/*
 * 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");