[U-Boot] [PATCH 08/55] i2c: Add a mux for GPIO-based I2C bus arbitration

[Heiko Schocher]

Hello Simon,

Am 03.07.2015 um 02:15 schrieb Simon Glass:
> While I2C supports multi-master buses this is difficult to get right. This

What do you mean here? Where are the problems? You have an i2c mux, or?

> driver provides a scheme based on two 'claim' GPIOs, one driven by the AP
> and one driver by the EC. With these they can communicate and reliably

What is AP and EC ?

> share the bus. This scheme has minimal overhead and involves very little
> code. It is used on snow to permit the EC and the AP to share access to
> the main system PMIC and battery. The scheme can survive reboots by either
> side without difficulty.
>
> Signed-off-by: Simon Glass <sjg at chromium.org>
> ---
>
>   drivers/i2c/muxes/Kconfig                  |   9 ++
>   drivers/i2c/muxes/Makefile                 |   1 +
>   drivers/i2c/muxes/i2c-arb-gpio-challenge.c | 147 +++++++++++++++++++++++++++++

Nice!

Could you add a readme, which explains this a little bit more?

>   3 files changed, 157 insertions(+)
>   create mode 100644 drivers/i2c/muxes/i2c-arb-gpio-challenge.c
>
> diff --git a/drivers/i2c/muxes/Kconfig b/drivers/i2c/muxes/Kconfig
> index a05b32d..bd3e078 100644
> --- a/drivers/i2c/muxes/Kconfig
> +++ b/drivers/i2c/muxes/Kconfig
> @@ -6,3 +6,12 @@ config I2C_MUX
>   	  one of several buses using some sort of control mechanism. The
>   	  bus select is handled automatically when that bus is accessed,
>   	  using a suitable I2C MUX driver.
> +
> +config I2C_ARB_GPIO_CHALLENGE
> +        bool "GPIO-based I2C arbitration"
> +        depends on I2C_MUX
> +        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.
> diff --git a/drivers/i2c/muxes/Makefile b/drivers/i2c/muxes/Makefile
> index 7583e3a..612cc27 100644
> --- a/drivers/i2c/muxes/Makefile
> +++ b/drivers/i2c/muxes/Makefile
> @@ -3,4 +3,5 @@
>   #
>   # SPDX-License-Identifier:	GPL-2.0+
>   #
> +obj-$(CONFIG_I2C_ARB_GPIO_CHALLENGE) += i2c-arb-gpio-challenge.o
>   obj-$(CONFIG_I2C_MUX) += i2c-mux-uclass.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 0000000..3f072c7
> --- /dev/null
> +++ b/drivers/i2c/muxes/i2c-arb-gpio-challenge.c
> @@ -0,0 +1,147 @@
> +/*
> + * Copyright (c) 2015 Google, Inc
> + * Written by Simon Glass <sjg at 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);

What is "our" and "their"?

bye,
Heiko
> +	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),
> +};
>

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