Re: [PATCH 1/2] Initial support for Allwinner's Security ID fuses

[Maxime Ripard <maxime.ripard@free-electrons.com>]

Hi Oliver,

Le 17/05/2013 15:35, Oliver Schinagl a écrit :
> From: Oliver Schinagl <oliver@schinagl.nl>
> 
> Allwinner has electric fuses (efuse) on their line of chips. This driver
> reads those fuses and exports them as a sysfs node. Also a symbol is exported
> for in-kernel useage.
> 
> While initially these fuses are used to somewhat determin the chipID, these
> appear to be writeable by the user and thus can be used for other purpouses.
> For example storing a 128 bit root key, a unique serial number, which could
> then even be used as a MAC address.
> 
> Because writing to e-fuses can be potentially dangerous, and are certainly
> not as often writable (if at all) as flash memory, these shouldn't be easily
> changeable, hence only a read-only mode. An offline tool to write the fuses
> is in the works.
> 
> Currently supported are the following known chips:
> Allwinner sun4i (A10)
> Allwinner sun5i (A10s A13)
> Allwinner sun6i (A31, A31s)
> Allwinner sun7i (A20)

Since I don't think those patches have been tested on sun6i/sun7i, and
that there's not even kernel support for those, maybe it's not worth
mentionning them?

> 
> Signed-off-by: Oliver Schinagl <oliver@schinagl.nl>
> ---
>  drivers/misc/eeprom/Kconfig     |  19 ++++
>  drivers/misc/eeprom/Makefile    |   1 +
>  drivers/misc/eeprom/sunxi_sid.c | 218 ++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 238 insertions(+)
>  create mode 100644 drivers/misc/eeprom/sunxi_sid.c
> 
> diff --git a/drivers/misc/eeprom/Kconfig b/drivers/misc/eeprom/Kconfig
> index 04f2e1f..c9ddda5 100644
> --- a/drivers/misc/eeprom/Kconfig
> +++ b/drivers/misc/eeprom/Kconfig
> @@ -96,4 +96,23 @@ config EEPROM_DIGSY_MTC_CFG
>  
>  	  If unsure, say N.
>  
> +config EEPROM_SUNXI_SID
> +	tristate "Allwinner sunxi security ID support"
> +	depends on ARCH_SUNXI && SYSFS
> +	help
> +	  This is a driver for the 'security ID' available on various Allwinner
> +	  devices. Currently supported are:
> +		sun4i (A10)
> +		sun5i (A10s, A12, A13)
> +		sun6i (A31)
> +		sun7i (A20)

Same things here.

> +
> +	  Due to the potential risks involved with changing e-fuses,
> +	  this driver is read-only
> +
> +	  For more information visit http://linux-sunxi.org/SID
> +
> +	  This driver can also be built as a module. If so, the module
> +	  will be called sunxi_sid.
> +
>  endmenu
> diff --git a/drivers/misc/eeprom/Makefile b/drivers/misc/eeprom/Makefile
> index fc1e81d..9507aec 100644
> --- a/drivers/misc/eeprom/Makefile
> +++ b/drivers/misc/eeprom/Makefile
> @@ -4,4 +4,5 @@ obj-$(CONFIG_EEPROM_LEGACY)	+= eeprom.o
>  obj-$(CONFIG_EEPROM_MAX6875)	+= max6875.o
>  obj-$(CONFIG_EEPROM_93CX6)	+= eeprom_93cx6.o
>  obj-$(CONFIG_EEPROM_93XX46)	+= eeprom_93xx46.o
> +obj-$(CONFIG_EEPROM_SUNXI_SID)	+= sunxi_sid.o
>  obj-$(CONFIG_EEPROM_DIGSY_MTC_CFG) += digsy_mtc_eeprom.o
> diff --git a/drivers/misc/eeprom/sunxi_sid.c b/drivers/misc/eeprom/sunxi_sid.c
> new file mode 100644
> index 0000000..953f137
> --- /dev/null
> +++ b/drivers/misc/eeprom/sunxi_sid.c
> @@ -0,0 +1,218 @@
> +/*
> + * Copyright (c) 2013 Oliver Schinagl
> + * http://www.linux-sunxi.org
> + *
> + * Oliver Schinagl <oliver@schinagl.nl>
> + *
> + * 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.
> + *
> + * This driver exposes the Allwinner security ID, a 128 bit eeprom, in chunks
> + * of 8 bytes.

16 bytes or 8 bits? because 8 bytes != 128 bits.

> + */
> +
> +#include <linux/compiler.h>
> +#include <linux/device.h>
> +#include <linux/errno.h>
> +#include <linux/export.h>
> +#include <linux/fs.h>
> +#include <linux/init.h>
> +#include <linux/io.h>
> +#include <linux/kobject.h>
> +#include <linux/module.h>
> +#include <linux/of_address.h>
> +#include <linux/platform_device.h>
> +#include <linux/stat.h>
> +#include <linux/sysfs.h>
> +#include <linux/types.h>
> +
> +
> +#define DRV_NAME "sunxi-sid"
> +#define DRV_VERSION "1.0"
> +
> +/* Register offsets */
> +#define SUNXI_SID_KEY0 0x00
> +#define SUNXI_SID_KEY1 0x04
> +#define SUNXI_SID_KEY2 0x08
> +#define SUNXI_SID_KEY3 0x0c
> +
> +/* There are 4 32-bit keys */
> +#define SUNXI_SID_KEYS 4
> +/* and 4 32-bit keys per 32-bit key */

The comment is wrong here.

> +#define SUNXI_SID_SIZE (SUNXI_SID_KEYS * 4)
> +
> +#if (SUNXI_SID_SIZE > PAGE_SIZE)
> +#error "SUNXI_SID_SIZE is larger then the target's PAGE_SIZE, ENOMEM."
> +#endif

Hmmmm, I don't follow you here, what's the relation between your driver
and PAGE_SIZE?

> +
> +static u8 keys_lut[] = {
> +	SUNXI_SID_KEY0,
> +	SUNXI_SID_KEY1,
> +	SUNXI_SID_KEY2,
> +	SUNXI_SID_KEY3,
> +};
> +
> +struct sid_priv {
> +	void __iomem *sid_base;
> +};
> +
> +struct sid_priv *p;

What's the point of having a structure here? And why putting a global
value, !static, with a generic name, while you could have an
instance-specific one?

struct file has a private_data field, use it.

> +
> +
> +/* We read the entire key, using a look up table. Returned is only the
> + * requested byte. This is of course slower then it could be and uses 4 times
> + * more reads as needed but keeps code a little simpler.
> + */
> +u8 sunxi_sid_read_byte(const int key)
> +{
> +	u32 sid_key;
> +	u8 ret;
> +
> +	ret = 0;
> +	if (likely((key <= SUNXI_SID_SIZE))) {
> +		sid_key = ioread32(p->sid_base + keys_lut[key >> 2]);
> +		switch (key % 4) {
> +		case 0:
> +			ret = (sid_key >> 24) & 0xff;
> +			break;
> +		case 1:
> +			ret = (sid_key >> 16) & 0xff;
> +			break;
> +		case 2:
> +			ret = (sid_key >> 8) & 0xff;
> +			break;
> +		case 3:
> +			ret = sid_key & 0xff;
> +			break;
> +		}
> +	}

Come on, you can do better. This lookup table is useless.

Also, why the first key is the one with the MSBs?
I'd expect that the key 0 is the one holding the LSBs.

> +
> +	return ret;
> +}
> +
> +static ssize_t sid_read(struct file *fd, struct kobject *kobj,
> +			struct bin_attribute *attr, char *buf,
> +			loff_t pos, size_t size)
> +{
> +	ssize_t ret;
> +	struct device *dev;
> +	struct sid_priv *priv;
> +	int i;
> +
> +	ret = -EPERM;
> +	dev = kobj_to_dev(kobj);
> +	priv = dev_get_drvdata(dev);
> +
> +	if ((likely(size > 0)) && ((size + pos) <= SUNXI_SID_SIZE)) {
> +		for (i = 0; i < size; i++) {
> +			buf[i] = sunxi_sid_read_byte(pos + i);
> +		}
> +		if (i < PAGE_SIZE) {
> +			buf[i] = '\0';
> +			ret = (ssize_t)size;
> +		} else {
> +			ret = -ENOMEM;
> +		}

Hmmmm, what? Why returning \0 here? It's not a string, it's binary data.
What's the relation with PAGE_SIZE again?

Just return the number of bytes read, that's it.

> +	} else {
> +		buf[0] = '\0';
> +		ret = 0;
> +	}
> +
> +	return ret;
> +}
> +
> +static struct of_device_id sid_of_match[] = {
> +	{
> +		.compatible = "allwinner,sun4i-sid",
> +	},
> +	{/* sentinel */}
> +};
> +MODULE_DEVICE_TABLE(of, sid_of_match);
> +
> +static struct bin_attribute sid_bin_attr = {
> +	.attr = {
> +		.name = "key",
> +		.mode = S_IRUGO,
> +	},
> +	.size = SUNXI_SID_SIZE,
> +	.read = sid_read,
> +};
> +
> +static int sid_remove(struct platform_device *pdev)
> +{
> +	struct device *dev = &pdev->dev;
> +	struct sid_priv *priv;
> +
> +	priv = dev_get_drvdata(dev);
> +	device_remove_bin_file(dev, &sid_bin_attr);
> +	iounmap(priv->sid_base);
> +	devm_kfree(dev, priv);
> +	return 0;
> +}
> +
> +static int __init sid_probe(struct platform_device *pdev)
> +{
> +	int ret;
> +	struct device *dev = &pdev->dev;
> +	struct sid_priv *priv;
> +
> +	priv = devm_kzalloc(dev, sizeof(*priv), GFP_KERNEL);
> +	p = priv;
> +
> +	dev_set_drvdata(dev, priv);
> +
> +	if (!priv) {
> +		dev_err(dev, "Unable to allocate device private data\n");
> +		ret = -ENOMEM;
> +		goto exit;
> +	}

Isn't it a bit weird to check for the memory allocation after using the
variable. Also, you don't really need the dev_err, since if the kernel
fails to allocate some memory, it will tell you anyway.

> +	priv->sid_base = of_iomap(dev->of_node, 0);
> +	if (!priv->sid_base) {
> +		dev_err(dev, "Unable to map memory region\n");
> +		ret = -ENOMEM;
> +		goto exit_free;
> +	}
> +
> +	ret = device_create_bin_file(dev, &sid_bin_attr);
> +	if (ret) {
> +		dev_err(dev, "Unable to create sysfs bin entry\n");
> +		goto exit_unmap;
> +	}

Hmmm, maybe it's not worth all these gotos just for an iounmap, I'd
probably return right away, but that's your call.

> +	dev_info(dev, "Sunxi security ID driver loaded successfully.\n");
> +
> +	return 0;
> +
> +
> +exit_unmap:
> +	iounmap(priv->sid_base);
> +exit_free:
> +	devm_kfree(dev, priv);
> +exit:
> +	return ret;
> +}
> +
> +static struct platform_driver sid_driver = {
> +	.probe = sid_probe,
> +	.remove = sid_remove,
> +	.driver = {
> +		.name = DRV_NAME,
> +		.owner = THIS_MODULE,
> +		.of_match_table = sid_of_match,
> +	},
> +};
> +module_platform_driver(sid_driver);
> +
> +
> +MODULE_AUTHOR("Oliver Schinagl <oliver@schinagl.nl>");
> +MODULE_DESCRIPTION("Allwinner sunxi security id driver");
> +MODULE_VERSION(DRV_VERSION);
> +MODULE_LICENSE("GPL");
> 

Thanks for this driver!
Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com