Re: [patch/rfc 2/4] pcf875x I2C GPIO expander driver

[Jean Delvare]

Hi David,

Sorry for the late review.

Note that I can't test your code.

On Mon, 29 Oct 2007 18:51:48 -0700, David Brownell wrote:
> This is a new-style I2C driver for some common 8 and 16 bit I2C based
> GPIO expanders:  pcf8574 and pcf8575.  Since it's a new-style driver,
> it's configured as part of board-specific init ... eliminating the
> need for error-prone manual configuration of module parameters.
> 
> The driver exposes the GPIO signals using the platform-neutral GPIO
> programming interface, so they are easily accessed by other kernel
> code.  The lack of such a flexible kernel API is what has ensured
> the proliferation of board-specific hacks for these chips... stuff
> that rarely makes it upstream since it's so ugly.  This driver will
> let such board-specific code use standard GPIO calls.
> 
> Signed-off-by: David Brownell <dbrownell@xxxxxxxxxxxxxxxxxxxxx>
> ---
> Note that there's currently a drivers/i2c/chips/pcf8574.c driver.

By now there's also a drivers/i2c/chips/pcf8575.c driver.

> 
> Key differences include:  this one talks to other kernel code so
> it can use the GPIOs "normally", but that one talks to userspace
> through sysfs.  Also, this one is a "new style" I2C driver, so it's
> smaller and doesn't need all those error-prone module parameters.
> Plus, this one handles both 8 and 16 bit chip versions.
> 
>  drivers/i2c/chips/Kconfig   |   18 ++
>  drivers/i2c/chips/Makefile  |    1 
>  drivers/i2c/chips/pcf857x.c |  309 ++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/pcf857x.h     |   43 ++++++
>  4 files changed, 371 insertions(+)
> 
> --- a/drivers/i2c/chips/Kconfig	2007-10-28 21:04:06.000000000 -0700
> +++ b/drivers/i2c/chips/Kconfig	2007-10-29 14:16:01.000000000 -0700
> @@ -51,6 +51,24 @@ config SENSORS_EEPROM
>  	  This driver can also be built as a module.  If so, the module
>  	  will be called eeprom.
>  
> +config GPIO_PCF857X
> +	tristate "PCF875x GPIO expanders"
> +	depends on GPIO_LIB
> +	help
> +	  Say yes here to provide access to some I2C GPIO expanders which
> +	  may be used for additional digital outputs or inputs:
> +
> +	    - pcf8574, pcf8574a ... 8 bits, from NXP or TI
> +	    - pcf8575, pcf8575c ... 16 bits, from NXP or TI
> +
> +	  Your board setup code will need to declare the expanders in
> +	  use, and assign numbers to the GPIOs they expose.  Those GPIOs
> +	  can then be used from drivers and other kernel code, just like
> +	  other GPIOs, but only accessible from task contexts.
> +
> +	  This driver provides only an in-kernel interface to those GPIOs.
> +	  Any sysfs interface to userspace would be provided separately.

How?

> +
>  config SENSORS_PCF8574
>  	tristate "Philips PCF8574 and PCF8574A"
>  	depends on EXPERIMENTAL
> --- /dev/null	1970-01-01 00:00:00.000000000 +0000
> +++ b/include/linux/pcf857x.h	2007-10-28 21:09:49.000000000 -0700
> @@ -0,0 +1,43 @@
> +#ifndef __LINUX_PCF857X_H
> +#define __LINUX_PCF857X_H
> +
> +/**
> + * struct pcf857x_platform_data - data to set up pcf857x driver
> + * @gpio_base: number of the chip's first GPIO
> + * @n_latch: optional bit-inverse of initial output state

Strange name, and I can't make much sense of the description either.

> + * @context: optional parameter passed to setup() and teardown()
> + * @setup: optional callback issued once the GPIOs are valid
> + * @teardown: optional callback issued before the GPIOs are invvalidated

Would make more sense to list setup and teardown before context?

Typo: invalidated.

> + *
> + * In addition to the I2C_BOARD_INFO() state appropriate to each chip,
> + * the i2c_board_info used with the pcf875x driver must provide the
> + * chip "type" ("pcf8574", "pcf8574a", "pcf8575", "pcf8575c") and its
> + * platform_data (pointer to one of these structures) with at least
> + * the gpio_base value initialized.
> + *
> + * The @setup callback may be used with the kind of board-specific glue
> + * which hands the (now-valid) GPIOs to other drivers, or which puts
> + * devices in their initial states using these GPIOs.
> + *
> + * Since these GPIO chips don't have conventional registers recording
> + * whether a pin is used for input or output, or an output latch to
> + * record the values being driven, the n_latch value may be used to
> + * avoid intialization glitches.  Its inverted value initializes the

Typo: initialization.

> + * value into which bits are masked before they're written to the PCF
> + * chip.  That means that if it's left at zero, the chip is treated as
> + * if it came from power-up reset.

After reading this paragraph I still have no idea what n_latch does.
But maybe that's just me.

> + */
> +struct pcf857x_platform_data {
> +	unsigned	gpio_base;
> +	unsigned	n_latch;
> +
> +	void		*context;
> +	int		(*setup)(struct i2c_client *client,
> +					int gpio, unsigned ngpio,
> +					void *context);
> +	int		(*teardown)(struct i2c_client *client,
> +					int gpio, unsigned ngpio,
> +					void *context);
> +};
> +
> +#endif /* __LINUX_PCF857X_H */
> --- a/drivers/i2c/chips/Makefile	2007-10-28 21:04:06.000000000 -0700
> +++ b/drivers/i2c/chips/Makefile	2007-10-28 21:09:49.000000000 -0700
> @@ -11,6 +11,7 @@ obj-$(CONFIG_SENSORS_M41T00)	+= m41t00.o
>  obj-$(CONFIG_SENSORS_PCA9539)	+= pca9539.o
>  obj-$(CONFIG_SENSORS_PCF8574)	+= pcf8574.o
>  obj-$(CONFIG_SENSORS_PCF8591)	+= pcf8591.o
> +obj-$(CONFIG_GPIO_PCF857X)	+= pcf857x.o

For alphabetical order, it would go one line above.

>  obj-$(CONFIG_ISP1301_OMAP)	+= isp1301_omap.o
>  obj-$(CONFIG_TPS65010)		+= tps65010.o
>  obj-$(CONFIG_SENSORS_TLV320AIC23) += tlv320aic23.o
> --- /dev/null	1970-01-01 00:00:00.000000000 +0000
> +++ b/drivers/i2c/chips/pcf857x.c	2007-10-29 14:12:21.000000000 -0700
> @@ -0,0 +1,309 @@
> +/*
> + * pcf857x - driver for pcf857{4,4a,5,5c} I2C GPIO expanders

I recommend spelling out chip names completely, as it lets people grep
the kernel tree for chip names when they look for support.

> + *
> + * Copyright (C) 2007 David Brownell
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License as published by
> + * the Free Software Foundation; either version 2 of the License, or
> + * (at your option) any later version.
> + *
> + * 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.
> + *
> + * You should have received a copy of the GNU General Public License
> + * along with this program; if not, write to the Free Software
> + * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
> + */
> +
> +#include <linux/kernel.h>
> +#include <linux/i2c.h>
> +#include <linux/pcf857x.h>

I suspect that there will be many more such header files in the future.
Would it make sense to move them to include/linux/gpio?

> +
> +#include <asm/gpio.h>
> +
> +
> +/*
> + * The pcf857x chips only expose a one read register and one write register.

Typo: extra "a".

> + * Writing a "one" bit (to match the reset state) lets that pin be used as
> + * an input; it's not an open-drain model, but it acts a bit like that.
> + *
> + * Some other I2C GPIO expander chips (like the pca9534, pca9535, pca9555,
> + * pca9539, mcp23008, and mc23017) have a more complex register model with
> + * more conventional input circuitry, often using 0x20..0x27 addresses.
> + */
> +struct pcf857x {
> +	struct gpio_chip	chip;
> +	struct i2c_client	*client;
> +	unsigned		out;		/* software latch */
> +};
> +
> +/*-------------------------------------------------------------------------*/
> +
> +/* Talk to 8-bit I/O expander */
> +
> +static int pcf857x_input8(struct gpio_chip *chip, unsigned offset)
> +{
> +	struct pcf857x	*gpio = container_of(chip, struct pcf857x, chip);
> +
> +	gpio->out |= (1 << offset);
> +	return i2c_smbus_write_byte(gpio->client, (u8) gpio->out);

Useless cast.

> +}
> +
> +static int pcf857x_get8(struct gpio_chip *chip, unsigned offset)
> +{
> +	struct pcf857x	*gpio = container_of(chip, struct pcf857x, chip);
> +	s32		value;
> +
> +	value = i2c_smbus_read_byte(gpio->client);
> +	return (value < 0) ? 0 : !!(value & (1 << offset));

!!(value & (1 << offset))
is more efficiently written
(value >> offset) & 1

> +}
> +
> +static int pcf857x_output8(struct gpio_chip *chip, unsigned offset, int value)
> +{
> +	struct pcf857x	*gpio = container_of(chip, struct pcf857x, chip);
> +	unsigned	bit = 1 << offset;
> +
> +	if (value)
> +		gpio->out |= bit;
> +	else
> +		gpio->out &= ~bit;
> +	return i2c_smbus_write_byte(gpio->client, (u8) gpio->out);

Useless cast.

> +}
> +
> +static void pcf857x_set8(struct gpio_chip *chip, unsigned offset, int value)
> +{
> +	pcf857x_output8(chip, offset, value);
> +}

It would be more efficient to drop pcf857x_set8 altogether and do
gpio->chip.set = pcf857x_output8.

Many of the comments above apply to the 16-bit functions below as well.

> +
> +/*-------------------------------------------------------------------------*/
> +
> +/* Talk to 16-bit I/O expander */
> +
> +static int i2c_write_le16(struct i2c_client *client, u16 word)
> +{
> +	u8 buf[2] = { word & 0xff, word >> 8, };

Stray comma.

> +	int status;
> +
> +	status = i2c_master_send(client, buf, 2);
> +	return (status < 0) ? status : 0;
> +}
> +
> +static int i2c_read_le16(struct i2c_client *client)
> +{
> +	u8 buf[2];
> +	int status;
> +
> +	status = i2c_master_recv(client, buf, 2);
> +	if (status < 0)
> +		return status;
> +	return (buf[1] << 8) | buf[0];
> +}
> +
> +static int pcf857x_input16(struct gpio_chip *chip, unsigned offset)
> +{
> +	struct pcf857x	*gpio = container_of(chip, struct pcf857x, chip);
> +
> +	gpio->out |= (1 << offset);
> +	return i2c_write_le16(gpio->client, (u16) gpio->out);
> +}
> +
> +static int pcf857x_get16(struct gpio_chip *chip, unsigned offset)
> +{
> +	struct pcf857x	*gpio = container_of(chip, struct pcf857x, chip);
> +	int		value;
> +
> +	value = i2c_read_le16(gpio->client);
> +	return (value < 0) ? 0 : !!(value & (1 << offset));
> +}
> +
> +static int pcf857x_output16(struct gpio_chip *chip, unsigned offset, int value)
> +{
> +	struct pcf857x	*gpio = container_of(chip, struct pcf857x, chip);
> +	unsigned	bit = 1 << offset;
> +
> +	if (value)
> +		gpio->out |= bit;
> +	else
> +		gpio->out &= ~bit;
> +	return i2c_write_le16(gpio->client, (u16) gpio->out);
> +}
> +
> +static void pcf857x_set16(struct gpio_chip *chip, unsigned offset, int value)
> +{
> +	pcf857x_output16(chip, offset, value);
> +}
> +
> +/*-------------------------------------------------------------------------*/
> +
> +static int pcf857x_probe(struct i2c_client *client)
> +{
> +	struct pcf857x_platform_data	*pdata;
> +	struct pcf857x			*gpio;
> +	int				status = 0;

Useless initialization.

> +
> +	pdata = client->dev.platform_data;
> +	if (!pdata)
> +		return -ENODEV;
> +
> +	/* Allocate, initialize, and register this gpio_chip. */
> +	gpio = kzalloc(sizeof *gpio, GFP_KERNEL);

You need to include <linux/slab.h> to have access to this function.

> +	if (!gpio)
> +		return -ENOMEM;
> +
> +	gpio->chip.base = pdata->gpio_base;
> +	gpio->chip.can_sleep = 1;
> +
> +	/* NOTE:  the OnSemi jlc1562b is also largely compatible with
> +	 * these parts, notably for output.  It has a low-resolution
> +	 * DAC instead of pin change IRQs; and its inputs can be the
> +	 * result of comparators.
> +	 */
> +
> +	/* '74a addresses are 0x38..0x3f; '74 uses 0x20..0x27 */
> +	if (strcmp(client->name, "pcf8574a") == 0
> +			|| strcmp(client->name, "pcf8574") == 0) {
> +		gpio->chip.ngpio = 8;
> +		gpio->chip.direction_input = pcf857x_input8;
> +		gpio->chip.get = pcf857x_get8;
> +		gpio->chip.direction_output = pcf857x_output8;
> +		gpio->chip.set = pcf857x_set8;
> +
> +		if (!i2c_check_functionality(client->adapter,
> +				I2C_FUNC_SMBUS_BYTE))
> +			status = -EIO;

You set status here...

> +
> +		/* fail if there's no chip present */
> +		status = i2c_smbus_read_byte(client);

... but overwrite it immediately.

> +
> +	/* '75/'75c addresses are 0x20..0x27, just like the '74;
> +	 * the '75c doesn't have a current source pulling high.
> +	 */
> +	} else if (strcmp(client->name, "pcf8575c") == 0
> +			|| strcmp(client->name, "pcf8575") == 0) {
> +		gpio->chip.ngpio = 16;
> +		gpio->chip.direction_input = pcf857x_input16;
> +		gpio->chip.get = pcf857x_get16;
> +		gpio->chip.direction_output = pcf857x_output16;
> +		gpio->chip.set = pcf857x_set16;
> +
> +		if (!i2c_check_functionality(client->adapter, I2C_FUNC_I2C))
> +			status = -EIO;
> +
> +		/* fail if there's no chip present */
> +		status = i2c_read_le16(client);

Same problem here.

> +
> +	} else
> +		status = -ENODEV;
> +
> +	if (status < 0) {
> +		dev_dbg(&client->dev, "probe error %d for '%s'\n",
> +				status, client->name);
> +		kfree(gpio);
> +		return status;

Might make sense to move this to a common error path, as you do it more
than once.

> +	}
> +
> +	gpio->chip.label = client->name;
> +
> +	gpio->client = client;
> +	i2c_set_clientdata(client, gpio);
> +
> +	/* NOTE:  these chips have strange "pseudo-bidirectional" I/O pins.
> +	 * We can't actually know whether a pin is configured (a) as output
> +	 * and driving the signal low, or (b) as input and reporting a low
> +	 * value ... without knowing the last value written since the chip
> +	 * came out of reset (if any).  We can't read the latched output.
> +	 *
> +	 * In short, the only reliable solution for setting up pin direction
> +	 * is to do it explicitly.  The setup() method can do that.
> +	 *
> +	 * We use pdata->n_latch to avoid trouble.  In the typical case it's
> +	 * left initialized to zero; our software copy of the "latch" then
> +	 * matches the chip's reset state.  But there may be cases where a
> +	 * system must drive some pins low, without transient glitching.
> +	 * Handle those cases by assigning n_latch to a nonzero value.
> +	 */
> +	gpio->out = ~pdata->n_latch;
> +
> +	status = gpiochip_add(&gpio->chip);
> +	if (status < 0) {
> +		kfree(gpio);
> +		return status;
> +	}
> +
> +	/* NOTE: these chips can issue "some pin-changed" IRQs, which we
> +	 * don't yet even try to use.  Among other issues, the relevant
> +	 * genirq state isn't available to modular drivers; and most irq
> +	 * methods can't be called from sleeping contexts.
> +	 */
> +
> +	dev_info(&client->dev, "gpios %d..%d on a %s%s\n",
> +			gpio->chip.base,
> +			gpio->chip.base + gpio->chip.ngpio - 1,
> +			client->name,
> +			client->irq ? " (irq ignored)" : "");
> +
> +	/* Let platform code set up the GPIOs and their users.
> +	 * Now is the first time anyone can use them.
> +	 */
> +	if (pdata->setup) {
> +		status = pdata->setup(client,
> +				gpio->chip.base, gpio->chip.ngpio,
> +				pdata->context);
> +		if (status < 0)
> +			dev_dbg(&client->dev, "setup --> %d\n", status);
> +	}
> +
> +	return 0;
> +}
> +
> +static int pcf857x_remove(struct i2c_client *client)
> +{
> +	struct pcf857x_platform_data	*pdata = client->dev.platform_data;
> +	struct pcf857x			*gpio = i2c_get_clientdata(client);
> +	int				status = 0;
> +
> +	if (pdata->teardown) {
> +		status = pdata->teardown(client,
> +				gpio->chip.base, gpio->chip.ngpio,
> +				pdata->context);
> +		if (status < 0) {
> +			dev_err(&client->dev, "%s --> %d\n",
> +					"teardown", status);

Why %s instead of hard-coding "teardown"?

Why is this an error message while a failing setup() at initialization
time only deserves a debug message?

> +			return status;
> +		}
> +	}
> +
> +	status = gpiochip_remove(&gpio->chip);
> +	if (status == 0)
> +		kfree(gpio);
> +	else
> +		dev_err(&client->dev, "%s --> %d\n", "remove", status);
> +	return status;
> +}
> +
> +static struct i2c_driver pcf857x_driver = {
> +	.driver = {
> +		.name	= "pcf857x",
> +		.owner	= THIS_MODULE,
> +	},
> +	.probe	= pcf857x_probe,
> +	.remove	= pcf857x_remove,
> +};
> +
> +static int __init pcf857x_init(void)
> +{
> +	return i2c_add_driver(&pcf857x_driver);
> +}
> +/* we want GPIOs to be ready at device_initcall() time */
> +subsys_initcall(pcf857x_init);
> +
> +static void __exit pcf857x_exit(void)
> +{
> +	i2c_del_driver(&pcf857x_driver);
> +}
> +module_exit(pcf857x_exit);
> +
> +MODULE_LICENSE("GPL");

No MODULE_AUTHOR? No entry in MAINTAINERS?

-- 
Jean Delvare
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