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 */
i2c_arbitrator_select(struct i2c_mux_core * muxc,u32 chan)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 */
i2c_arbitrator_deselect(struct i2c_mux_core * muxc,u32 chan)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
i2c_arbitrator_probe(struct platform_device * pdev)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
i2c_arbitrator_remove(struct platform_device * pdev)177 static int 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 return 0;
184 }
185
186 static const struct of_device_id i2c_arbitrator_of_match[] = {
187 { .compatible = "i2c-arb-gpio-challenge", },
188 {},
189 };
190 MODULE_DEVICE_TABLE(of, i2c_arbitrator_of_match);
191
192 static struct platform_driver i2c_arbitrator_driver = {
193 .probe = i2c_arbitrator_probe,
194 .remove = i2c_arbitrator_remove,
195 .driver = {
196 .name = "i2c-arb-gpio-challenge",
197 .of_match_table = i2c_arbitrator_of_match,
198 },
199 };
200
201 module_platform_driver(i2c_arbitrator_driver);
202
203 MODULE_DESCRIPTION("GPIO-based I2C Arbitration");
204 MODULE_AUTHOR("Doug Anderson <dianders@chromium.org>");
205 MODULE_LICENSE("GPL v2");
206 MODULE_ALIAS("platform:i2c-arb-gpio-challenge");
207
