eeprom: New ee1004 driver for DDR4 memory

The EEPROMs which hold the SPD data on DDR4 memory modules are no
longer standard AT24C02-compatible EEPROMs. They are 512-byte EEPROMs
which use only 1 I2C address for data access. You need to switch
between the lower page and the upper page of data by sending commands
on the SMBus.

Signed-off-by: Jean Delvare <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/misc/eeprom/Kconfig

···
 	  This driver can also be built as a module. If so, the module
 	  will be called idt_89hpesx.
 
+config EEPROM_EE1004
+	tristate "SPD EEPROMs on DDR4 memory modules"
+	depends on I2C && SYSFS
+	help
+	  Enable this driver to get read support to SPD EEPROMs following
+	  the JEDEC EE1004 standard. These are typically found on DDR4
+	  SDRAM memory modules.
+
+	  This driver can also be built as a module.  If so, the module
+	  will be called ee1004.
+
 endmenu

drivers/misc/eeprom/Makefile

···
 obj-$(CONFIG_EEPROM_93XX46)	+= eeprom_93xx46.o
 obj-$(CONFIG_EEPROM_DIGSY_MTC_CFG) += digsy_mtc_eeprom.o
 obj-$(CONFIG_EEPROM_IDT_89HPESX) += idt_89hpesx.o
+obj-$(CONFIG_EEPROM_EE1004)	+= ee1004.o

drivers/misc/eeprom/ee1004.c

···
+/*
+ * 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");