i2c: mux: Add i2c-arb-gpio-challenge 'mux' driver

The i2c-arb-gpio-challenge driver implements an I2C arbitration scheme
where masters need to claim the bus with a GPIO before they can start
a transaction.  This should generally only be used when standard I2C
multimaster isn't appropriate for some reason (errata/bugs).

This driver is based on code that Simon Glass added to the i2c-s3c2410
driver in the Chrome OS kernel 3.4 tree.  The current incarnation as a
mux driver is as suggested by Grant Likely.  See
<https://patchwork.kernel.org/patch/1877311/> for some history.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Naveen Krishna Chatradhi <ch.naveen@samsung.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

4 files changed, 345 insertions, 0 deletions

diff --git a/Documentation/devicetree/bindings/i2c/i2c-arb-gpio-challenge.txt b/Documentation/devicetree/bindings/i2c/i2c-arb-gpio-challenge.txt
new file mode 100644
index 000000000000..1ac8ea8ade1d
--- /dev/null
+++ b/Documentation/devicetree/bindings/i2c/i2c-arb-gpio-challenge.txt
@@ -0,0 +1,80 @@
+GPIO-based I2C Arbitration Using a Challenge & Response Mechanism
+=================================================================
+This uses GPIO lines and a challenge & response mechanism to arbitrate who is
+the master of an I2C bus in a multimaster situation.
+
+In many cases using GPIOs to arbitrate is not needed and a design can use
+the standard I2C multi-master rules.  Using GPIOs is generally useful in
+the case where there is a device on the bus that has errata and/or bugs
+that makes standard multimaster mode not feasible.
+
+
+Algorithm:
+
+All masters on the bus have a 'bus claim' line which is an output that the
+others can see. These are all active low with pull-ups enabled.  We'll
+describe these lines as:
+
+- OUR_CLAIM: output from us signaling to other hosts that we want the bus
+- THEIR_CLAIMS: output from others signaling that they want the bus
+
+The basic algorithm is to assert your line when you want the bus, then make
+sure that the other side doesn't want it also.  A detailed explanation is best
+done with an example.
+
+Let's say we want to claim the bus.  We:
+1. Assert OUR_CLAIM.
+2. Waits a little bit for the other sides to notice (slew time, say 10
+   microseconds).
+3. Check THEIR_CLAIMS.  If none are asserted then the we have the bus and we are
+   done.
+4. Otherwise, wait for a few milliseconds and see if THEIR_CLAIMS are released.
+5. If not, back off, release the claim and wait for a few more milliseconds.
+6. Go back to 1 (until retry time has expired).
+
+
+Required properties:
+- compatible: i2c-arb-gpio-challenge
+- our-claim-gpio: The GPIO that we use to claim the bus.
+- their-claim-gpios: The GPIOs that the other sides use to claim the bus.
+  Note that some implementations may only support a single other master.
+- Standard I2C mux properties. See mux.txt in this directory.
+- Single I2C child bus node at reg 0. See mux.txt in this directory.
+
+Optional properties:
+- slew-delay-us: microseconds to wait for a GPIO to go high. Default is 10 us.
+- wait-retry-us: we'll attempt another claim after this many microseconds.
+    Default is 3000 us.
+- wait-free-us: we'll give up after this many microseconds. Default is 50000 us.
+
+
+Example:
+        i2c@12CA0000 {
+                compatible = "acme,some-i2c-device";
+                #address-cells = <1>;
+                #size-cells = <0>;
+        };
+
+        i2c-arbitrator {
+                compatible = "i2c-arb-gpio-challenge";
+                #address-cells = <1>;
+                #size-cells = <0>;
+
+                i2c-parent = <&{/i2c@12CA0000}>;
+
+                our-claim-gpio = <&gpf0 3 1>;
+                their-claim-gpios = <&gpe0 4 1>;
+                slew-delay-us = <10>;
+                wait-retry-us = <3000>;
+                wait-free-us = <50000>;
+
+                i2c@0 {
+                        reg = <0>;
+                        #address-cells = <1>;
+                        #size-cells = <0>;
+
+                        i2c@52 {
+                                // Normal I2C device
+                        };
+                };
+        };
diff --git a/drivers/i2c/muxes/Kconfig b/drivers/i2c/muxes/Kconfig
index 0be5b83c08fa..5faf244d2476 100644
--- a/drivers/i2c/muxes/Kconfig
+++ b/drivers/i2c/muxes/Kconfig
@@ -5,6 +5,18 @@
 menu "Multiplexer I2C Chip support"
         depends on I2C_MUX
 
+config I2C_ARB_GPIO_CHALLENGE
+        tristate "GPIO-based I2C arbitration"
+        depends on GENERIC_GPIO && OF
+        help
+          If you say yes to this option, support will be included for an
+          I2C multimaster arbitration scheme using GPIOs and a challenge &
+          response mechanism where masters have to claim the bus by asserting
+          a GPIO.
+
+          This driver can also be built as a module.  If so, the module
+          will be called i2c-arb-gpio-challenge.
+
 config I2C_MUX_GPIO
         tristate "GPIO-based I2C multiplexer"
         depends on GENERIC_GPIO
diff --git a/drivers/i2c/muxes/Makefile b/drivers/i2c/muxes/Makefile
index 76da8692afff..465778b5d5dc 100644
--- a/drivers/i2c/muxes/Makefile
+++ b/drivers/i2c/muxes/Makefile
@@ -1,6 +1,8 @@
 #
 # Makefile for multiplexer I2C chip drivers.
 
+obj-$(CONFIG_I2C_ARB_GPIO_CHALLENGE)    += i2c-arb-gpio-challenge.o
+
 obj-$(CONFIG_I2C_MUX_GPIO)      += i2c-mux-gpio.o
 obj-$(CONFIG_I2C_MUX_PCA9541)   += i2c-mux-pca9541.o
 obj-$(CONFIG_I2C_MUX_PCA954x)   += i2c-mux-pca954x.o
diff --git a/drivers/i2c/muxes/i2c-arb-gpio-challenge.c b/drivers/i2c/muxes/i2c-arb-gpio-challenge.c
new file mode 100644
index 000000000000..210b6f7b9028
--- /dev/null
+++ b/drivers/i2c/muxes/i2c-arb-gpio-challenge.c
@@ -0,0 +1,251 @@
+/*
+ * GPIO-based I2C Arbitration Using a Challenge & Response Mechanism
+ *
+ * Copyright (C) 2012 Google, Inc
+ *
+ * This software is licensed under the terms of the GNU General Public
+ * License version 2, as published by the Free Software Foundation, and
+ * may be copied, distributed, and modified under those terms.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ */
+
+#include <linux/delay.h>
+#include <linux/gpio.h>
+#include <linux/kernel.h>
+#include <linux/i2c.h>
+#include <linux/i2c-mux.h>
+#include <linux/init.h>
+#include <linux/module.h>
+#include <linux/of_i2c.h>
+#include <linux/of_gpio.h>
+#include <linux/platform_device.h>
+#include <linux/slab.h>
+
+
+/**
+ * struct i2c_arbitrator_data - Driver data for I2C arbitrator
+ *
+ * @parent: Parent adapter
+ * @child: Child bus
+ * @our_gpio: GPIO we'll use to claim.
+ * @our_gpio_release: 0 if active high; 1 if active low; AKA if the GPIO ==
+ *   this then consider it released.
+ * @their_gpio: GPIO that the other side will use to claim.
+ * @their_gpio_release: 0 if active high; 1 if active low; AKA if the GPIO ==
+ *   this then consider it released.
+ * @slew_delay_us: microseconds to wait for a GPIO to go high.
+ * @wait_retry_us: we'll attempt another claim after this many microseconds.
+ * @wait_free_us: we'll give up after this many microseconds.
+ */
+
+struct i2c_arbitrator_data {
+        struct i2c_adapter *parent;
+        struct i2c_adapter *child;
+        int our_gpio;
+        int our_gpio_release;
+        int their_gpio;
+        int their_gpio_release;
+        unsigned int slew_delay_us;
+        unsigned int wait_retry_us;
+        unsigned int wait_free_us;
+};
+
+
+/**
+ * i2c_arbitrator_select - claim the I2C bus
+ *
+ * Use the GPIO-based signalling protocol; return -EBUSY if we fail.
+ */
+static int i2c_arbitrator_select(struct i2c_adapter *adap, void *data, u32 chan)
+{
+        const struct i2c_arbitrator_data *arb = data;
+        unsigned long stop_retry, stop_time;
+
+        /* Start a round of trying to claim the bus */
+        stop_time = jiffies + usecs_to_jiffies(arb->wait_free_us) + 1;
+        do {
+                /* Indicate that we want to claim the bus */
+                gpio_set_value(arb->our_gpio, !arb->our_gpio_release);
+                udelay(arb->slew_delay_us);
+
+                /* Wait for the other master to release it */
+                stop_retry = jiffies + usecs_to_jiffies(arb->wait_retry_us) + 1;
+                while (time_before(jiffies, stop_retry)) {
+                        int gpio_val = !!gpio_get_value(arb->their_gpio);
+
+                        if (gpio_val == arb->their_gpio_release) {
+                                /* We got it, so return */
+                                return 0;
+                        }
+
+                        usleep_range(50, 200);
+                }
+
+                /* It didn't release, so give up, wait, and try again */
+                gpio_set_value(arb->our_gpio, arb->our_gpio_release);
+
+                usleep_range(arb->wait_retry_us, arb->wait_retry_us * 2);
+        } while (time_before(jiffies, stop_time));
+
+        /* Give up, release our claim */
+        gpio_set_value(arb->our_gpio, arb->our_gpio_release);
+        udelay(arb->slew_delay_us);
+        dev_err(&adap->dev, "Could not claim bus, timeout\n");
+        return -EBUSY;
+}
+
+/**
+ * i2c_arbitrator_deselect - release the I2C bus
+ *
+ * Release the I2C bus using the GPIO-based signalling protocol.
+ */
+static int i2c_arbitrator_deselect(struct i2c_adapter *adap, void *data,
+                                   u32 chan)
+{
+        const struct i2c_arbitrator_data *arb = data;
+
+        /* Release the bus and wait for the other master to notice */
+        gpio_set_value(arb->our_gpio, arb->our_gpio_release);
+        udelay(arb->slew_delay_us);
+
+        return 0;
+}
+
+static int i2c_arbitrator_probe(struct platform_device *pdev)
+{
+        struct device *dev = &pdev->dev;
+        struct device_node *np = dev->of_node;
+        struct device_node *parent_np;
+        struct i2c_arbitrator_data *arb;
+        enum of_gpio_flags gpio_flags;
+        unsigned long out_init;
+        int ret;
+
+        /* We only support probing from device tree; no platform_data */
+        if (!np) {
+                dev_err(dev, "Cannot find device tree node\n");
+                return -ENODEV;
+        }
+        if (dev->platform_data) {
+                dev_err(dev, "Platform data is not supported\n");
+                return -EINVAL;
+        }
+
+        arb = devm_kzalloc(dev, sizeof(*arb), GFP_KERNEL);
+        if (!arb) {
+                dev_err(dev, "Cannot allocate i2c_arbitrator_data\n");
+                return -ENOMEM;
+        }
+        platform_set_drvdata(pdev, arb);
+
+        /* Request GPIOs */
+        ret = of_get_named_gpio_flags(np, "our-claim-gpio", 0, &gpio_flags);
+        if (!gpio_is_valid(ret)) {
+                if (ret != -EPROBE_DEFER)
+                        dev_err(dev, "Error getting our-claim-gpio\n");
+                return ret;
+        }
+        arb->our_gpio = ret;
+        arb->our_gpio_release = !!(gpio_flags & OF_GPIO_ACTIVE_LOW);
+        out_init = (gpio_flags & OF_GPIO_ACTIVE_LOW) ?
+                GPIOF_OUT_INIT_HIGH : GPIOF_OUT_INIT_LOW;
+        ret = devm_gpio_request_one(dev, arb->our_gpio, out_init,
+                                    "our-claim-gpio");
+        if (ret) {
+                if (ret != -EPROBE_DEFER)
+                        dev_err(dev, "Error requesting our-claim-gpio\n");
+                return ret;
+        }
+
+        ret = of_get_named_gpio_flags(np, "their-claim-gpios", 0, &gpio_flags);
+        if (!gpio_is_valid(ret)) {
+                if (ret != -EPROBE_DEFER)
+                        dev_err(dev, "Error getting their-claim-gpio\n");
+                return ret;
+        }
+        arb->their_gpio = ret;
+        arb->their_gpio_release = !!(gpio_flags & OF_GPIO_ACTIVE_LOW);
+        ret = devm_gpio_request_one(dev, arb->their_gpio, GPIOF_IN,
+                                    "their-claim-gpio");
+        if (ret) {
+                if (ret != -EPROBE_DEFER)
+                        dev_err(dev, "Error requesting their-claim-gpio\n");
+                return ret;
+        }
+
+        /* At the moment we only support a single two master (us + 1 other) */
+        if (gpio_is_valid(of_get_named_gpio(np, "their-claim-gpios", 1))) {
+                dev_err(dev, "Only one other master is supported\n");
+                return -EINVAL;
+        }
+
+        /* Arbitration parameters */
+        if (of_property_read_u32(np, "slew-delay-us", &arb->slew_delay_us))
+                arb->slew_delay_us = 10;
+        if (of_property_read_u32(np, "wait-retry-us", &arb->wait_retry_us))
+                arb->wait_retry_us = 3000;
+        if (of_property_read_u32(np, "wait-free-us", &arb->wait_free_us))
+                arb->wait_free_us = 50000;
+
+        /* Find our parent */
+        parent_np = of_parse_phandle(np, "i2c-parent", 0);
+        if (!parent_np) {
+                dev_err(dev, "Cannot parse i2c-parent\n");
+                return -EINVAL;
+        }
+        arb->parent = of_find_i2c_adapter_by_node(parent_np);
+        if (!arb->parent) {
+                dev_err(dev, "Cannot find parent bus\n");
+                return -EINVAL;
+        }
+
+        /* Actually add the mux adapter */
+        arb->child = i2c_add_mux_adapter(arb->parent, dev, arb, 0, 0, 0,
+                                         i2c_arbitrator_select,
+                                         i2c_arbitrator_deselect);
+        if (!arb->child) {
+                dev_err(dev, "Failed to add adapter\n");
+                ret = -ENODEV;
+                i2c_put_adapter(arb->parent);
+        }
+
+        return ret;
+}
+
+static int i2c_arbitrator_remove(struct platform_device *pdev)
+{
+        struct i2c_arbitrator_data *arb = platform_get_drvdata(pdev);
+
+        i2c_del_mux_adapter(arb->child);
+        i2c_put_adapter(arb->parent);
+
+        return 0;
+}
+
+static const struct of_device_id i2c_arbitrator_of_match[] = {
+        { .compatible = "i2c-arb-gpio-challenge", },
+        {},
+};
+MODULE_DEVICE_TABLE(of, i2c_arbitrator_of_match);
+
+static struct platform_driver i2c_arbitrator_driver = {
+        .probe  = i2c_arbitrator_probe,
+        .remove = i2c_arbitrator_remove,
+        .driver = {
+                .owner  = THIS_MODULE,
+                .name   = "i2c-arb-gpio-challenge",
+                .of_match_table = of_match_ptr(i2c_arbitrator_of_match),
+        },
+};
+
+module_platform_driver(i2c_arbitrator_driver);
+
+MODULE_DESCRIPTION("GPIO-based I2C Arbitration");
+MODULE_AUTHOR("Doug Anderson <dianders@chromium.org>");
+MODULE_LICENSE("GPL v2");
+MODULE_ALIAS("platform:i2c-arb-gpio-challenge");