.
38  * @wait_retry_us: we'll attempt another claim after this many microseconds.
39  * @wait_free_us: we'll give up after this many microseconds.
40  */
41 
42 struct i2c_arbitrator_data {
43 	int our_gpio;
44 	int our_gpio_release;
45 	int their_gpio;
46 	int their_gpio_release;
47 	unsigned int slew_delay_us;
48 	unsigned int wait_retry_us;
49 	unsigned int wait_free_us;
50 };
51 
52 
53 /**
54  * i2c_arbitrator_select - claim the I2C bus
55  *
56  * Use the GPIO-based signalling protocol; return -EBUSY if we fail.
57  */
58 static int i2c_arbitrator_select(struct i2c_mux_core *muxc, u32 chan)
59 {
60 	const struct i2c_arbitrator_data *arb = i2c_mux_priv(muxc);
61 	unsigned long stop_retry, stop_time;
62 
63 	/* Start a round of trying to claim the bus */
64 	stop_time = jiffies + usecs_to_jiffies(arb->wait_free_us) + 1;
65 	do {
66 		/* Indicate that we want to claim the bus */
67 		gpio_set_value(arb->our_gpio, !arb->our_gpio_release);
68 		udelay(arb->slew_delay_us);
69 
70 		/* Wait for the other master to release it */
71 		stop_retry = jiffies + usecs_to_jiffies(arb->wait_retry_us) + 1;
72 		while (time_before(jiffies, stop_retry)) {
73 			int gpio_val = !!gpio_get_value(arb->their_gpio);
74 
75 			if (gpio_val == arb->their_gpio_release) {
76 				/* We got it, so return */
77 				return 0;
78 			}
79 
80 			usleep_range(50, 200);
81 		}
82 
83 		/* It didn't release, so give up, wait, and try again */
84 		gpio_set_value(arb->our_gpio, arb->our_gpio_release);
85 
86 		usleep_range(arb->wait_retry_us, arb->wait_retry_us * 2);
87 	} while (time_before(jiffies, stop_time));
88 
89 	/* Give up, release our claim */
90 	gpio_set_value(arb->our_gpio, arb->our_gpio_release);
91 	udelay(arb->slew_delay_us);
92 	dev_err(muxc->dev, "Could not claim bus, timeout\n");
93 	return -EBUSY;
94 }
95 
96 /**
97  * i2c_arbitrator_deselect - release the I2C bus
98  *
99  * Release the I2C bus using the GPIO-based signalling protocol.
100  */
101 static int i2c_arbitrator_deselect(struct i2c_mux_core *muxc, u32 chan)
102 {
103 	const struct i2c_arbitrator_data *arb = i2c_mux_priv(muxc);
104 
105 	/* Release the bus and wait for the other master to notice */
106 	gpio_set_value(arb->our_gpio, arb->our_gpio_release);
107 	udelay(arb->slew_delay_us);
108 
109 	return 0;
110 }
111 
112 static int i2c_arbitrator_probe(struct platform_device *pdev)
113 {
114 	struct device *dev = &pdev->dev;
115 	struct device_node *np = dev->of_node;
116 	struct device_node *parent_np;
117 	struct i2c_mux_core *muxc;
118 	struct i2c_arbitrator_data *arb;
119 	enum of_gpio_flags gpio_flags;
120 	unsigned long out_init;
121 	int ret;
122 
123 	/* We only support probing from device tree; no platform_data */
124 	if (!np) {
125 		dev_err(dev, "Cannot find device tree node\n");
126 		return -ENODEV;
127 	}
128 	if (dev_get_platdata(dev)) {
129 		dev_err(dev, "Platform data is not supported\n");
130 		return -EINVAL;
131 	}
132 
133 	muxc = i2c_mux_alloc(NULL, dev, 1, sizeof(*arb), I2C_MUX_ARBITRATOR,
134 			     i2c_arbitrator_select, i2c_arbitrator_deselect);
135 	if (!muxc)
136 		return -ENOMEM;
137 	arb = i2c_mux_priv(muxc);
138 
139 	platform_set_drvdata(pdev, muxc);
140 
141 	/* Request GPIOs */
142 	ret = of_get_named_gpio_flags(np, "our-claim-gpio", 0, &gpio_flags);
143 	if (!gpio_is_valid(ret)) {
144 		if (ret != -EPROBE_DEFER)
145 			dev_err(dev, "Error getting our-claim-gpio\n");
146 		return ret;
147 	}
148 	arb->our_gpio = ret;
149 	arb->our_gpio_release = !!(gpio_flags & OF_GPIO_ACTIVE_LOW);
150 	out_init = (gpio_flags & OF_GPIO_ACTIVE_LOW) ?
151 		GPIOF_OUT_INIT_HIGH : GPIOF_OUT_INIT_LOW;
152 	ret = devm_gpio_request_one(dev, arb->our_gpio, out_init,
153 				    "our-claim-gpio");
154 	if (ret) {
155 		if (ret != -EPROBE_DEFER)
156 			dev_err(dev, "Error requesting our-claim-gpio\n");
157 		return ret;
158 	}
159 
160 	ret = of_get_named_gpio_flags(np, "their-claim-gpios", 0, &gpio_flags);
161 	if (!gpio_is_valid(ret)) {
162 		if (ret != -EPROBE_DEFER)
163 			dev_err(dev, "Error getting their-claim-gpio\n");
164 		return ret;
165 	}
166 	arb->their_gpio = ret;
167 	arb->their_gpio_release = !!(gpio_flags & OF_GPIO_ACTIVE_LOW);
168 	ret = devm_gpio_request_one(dev, arb->their_gpio, GPIOF_IN,
169 				    "their-claim-gpio");
170 	if (ret) {
171 		if (ret != -EPROBE_DEFER)
172 			dev_err(dev, "Error requesting their-claim-gpio\n");
173 		return ret;
174 	}
175 
176 	/* At the moment we only support a single two master (us + 1 other) */
177 	if (gpio_is_valid(of_get_named_gpio(np, "their-claim-gpios", 1))) {
178 		dev_err(dev, "Only one other master is supported\n");
179 		return -EINVAL;
180 	}
181 
182 	/* Arbitration parameters */
183 	if (of_property_read_u32(np, "slew-delay-us", &arb->slew_delay_us))
184 		arb->slew_delay_us = 10;
185 	if (of_property_read_u32(np, "wait-retry-us", &arb->wait_retry_us))
186 		arb->wait_retry_us = 3000;
187 	if (of_property_read_u32(np, "wait-free-us", &arb->wait_free_us))
188 		arb->wait_free_us = 50000;
189 
190 	/* Find our parent */
191 	parent_np = of_parse_phandle(np, "i2c-parent", 0);
192 	if (!parent_np) {
193 		dev_err(dev, "Cannot parse i2c-parent\n");
194 		return -EINVAL;
195 	}
196 	muxc->parent = of_get_i2c_adapter_by_node(parent_np);
197 	of_node_put(parent_np);
198 	if (!muxc->parent) {
199 		dev_err(dev, "Cannot find parent bus\n");
200 		return -EPROBE_DEFER;
201 	}
202 
203 	/* Actually add the mux adapter */
204 	ret = i2c_mux_add_adapter(muxc, 0, 0, 0);
205 	if (ret) {
206 		dev_err(dev, "Failed to add adapter\n");
207 		i2c_put_adapter(muxc->parent);
208 	}
209 
210 	return ret;
211 }
212 
213 static int i2c_arbitrator_remove(struct platform_device *pdev)
214 {
215 	struct i2c_mux_core *muxc = platform_get_drvdata(pdev);
216 
217 	i2c_mux_del_adapters(muxc);
218 	i2c_put_adapter(muxc->parent);
219 	return 0;
220 }
221 
222 static const struct of_device_id i2c_arbitrator_of_match[] = {
223 	{ .compatible = "i2c-arb-gpio-challenge", },
224 	{},
225 };
226 MODULE_DEVICE_TABLE(of, i2c_arbitrator_of_match);
227 
228 static struct platform_driver i2c_arbitrator_driver = {
229 	.probe	= i2c_arbitrator_probe,
230 	.remove	= i2c_arbitrator_remove,
231 	.driver	= {
232 		.name	= "i2c-arb-gpio-challenge",
233 		.of_match_table = i2c_arbitrator_of_match,
234 	},
235 };
236 
237 module_platform_driver(i2c_arbitrator_driver);
238 
239 MODULE_DESCRIPTION("GPIO-based I2C Arbitration");
240 MODULE_AUTHOR("Doug Anderson <dianders@chromium.org>");
241 MODULE_LICENSE("GPL v2");
242 MODULE_ALIAS("platform:i2c-arb-gpio-challenge");
243