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
 42struct 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 */
 58static 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 */
101static 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
112static 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
213static 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
222static const struct of_device_id i2c_arbitrator_of_match[] = {
223	{ .compatible = "i2c-arb-gpio-challenge", },
224	{},
225};
226MODULE_DEVICE_TABLE(of, i2c_arbitrator_of_match);
227
228static 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
237module_platform_driver(i2c_arbitrator_driver);
238
239MODULE_DESCRIPTION("GPIO-based I2C Arbitration");
240MODULE_AUTHOR("Doug Anderson <dianders@chromium.org>");
241MODULE_LICENSE("GPL v2");
242MODULE_ALIAS("platform:i2c-arb-gpio-challenge");