Date:	Wed, 19 Sep 2012 23:08:13 +0300
From:	Aaro Koskinen <aaro.koskinen@....fi>
To:	Wolfram Sang <w.sang@...gutronix.de>
Cc:	Jean Delvare <khali@...ux-fr.org>, linux-omap@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-i2c@...r.kernel.org
Subject: Re: [PATCH 1/4] i2c: introduce i2c-cbus driver

On Fri, Sep 14, 2012 at 12:08:06PM +0200, Wolfram Sang wrote:
> On Mon, Sep 03, 2012 at 11:23:22PM +0300, Aaro Koskinen wrote:
> > Add i2c driver to enable access to devices behind CBUS on Nokia Internet
> > Tablets.
> > 
> > The patch also adds CBUS I2C configuration for N8x0 which is one of the
> > users of this driver.
> > 
> > Cc: linux-i2c@...r.kernel.org
> > Acked-by: Felipe Balbi <balbi@...com>
> > Acked-by: Tony Lindgren <tony@...mide.com>
> > Signed-off-by: Aaro Koskinen <aaro.koskinen@....fi>
> 
> OK, I found the short paragrahp about CBUS in the I2C spec, so I2C might
> be an appropriate place. Still, before deciding if it should rather be
> in the core directory, I still have a few questions.

Thanks for your feedback. I will fix up the incorrect comments, and do
other improvements (e.g. use devm_* stuff) in the next version.

The questions about delays and bit counts are very valid, but it's
difficult to do anything with them due to lack of documentation and HW on
which to test - I have Nokia tablets to test the driver, and the driver
is very reliable, but I do not know of any other HW with CBUS. :-/

A.

> Also, does anybody know of a generic bit-banging implementation in the
> kernel which could be used here?
> 
> Jean: I'd appreciate your general opinion here, too.
> 
> > diff --git a/drivers/i2c/busses/i2c-cbus.c b/drivers/i2c/busses/i2c-cbus.c
> > new file mode 100644
> > index 0000000..bacf2a9
> > --- /dev/null
> > +++ b/drivers/i2c/busses/i2c-cbus.c
> > @@ -0,0 +1,369 @@
> > +/*
> > + * CBUS I2C driver for Nokia Internet Tablets.
> > + *
> > + * Copyright (C) 2004-2010 Nokia Corporation
> > + *
> > + * Based on code written by Juha Yrjölä, David Weinehall, Mikko Ylinen and
> > + * Felipe Balbi. Converted to I2C driver by Aaro Koskinen.
> > + *
> > + * This file is subject to the terms and conditions of the GNU General
> > + * Public License. See the file "COPYING" in the main directory of this
> > + * archive for more details.
> > + *
> > + * 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/io.h>
> > +#include <linux/i2c.h>
> > +#include <linux/gpio.h>
> > +#include <linux/init.h>
> > +#include <linux/slab.h>
> > +#include <linux/delay.h>
> > +#include <linux/errno.h>
> > +#include <linux/kernel.h>
> > +#include <linux/module.h>
> > +#include <linux/of_gpio.h>
> > +#include <linux/i2c-cbus.h>
> > +#include <linux/interrupt.h>
> > +#include <linux/platform_device.h>
> > +
> > +struct cbus_host {
> > +	/* host lock */
> > +	spinlock_t	lock;
> > +
> > +	struct device	*dev;
> > +
> > +	int		clk_gpio;
> > +	int		dat_gpio;
> > +	int		sel_gpio;
> > +};
> > +
> > +/**
> > + * cbus_send_bit - sends one bit over the bus
> > + * @host: the host we're using
> > + * @bit: one bit of information to send
> > + * @input: whether to set data pin as input after sending
> > + */
> > +static int cbus_send_bit(struct cbus_host *host, unsigned bit,
> > +		unsigned input)
> > +{
> > +	int ret = 0;
> > +
> > +	gpio_set_value(host->dat_gpio, bit ? 1 : 0);
> > +	gpio_set_value(host->clk_gpio, 1);
> > +
> > +	/* The data bit is read on the rising edge of CLK */
> 
> This comment doesn't belong to the if-block, or?
> 
> > +	if (input)
> > +		ret = gpio_direction_input(host->dat_gpio);
> 
> No minimum delay for the signal to be on the bus?
> 
> > +
> > +	gpio_set_value(host->clk_gpio, 0);
> > +
> > +	return ret;
> > +}
> > +
> > +/**
> > + * cbus_send_data - sends @len amount of data over the bus
> > + * @host: the host we're using
> > + * @data: the data to send
> > + * @len: size of the transfer
> > + * @input: whether to set data pin as input after sending
> > + */
> > +static int cbus_send_data(struct cbus_host *host, unsigned data, unsigned len,
> > +		unsigned input)
> > +{
> > +	int ret = 0;
> > +	int i;
> > +
> > +	for (i = len; i > 0; i--) {
> > +		ret = cbus_send_bit(host, data & (1 << (i - 1)),
> > +				input && (i == 1));
> > +		if (ret < 0)
> > +			goto out;
> > +	}
> > +
> > +out:
> > +	return ret;
> > +}
> > +
> > +/**
> > + * cbus_receive_bit - receives one bit from the bus
> > + * @host: the host we're using
> > + */
> > +static int cbus_receive_bit(struct cbus_host *host)
> > +{
> > +	int ret;
> > +
> > +	gpio_set_value(host->clk_gpio, 1);
> 
> No delays to ensure the signal has stabilized?
> 
> > +	ret = gpio_get_value(host->dat_gpio);
> > +	if (ret < 0)
> > +		goto out;
> > +	gpio_set_value(host->clk_gpio, 0);
> > +
> > +out:
> > +	return ret;
> > +}
> > +
> > +/**
> > + * cbus_receive_word - receives 16-bit word from the bus
> > + * @host: the host we're using
> > + */
> > +static int cbus_receive_word(struct cbus_host *host)
> > +{
> > +	int ret = 0;
> > +	int i;
> > +
> > +	for (i = 16; i > 0; i--) {
> > +		int bit = cbus_receive_bit(host);
> > +
> > +		if (bit < 0)
> > +			goto out;
> > +
> > +		if (bit)
> > +			ret |= 1 << (i - 1);
> > +	}
> > +
> > +out:
> > +	return ret;
> > +}
> > +
> > +/**
> > + * cbus_transfer - transfers data over the bus
> > + * @host: the host we're using
> > + * @rw: read/write flag
> > + * @dev: device address
> > + * @reg: register address
> > + * @data: if @rw == I2C_SBUS_WRITE data to send otherwise 0
> > + */
> > +static int cbus_transfer(struct cbus_host *host, char rw, unsigned dev,
> > +			 unsigned reg, unsigned data)
> > +{
> > +	unsigned long flags;
> > +	int ret;
> > +
> > +	/* We don't want interrupts disturbing our transfer */
> > +	spin_lock_irqsave(&host->lock, flags);
> > +
> > +	/* Reset state and start of transfer, SEL stays down during transfer */
> > +	gpio_set_value(host->sel_gpio, 0);
> > +
> > +	/* Set the DAT pin to output */
> > +	gpio_direction_output(host->dat_gpio, 1);
> > +
> > +	/* Send the device address */
> > +	ret = cbus_send_data(host, dev, 3, 0);
> 
> Is the device address always 3 bit? From the (admittetly sparse) I2C
> spec, I'd assume to be 7?
> 
> > +	if (ret < 0) {
> > +		dev_dbg(host->dev, "failed sending device addr\n");
> > +		goto out;
> > +	}
> > +
> > +	/* Send the rw flag */
> > +	ret = cbus_send_bit(host, rw == I2C_SMBUS_READ, 0);
> > +	if (ret < 0) {
> > +		dev_dbg(host->dev, "failed sending read/write flag\n");
> > +		goto out;
> > +	}
> > +
> > +	/* Send the device address */
> 
> Comment is wrong.
> 
> > +	ret = cbus_send_data(host, reg, 5, rw == I2C_SMBUS_READ);
> 
> Is reg always 5 bit?
> 
> > +	if (ret < 0) {
> > +		dev_dbg(host->dev, "failed sending register addr\n");
> > +		goto out;
> > +	}
> > +
> > +	if (rw == I2C_SMBUS_WRITE) {
> > +		ret = cbus_send_data(host, data, 16, 0);
> > +		if (ret < 0) {
> > +			dev_dbg(host->dev, "failed sending data\n");
> > +			goto out;
> > +		}
> > +	} else {
> 
> What about setting dat to input here and drop the parameter from
> cbus_data_send()?
> 
> > +		gpio_set_value(host->clk_gpio, 1);
> > +
> > +		ret = cbus_receive_word(host);
> > +		if (ret < 0) {
> > +			dev_dbg(host->dev, "failed receiving data\n");
> > +			goto out;
> > +		}
> > +	}
> > +
> > +	/* Indicate end of transfer, SEL goes up until next transfer */
> > +	gpio_set_value(host->sel_gpio, 1);
> > +	gpio_set_value(host->clk_gpio, 1);
> > +	gpio_set_value(host->clk_gpio, 0);
> > +
> > +out:
> > +	spin_unlock_irqrestore(&host->lock, flags);
> > +
> > +	return ret;
> > +}
> > +
> > +static int cbus_i2c_smbus_xfer(struct i2c_adapter	*adapter,
> > +			       u16			addr,
> > +			       unsigned short		flags,
> > +			       char			read_write,
> > +			       u8			command,
> > +			       int			size,
> > +			       union i2c_smbus_data	*data)
> > +{
> > +	struct cbus_host *chost = i2c_get_adapdata(adapter);
> > +	int ret;
> > +
> > +	if (size != I2C_SMBUS_WORD_DATA)
> > +		return -EINVAL;
> > +
> > +	ret = cbus_transfer(chost, read_write == I2C_SMBUS_READ, addr,
> > +			    command, data->word);
> > +	if (ret < 0)
> > +		return ret;
> > +
> > +	if (read_write == I2C_SMBUS_READ)
> > +		data->word = ret;
> > +
> > +	return 0;
> > +}
> > +
> > +static u32 cbus_i2c_func(struct i2c_adapter *adapter)
> > +{
> > +	return I2C_FUNC_SMBUS_READ_WORD_DATA | I2C_FUNC_SMBUS_WRITE_WORD_DATA;
> > +}
> > +
> > +static const struct i2c_algorithm cbus_i2c_algo = {
> > +	.smbus_xfer	= cbus_i2c_smbus_xfer,
> > +	.functionality	= cbus_i2c_func,
> > +};
> > +
> > +static int cbus_i2c_remove(struct platform_device *pdev)
> > +{
> > +	struct i2c_adapter *adapter = platform_get_drvdata(pdev);
> > +	struct cbus_host *chost = i2c_get_adapdata(adapter);
> > +	int ret;
> > +
> > +	ret = i2c_del_adapter(adapter);
> > +	if (ret)
> > +		return ret;
> > +	gpio_free(chost->clk_gpio);
> > +	gpio_free(chost->dat_gpio);
> > +	gpio_free(chost->sel_gpio);
> > +	kfree(chost);
> > +	kfree(adapter);
> 
> Please use devm_gpio_* and devm_kzalloc_*
> 
> > +	platform_set_drvdata(pdev, NULL);
> > +
> > +	return 0;
> > +}
> > +
> > +static int cbus_i2c_probe(struct platform_device *pdev)
> > +{
> > +	struct i2c_adapter *adapter;
> > +	struct cbus_host *chost;
> > +	struct gpio gpios[3];
> > +	int ret;
> > +
> > +	adapter = kzalloc(sizeof(struct i2c_adapter), GFP_KERNEL);
> > +	if (!adapter) {
> > +		ret = -ENOMEM;
> > +		goto error;
> > +	}
> > +
> > +	chost = kzalloc(sizeof(*chost), GFP_KERNEL);
> > +	if (!chost) {
> > +		ret = -ENOMEM;
> > +		goto error_mem;
> > +	}
> > +
> > +	if (pdev->dev.of_node) {
> > +		struct device_node *dnode = pdev->dev.of_node;
> > +		if (of_gpio_count(dnode) != 3) {
> > +			ret = -ENODEV;
> > +			goto error_mem;
> > +		}
> > +		chost->clk_gpio = of_get_gpio(dnode, 0);
> > +		chost->dat_gpio = of_get_gpio(dnode, 1);
> > +		chost->sel_gpio = of_get_gpio(dnode, 2);
> > +	} else if (pdev->dev.platform_data) {
> > +		struct i2c_cbus_platform_data *pdata = pdev->dev.platform_data;
> > +		chost->clk_gpio = pdata->clk_gpio;
> > +		chost->dat_gpio = pdata->dat_gpio;
> > +		chost->sel_gpio = pdata->sel_gpio;
> > +	} else {
> > +		ret = -ENODEV;
> > +		goto error_mem;
> > +	}
> > +
> > +	adapter->owner		= THIS_MODULE;
> > +	adapter->class		= I2C_CLASS_HWMON;
> > +	adapter->dev.parent	= &pdev->dev;
> > +	adapter->nr		= pdev->id;
> > +	adapter->timeout	= HZ;
> > +	adapter->algo		= &cbus_i2c_algo;
> > +	strlcpy(adapter->name, "CBUS I2C adapter", sizeof(adapter->name));
> > +
> > +	spin_lock_init(&chost->lock);
> > +	chost->dev = &pdev->dev;
> > +
> > +	gpios[0].gpio  = chost->clk_gpio;
> > +	gpios[0].flags = GPIOF_OUT_INIT_LOW;
> > +	gpios[0].label = "CBUS clk";
> > +
> > +	gpios[1].gpio  = chost->dat_gpio;
> > +	gpios[1].flags = GPIOF_IN;
> > +	gpios[1].label = "CBUS data";
> > +
> > +	gpios[2].gpio  = chost->sel_gpio;
> > +	gpios[2].flags = GPIOF_OUT_INIT_HIGH;
> > +	gpios[2].label = "CBUS sel";
> > +
> > +	ret = gpio_request_array(gpios, ARRAY_SIZE(gpios));
> > +	if (ret)
> > +		goto error_gpio;
> > +
> > +	gpio_set_value(chost->clk_gpio, 1);
> > +	gpio_set_value(chost->clk_gpio, 0);
> 
> What is that pulse needed for?
> 
> > +
> > +	i2c_set_adapdata(adapter, chost);
> > +	platform_set_drvdata(pdev, adapter);
> > +
> > +	ret = i2c_add_numbered_adapter(adapter);
> > +	if (ret)
> > +		goto error_i2c;
> > +
> > +	return 0;
> > +
> > +error_i2c:
> > +	gpio_free_array(gpios, ARRAY_SIZE(gpios));
> > +error_gpio:
> > +	kfree(chost);
> > +error_mem:
> > +	kfree(adapter);
> > +error:
> > +	return ret;
> > +}
> > +
> > +#if defined(CONFIG_OF)
> > +static const struct of_device_id i2c_cbus_dt_ids[] = {
> > +	{ .compatible = "i2c-cbus", },
> > +	{ }
> > +};
> > +MODULE_DEVICE_TABLE(of, i2c_cbus_dt_ids);
> > +#endif
> > +
> > +static struct platform_driver cbus_i2c_driver = {
> > +	.probe	= cbus_i2c_probe,
> > +	.remove	= cbus_i2c_remove,
> > +	.driver	= {
> > +		.owner	= THIS_MODULE,
> > +		.name	= "i2c-cbus",
> > +	},
> > +};
> > +module_platform_driver(cbus_i2c_driver);
> > +
> > +MODULE_ALIAS("platform:i2c-cbus");
> > +MODULE_DESCRIPTION("CBUS I2C driver");
> > +MODULE_AUTHOR("Juha Yrjölä");
> > +MODULE_AUTHOR("David Weinehall");
> > +MODULE_AUTHOR("Mikko Ylinen");
> > +MODULE_AUTHOR("Felipe Balbi");
> > +MODULE_AUTHOR("Aaro Koskinen <aaro.koskinen@....fi>");
> > +MODULE_LICENSE("GPL");
> 
> So much for now.
> 
> Regards,
> 
>    Wolfram
> 
> -- 
> Pengutronix e.K.                           | Wolfram Sang                |
> Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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