1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3  * GPIO-based I2C Arbitration Using a Challenge & Response Mechanism
4  *
5  * Copyright (C) 2012 Google, Inc
6  */
7 
8 #include <linux/delay.h>
9 #include <linux/gpio/consumer.h>
10 #include <linux/kernel.h>
11 #include <linux/i2c.h>
12 #include <linux/i2c-mux.h>
13 #include <linux/module.h>
14 #include <linux/platform_device.h>
15 #include <linux/slab.h>
16 
17 
18 /**
19  * struct i2c_arbitrator_data - Driver data for I2C arbitrator
20  *
21  * @our_gpio: GPIO descriptor we'll use to claim.
22  * @their_gpio: GPIO descriptor that the other side will use to claim.
23  * @slew_delay_us: microseconds to wait for a GPIO to go high.
24  * @wait_retry_us: we'll attempt another claim after this many microseconds.
25  * @wait_free_us: we'll give up after this many microseconds.
26  */
27 
28 struct i2c_arbitrator_data {
29 	struct gpio_desc *our_gpio;
30 	struct gpio_desc *their_gpio;
31 	unsigned int slew_delay_us;
32 	unsigned int wait_retry_us;
33 	unsigned int wait_free_us;
34 };
35 
36 
37 /*
38  * i2c_arbitrator_select - claim the I2C bus
39  *
40  * Use the GPIO-based signalling protocol; return -EBUSY if we fail.
41  */
42 static int i2c_arbitrator_select(struct i2c_mux_core *muxc, u32 chan)
43 {
44 	const struct i2c_arbitrator_data *arb = i2c_mux_priv(muxc);
45 	unsigned long stop_retry, stop_time;
46 
47 	/* Start a round of trying to claim the bus */
48 	stop_time = jiffies + usecs_to_jiffies(arb->wait_free_us) + 1;
49 	do {
50 		/* Indicate that we want to claim the bus */
51 		gpiod_set_value(arb->our_gpio, 1);
52 		udelay(arb->slew_delay_us);
53 
54 		/* Wait for the other master to release it */
55 		stop_retry = jiffies + usecs_to_jiffies(arb->wait_retry_us) + 1;
56 		while (time_before(jiffies, stop_retry)) {
57 			int gpio_val = gpiod_get_value(arb->their_gpio);
58 
59 			if (!gpio_val) {
60 				/* We got it, so return */
61 				return 0;
62 			}
63 
64 			usleep_range(50, 200);
65 		}
66 
67 		/* It didn't release, so give up, wait, and try again */
68 		gpiod_set_value(arb->our_gpio, 0);
69 
70 		usleep_range(arb->wait_retry_us, arb->wait_retry_us * 2);
71 	} while (time_before(jiffies, stop_time));
72 
73 	/* Give up, release our claim */
74 	gpiod_set_value(arb->our_gpio, 0);
75 	udelay(arb->slew_delay_us);
76 	dev_err(muxc->dev, "Could not claim bus, timeout\n");
77 	return -EBUSY;
78 }
79 
80 /*
81  * i2c_arbitrator_deselect - release the I2C bus
82  *
83  * Release the I2C bus using the GPIO-based signalling protocol.
84  */
85 static int i2c_arbitrator_deselect(struct i2c_mux_core *muxc, u32 chan)
86 {
87 	const struct i2c_arbitrator_data *arb = i2c_mux_priv(muxc);
88 
89 	/* Release the bus and wait for the other master to notice */
90 	gpiod_set_value(arb->our_gpio, 0);
91 	udelay(arb->slew_delay_us);
92 
93 	return 0;
94 }
95 
96 static int i2c_arbitrator_probe(struct platform_device *pdev)
97 {
98 	struct device *dev = &pdev->dev;
99 	struct device_node *np = dev->of_node;
100 	struct device_node *parent_np;
101 	struct i2c_mux_core *muxc;
102 	struct i2c_arbitrator_data *arb;
103 	struct gpio_desc *dummy;
104 	int ret;
105 
106 	/* We only support probing from device tree; no platform_data */
107 	if (!np) {
108 		dev_err(dev, "Cannot find device tree node\n");
109 		return -ENODEV;
110 	}
111 	if (dev_get_platdata(dev)) {
112 		dev_err(dev, "Platform data is not supported\n");
113 		return -EINVAL;
114 	}
115 
116 	muxc = i2c_mux_alloc(NULL, dev, 1, sizeof(*arb), I2C_MUX_ARBITRATOR,
117 			     i2c_arbitrator_select, i2c_arbitrator_deselect);
118 	if (!muxc)
119 		return -ENOMEM;
120 	arb = i2c_mux_priv(muxc);
121 
122 	platform_set_drvdata(pdev, muxc);
123 
124 	/* Request GPIOs, our GPIO as unclaimed to begin with */
125 	arb->our_gpio = devm_gpiod_get(dev, "our-claim", GPIOD_OUT_LOW);
126 	if (IS_ERR(arb->our_gpio)) {
127 		dev_err(dev, "could not get \"our-claim\" GPIO (%ld)\n",
128 			PTR_ERR(arb->our_gpio));
129 		return PTR_ERR(arb->our_gpio);
130 	}
131 
132 	arb->their_gpio = devm_gpiod_get(dev, "their-claim", GPIOD_IN);
133 	if (IS_ERR(arb->their_gpio)) {
134 		dev_err(dev, "could not get \"their-claim\" GPIO (%ld)\n",
135 			PTR_ERR(arb->their_gpio));
136 		return PTR_ERR(arb->their_gpio);
137 	}
138 
139 	/* At the moment we only support a single two master (us + 1 other) */
140 	dummy = devm_gpiod_get_index(dev, "their-claim", 1, GPIOD_IN);
141 	if (!IS_ERR(dummy)) {
142 		dev_err(dev, "Only one other master is supported\n");
143 		return -EINVAL;
144 	} else if (PTR_ERR(dummy) == -EPROBE_DEFER) {
145 		return -EPROBE_DEFER;
146 	}
147 
148 	/* Arbitration parameters */
149 	if (of_property_read_u32(np, "slew-delay-us", &arb->slew_delay_us))
150 		arb->slew_delay_us = 10;
151 	if (of_property_read_u32(np, "wait-retry-us", &arb->wait_retry_us))
152 		arb->wait_retry_us = 3000;
153 	if (of_property_read_u32(np, "wait-free-us", &arb->wait_free_us))
154 		arb->wait_free_us = 50000;
155 
156 	/* Find our parent */
157 	parent_np = of_parse_phandle(np, "i2c-parent", 0);
158 	if (!parent_np) {
159 		dev_err(dev, "Cannot parse i2c-parent\n");
160 		return -EINVAL;
161 	}
162 	muxc->parent = of_get_i2c_adapter_by_node(parent_np);
163 	of_node_put(parent_np);
164 	if (!muxc->parent) {
165 		dev_err(dev, "Cannot find parent bus\n");
166 		return -EPROBE_DEFER;
167 	}
168 
169 	/* Actually add the mux adapter */
170 	ret = i2c_mux_add_adapter(muxc, 0, 0, 0);
171 	if (ret)
172 		i2c_put_adapter(muxc->parent);
173 
174 	return ret;
175 }
176 
177 static void i2c_arbitrator_remove(struct platform_device *pdev)
178 {
179 	struct i2c_mux_core *muxc = platform_get_drvdata(pdev);
180 
181 	i2c_mux_del_adapters(muxc);
182 	i2c_put_adapter(muxc->parent);
183 }
184 
185 static const struct of_device_id i2c_arbitrator_of_match[] = {
186 	{ .compatible = "i2c-arb-gpio-challenge", },
187 	{},
188 };
189 MODULE_DEVICE_TABLE(of, i2c_arbitrator_of_match);
190 
191 static struct platform_driver i2c_arbitrator_driver = {
192 	.probe	= i2c_arbitrator_probe,
193 	.remove_new = i2c_arbitrator_remove,
194 	.driver	= {
195 		.name	= "i2c-arb-gpio-challenge",
196 		.of_match_table = i2c_arbitrator_of_match,
197 	},
198 };
199 
200 module_platform_driver(i2c_arbitrator_driver);
201 
202 MODULE_DESCRIPTION("GPIO-based I2C Arbitration");
203 MODULE_AUTHOR("Doug Anderson <dianders@chromium.org>");
204 MODULE_LICENSE("GPL v2");
205 MODULE_ALIAS("platform:i2c-arb-gpio-challenge");
206