/*
 * ee1004 - driver for DDR4 SPD EEPROMs
 *
 * Copyright (C) 2017 Jean Delvare
 *
 * Based on the at24 driver:
 * Copyright (C) 2005-2007 David Brownell
 * Copyright (C) 2008 Wolfram Sang, Pengutronix
 *
 * 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.
 */

#include <linux/i2c.h>
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/mod_devicetable.h>
#include <linux/module.h>
#include <linux/mutex.h>

/*
 * DDR4 memory modules use special EEPROMs following the Jedec EE1004
 * specification. These are 512-byte EEPROMs using a single I2C address
 * in the 0x50-0x57 range for data. One of two 256-byte page is selected
 * by writing a command to I2C address 0x36 or 0x37 on the same I2C bus.
 *
 * Therefore we need to request these 2 additional addresses, and serialize
 * access to all such EEPROMs with a single mutex.
 *
 * We assume it is safe to read up to 32 bytes at once from these EEPROMs.
 * We use SMBus access even if I2C is available, these EEPROMs are small
 * enough, and reading from them infrequent enough, that we favor simplicity
 * over performance.
 */

#define EE1004_ADDR_SET_PAGE		0x36
#define EE1004_EEPROM_SIZE		512
#define EE1004_PAGE_SIZE		256
#define EE1004_PAGE_SHIFT		8

/*
 * Mutex protects ee1004_set_page and ee1004_dev_count, and must be held
 * from page selection to end of read.
 */
static DEFINE_MUTEX(ee1004_bus_lock);
static struct i2c_client *ee1004_set_page[2];
static unsigned int ee1004_dev_count;
static int ee1004_current_page;

static const struct i2c_device_id ee1004_ids[] = {
	{ "ee1004", 0 },
	{ }
};
MODULE_DEVICE_TABLE(i2c, ee1004_ids);

/*-------------------------------------------------------------------------*/

static ssize_t ee1004_eeprom_read(struct i2c_client *client, char *buf,
				  unsigned int offset, size_t count)
{
	int status;

	if (count > I2C_SMBUS_BLOCK_MAX)
		count = I2C_SMBUS_BLOCK_MAX;
	/* Can't cross page boundaries */
	if (unlikely(offset + count > EE1004_PAGE_SIZE))
		count = EE1004_PAGE_SIZE - offset;

	status = i2c_smbus_read_i2c_block_data_or_emulated(client, offset,
							   count, buf);
	dev_dbg(&client->dev, "read %zu@%d --> %d\n", count, offset, status);

	return status;
}

static ssize_t ee1004_read(struct file *filp, struct kobject *kobj,
			   struct bin_attribute *bin_attr,
			   char *buf, loff_t off, size_t count)
{
	struct device *dev = kobj_to_dev(kobj);
	struct i2c_client *client = to_i2c_client(dev);
	size_t requested = count;
	int page;

	if (unlikely(!count))
		return count;

	page = off >> EE1004_PAGE_SHIFT;
	if (unlikely(page > 1))
		return 0;
	off &= (1 << EE1004_PAGE_SHIFT) - 1;

	/*
	 * Read data from chip, protecting against concurrent access to
	 * other EE1004 SPD EEPROMs on the same adapter.
	 */
	mutex_lock(&ee1004_bus_lock);

	while (count) {
		int status;

		/* Select page */
		if (page != ee1004_current_page) {
			/* Data is ignored */
			status = i2c_smbus_write_byte(ee1004_set_page[page],
						      0x00);
			if (status < 0) {
				dev_err(dev, "Failed to select page %d (%d)\n",
					page, status);
				mutex_unlock(&ee1004_bus_lock);
				return status;
			}
			dev_dbg(dev, "Selected page %d\n", page);
			ee1004_current_page = page;
		}

		status = ee1004_eeprom_read(client, buf, off, count);
		if (status < 0) {
			mutex_unlock(&ee1004_bus_lock);
			return status;
		}
		buf += status;
		off += status;
		count -= status;

		if (off == EE1004_PAGE_SIZE) {
			page++;
			off = 0;
		}
	}

	mutex_unlock(&ee1004_bus_lock);

	return requested;
}

static const struct bin_attribute eeprom_attr = {
	.attr = {
		.name = "eeprom",
		.mode = 0444,
	},
	.size = EE1004_EEPROM_SIZE,
	.read = ee1004_read,
};

static int ee1004_probe(struct i2c_client *client,
			const struct i2c_device_id *id)
{
	int err, cnr = 0;
	const char *slow = NULL;

	/* Make sure we can operate on this adapter */
	if (!i2c_check_functionality(client->adapter,
				     I2C_FUNC_SMBUS_READ_BYTE |
				     I2C_FUNC_SMBUS_READ_I2C_BLOCK)) {
		if (i2c_check_functionality(client->adapter,
				     I2C_FUNC_SMBUS_READ_BYTE |
				     I2C_FUNC_SMBUS_READ_WORD_DATA))
			slow = "word";
		else if (i2c_check_functionality(client->adapter,
				     I2C_FUNC_SMBUS_READ_BYTE |
				     I2C_FUNC_SMBUS_READ_BYTE_DATA))
			slow = "byte";
		else
			return -EPFNOSUPPORT;
	}

	/* Use 2 dummy devices for page select command */
	mutex_lock(&ee1004_bus_lock);
	if (++ee1004_dev_count == 1) {
		for (cnr = 0; cnr < 2; cnr++) {
			ee1004_set_page[cnr] = i2c_new_dummy(client->adapter,
						EE1004_ADDR_SET_PAGE + cnr);
			if (!ee1004_set_page[cnr]) {
				dev_err(&client->dev,
					"address 0x%02x unavailable\n",
					EE1004_ADDR_SET_PAGE + cnr);
				err = -EADDRINUSE;
				goto err_clients;
			}
		}
	} else if (i2c_adapter_id(client->adapter) !=
		   i2c_adapter_id(ee1004_set_page[0]->adapter)) {
		dev_err(&client->dev,
			"Driver only supports devices on a single I2C bus\n");
		err = -EOPNOTSUPP;
		goto err_clients;
	}

	/* Remember current page to avoid unneeded page select */
	err = i2c_smbus_read_byte(ee1004_set_page[0]);
	if (err == -ENXIO) {
		/* Nack means page 1 is selected */
		ee1004_current_page = 1;
	} else if (err < 0) {
		/* Anything else is a real error, bail out */
		goto err_clients;
	} else {
		/* Ack means page 0 is selected, returned value meaningless */
		ee1004_current_page = 0;
	}
	dev_dbg(&client->dev, "Currently selected page: %d\n",
		ee1004_current_page);
	mutex_unlock(&ee1004_bus_lock);

	/* Create the sysfs eeprom file */
	err = sysfs_create_bin_file(&client->dev.kobj, &eeprom_attr);
	if (err)
		goto err_clients_lock;

	dev_info(&client->dev,
		 "%u byte EE1004-compliant SPD EEPROM, read-only\n",
		 EE1004_EEPROM_SIZE);
	if (slow)
		dev_notice(&client->dev,
			   "Falling back to %s reads, performance will suffer\n",
			   slow);

	return 0;

 err_clients_lock:
	mutex_lock(&ee1004_bus_lock);
 err_clients:
	if (--ee1004_dev_count == 0) {
		for (cnr--; cnr >= 0; cnr--) {
			i2c_unregister_device(ee1004_set_page[cnr]);
			ee1004_set_page[cnr] = NULL;
		}
	}
	mutex_unlock(&ee1004_bus_lock);

	return err;
}

static int ee1004_remove(struct i2c_client *client)
{
	int i;

	sysfs_remove_bin_file(&client->dev.kobj, &eeprom_attr);

	/* Remove page select clients if this is the last device */
	mutex_lock(&ee1004_bus_lock);
	if (--ee1004_dev_count == 0) {
		for (i = 0; i < 2; i++) {
			i2c_unregister_device(ee1004_set_page[i]);
			ee1004_set_page[i] = NULL;
		}
	}
	mutex_unlock(&ee1004_bus_lock);

	return 0;
}

/*-------------------------------------------------------------------------*/

static struct i2c_driver ee1004_driver = {
	.driver = {
		.name = "ee1004",
	},
	.probe = ee1004_probe,
	.remove = ee1004_remove,
	.id_table = ee1004_ids,
};

static int __init ee1004_init(void)
{
	return i2c_add_driver(&ee1004_driver);
}
module_init(ee1004_init);

static void __exit ee1004_exit(void)
{
	i2c_del_driver(&ee1004_driver);
}
module_exit(ee1004_exit);

MODULE_DESCRIPTION("Driver for EE1004-compliant DDR4 SPD EEPROMs");
MODULE_AUTHOR("Jean Delvare");
MODULE_LICENSE("GPL");