--- /dev/null
+I2C Bus Arbitration
+===================
+
+While I2C supports multi-master buses this is difficult to get right.
+The implementation on the master side in software is quite complex.
+Clock-stretching and the arbitrary time that an I2C transaction can take
+make it difficult to share the bus fairly in the face of high traffic.
+When one or more masters can be reset independently part-way through a
+transaction it is hard to know the state of the bus.
+
+U-Boot provides a scheme based on two 'claim' GPIOs, one driven by the
+AP (Application Processor, meaning the main CPU) and one driven by the EC
+(Embedded Controller, a small CPU aimed at handling system tasks). With
+these they can communicate and reliably share the bus. This scheme has
+minimal overhead and involves very little code. The scheme can survive
+reboots by either side without difficulty.
+
+Since U-Boot runs on the AP, the terminology used is 'our' claim GPIO,
+meaning the AP's, and 'their' claim GPIO, meaning the EC's. This terminology
+is used by the device tree bindings in Linux also.
+
+The driver is implemented as an I2C mux, as it is in Linux. See
+i2c-arb-gpio-challenge for the implementation.
+
+GPIO lines are shared between the AP and EC to manage the bus. The AP and EC
+each have a 'bus claim' line, which is an output that the other can see.
+
+- AP_CLAIM: output from AP, signalling to the EC that the AP wants the bus
+- EC_CLAIM: output from EC, signalling to the AP that the EC wants 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 the AP wants to claim the bus. It:
+
+1. Asserts AP_CLAIM
+2. Waits a little bit for the other side to notice (slew time)
+3. Checks EC_CLAIM. If this is not asserted, then the AP has the bus, and we
+ are done
+4. Otherwise, wait for a few milliseconds (retry time) and see if EC_CLAIM is
+ released
+5. If not, back off, release the claim and wait for a few more milliseconds
+ (retry time again)
+6. Go back to 1 if things don't look wedged (wait time has expired)
+7. Panic. The other side is hung with the CLAIM line set.
+
+The same algorithm applies on the EC.
+
+To release the bus, just de-assert the claim line.
+
+Typical delays are:
+- slew time 10 us
+- retry time 3 ms
+- wait time - 50ms
+
+In general the traffic is fairly light, and in particular the EC wants access
+to the bus quite rarely (maybe every 10s or 30s to check the battery). This
+scheme works very nicely with very low contention. There is only a 10 us
+wait for access to the bus assuming that the other side isn't using it.
--- /dev/null
+/*
+ * Copyright (c) 2015 Google, Inc
+ * Written by Simon Glass <sjg@chromium.org>
+ *
+ * SPDX-License-Identifier: GPL-2.0+
+ */
+
+#include <common.h>
+#include <dm.h>
+#include <errno.h>
+#include <i2c.h>
+#include <asm/gpio.h>
+
+DECLARE_GLOBAL_DATA_PTR;
+
+struct i2c_arbitrator_priv {
+ struct gpio_desc ap_claim;
+ struct gpio_desc ec_claim;
+ uint slew_delay_us;
+ uint wait_retry_ms;
+ uint wait_free_ms;
+};
+
+int i2c_arbitrator_deselect(struct udevice *mux, struct udevice *bus,
+ uint channel)
+{
+ struct i2c_arbitrator_priv *priv = dev_get_priv(mux);
+ int ret;
+
+ debug("%s: %s\n", __func__, mux->name);
+ ret = dm_gpio_set_value(&priv->ap_claim, 0);
+ udelay(priv->slew_delay_us);
+
+ return ret;
+}
+
+int i2c_arbitrator_select(struct udevice *mux, struct udevice *bus,
+ uint channel)
+{
+ struct i2c_arbitrator_priv *priv = dev_get_priv(mux);
+ unsigned start;
+ int ret;
+
+ debug("%s: %s\n", __func__, mux->name);
+ /* Start a round of trying to claim the bus */
+ start = get_timer(0);
+ do {
+ unsigned start_retry;
+ int waiting = 0;
+
+ /* Indicate that we want to claim the bus */
+ ret = dm_gpio_set_value(&priv->ap_claim, 1);
+ if (ret)
+ goto err;
+ udelay(priv->slew_delay_us);
+
+ /* Wait for the EC to release it */
+ start_retry = get_timer(0);
+ while (get_timer(start_retry) < priv->wait_retry_ms) {
+ ret = dm_gpio_get_value(&priv->ec_claim);
+ if (ret < 0) {
+ goto err;
+ } else if (!ret) {
+ /* We got it, so return */
+ return 0;
+ }
+
+ if (!waiting)
+ waiting = 1;
+ }
+
+ /* It didn't release, so give up, wait, and try again */
+ ret = dm_gpio_set_value(&priv->ap_claim, 0);
+ if (ret)
+ goto err;
+
+ mdelay(priv->wait_retry_ms);
+ } while (get_timer(start) < priv->wait_free_ms);
+
+ /* Give up, release our claim */
+ printf("I2C: Could not claim bus, timeout %lu\n", get_timer(start));
+ ret = -ETIMEDOUT;
+ ret = 0;
+err:
+ return ret;
+}
+
+static int i2c_arbitrator_probe(struct udevice *dev)
+{
+ struct i2c_arbitrator_priv *priv = dev_get_priv(dev);
+ const void *blob = gd->fdt_blob;
+ int node = dev->of_offset;
+ int ret;
+
+ debug("%s: %s\n", __func__, dev->name);
+ priv->slew_delay_us = fdtdec_get_int(blob, node, "slew-delay-us", 0);
+ priv->wait_retry_ms = fdtdec_get_int(blob, node, "wait-retry-us", 0) /
+ 1000;
+ priv->wait_free_ms = fdtdec_get_int(blob, node, "wait-free-us", 0) /
+ 1000;
+ ret = gpio_request_by_name(dev, "our-claim-gpio", 0, &priv->ap_claim,
+ GPIOD_IS_OUT);
+ if (ret)
+ goto err;
+ ret = gpio_request_by_name(dev, "their-claim-gpios", 0, &priv->ec_claim,
+ GPIOD_IS_IN);
+ if (ret)
+ goto err_ec_gpio;
+
+ return 0;
+
+err_ec_gpio:
+ dm_gpio_free(dev, &priv->ap_claim);
+err:
+ debug("%s: ret=%d\n", __func__, ret);
+ return ret;
+}
+
+static int i2c_arbitrator_remove(struct udevice *dev)
+{
+ struct i2c_arbitrator_priv *priv = dev_get_priv(dev);
+
+ dm_gpio_free(dev, &priv->ap_claim);
+ dm_gpio_free(dev, &priv->ec_claim);
+
+ return 0;
+}
+
+static const struct i2c_mux_ops i2c_arbitrator_ops = {
+ .select = i2c_arbitrator_select,
+ .deselect = i2c_arbitrator_deselect,
+};
+
+static const struct udevice_id i2c_arbitrator_ids[] = {
+ { .compatible = "i2c-arb-gpio-challenge" },
+ { }
+};
+
+U_BOOT_DRIVER(i2c_arbitrator) = {
+ .name = "i2c_arbitrator",
+ .id = UCLASS_I2C_MUX,
+ .of_match = i2c_arbitrator_ids,
+ .probe = i2c_arbitrator_probe,
+ .remove = i2c_arbitrator_remove,
+ .ops = &i2c_arbitrator_ops,
+ .priv_auto_alloc_size = sizeof(struct i2c_arbitrator_priv),
+};
projects / u-boot / commitdiff