[PATCH v3 1/4] firmware: introduce sysfs driver for QEMU's fw_cfg device

[lersek@redhat.com (Laszlo Ersek)]

On 10/04/15 01:28, Gabriel L. Somlo wrote:
> From: Gabriel Somlo <somlo@cmu.edu>
> 
> Make fw_cfg entries of type "file" available via sysfs. Entries
> are listed under /sys/firmware/qemu_fw_cfg/by_key, in folders
> named after each entry's selector key. Filename, selector value,
> and size read-only attributes are included for each entry. Also,
> a "raw" attribute allows retrieval of the full binary content of
> each entry.
> 
> This patch also provides a documentation file outlining the
> guest-side "hardware" interface exposed by the QEMU fw_cfg device.
> 
> Signed-off-by: Gabriel Somlo <somlo@cmu.edu>
> ---
>  .../ABI/testing/sysfs-firmware-qemu_fw_cfg         | 167 ++++++++
>  drivers/firmware/Kconfig                           |  10 +
>  drivers/firmware/Makefile                          |   1 +
>  drivers/firmware/qemu_fw_cfg.c                     | 456 +++++++++++++++++++++
>  4 files changed, 634 insertions(+)
>  create mode 100644 Documentation/ABI/testing/sysfs-firmware-qemu_fw_cfg
>  create mode 100644 drivers/firmware/qemu_fw_cfg.c
> 
> diff --git a/Documentation/ABI/testing/sysfs-firmware-qemu_fw_cfg b/Documentation/ABI/testing/sysfs-firmware-qemu_fw_cfg
> new file mode 100644
> index 0000000..f1ef44e
> --- /dev/null
> +++ b/Documentation/ABI/testing/sysfs-firmware-qemu_fw_cfg
> @@ -0,0 +1,167 @@
> +What:		/sys/firmware/qemu_fw_cfg/
> +Date:		August 2015
> +Contact:	Gabriel Somlo <somlo@cmu.edu>
> +Description:
> +		Several different architectures supported by QEMU (x86, arm,
> +		sun4*, ppc/mac) are provisioned with a firmware configuration
> +		(fw_cfg) device, used by the host to provide configuration data
> +		to the starting guest. While most of this data is meant for use
> +		by the guest firmware, starting with QEMU v2.4, guest VMs may
> +		be given arbitrary fw_cfg entries supplied directly on the
> +		command line, which therefore may be of interest to userspace.
> +
> +		=== Guest-side Hardware Interface ===
> +
> +		The fw_cfg device is available to guest VMs as a register pair
> +		(control and data), accessible as either a IO ports or as MMIO
> +		addresses, depending on the architecture.
> +
> +		--- Control Register ---
> +
> +		Width: 16-bit
> +		Access: Write-Only
> +		Endianness: LE (if IOport) or BE (if MMIO)
> +
> +		A write to the control register selects the index for one of
> +		the firmware configuration items (or "blobs") available on the
> +		fw_cfg device, which can subsequently be read from the data
> +		register.
> +
> +		Each time the control register is written, an data offset
> +		internal to the fw_cfg device will be set to zero. This data
> +		offset impacts which portion of the selected fw_cfg blob is
> +		accessed by reading the data register, as explained below.
> +
> +		--- Data Register ---
> +
> +		Width: 8-bit (if IOport), or 8/16/32/64-bit (if MMIO)
> +		Access: Read-Only
> +		Endianness: string preserving
> +
> +		The data register allows access to an array of bytes which
> +		represent the fw_cfg blob last selected by a write to the
> +		control register.
> +
> +		Immediately following a write to the control register, the data
> +		offset will be set to zero. Each successful read access to the
> +		data register will increment the data offset by the appropriate
> +		access width.
> +
> +		Each fw_cfg blob has a maximum associated data length. Once the
> +		data offset exceeds this maximum length, any subsequent reads
> +		via the data register will return 0x00.
> +
> +		An N-byte wide read of the data register will return the next
> +		available N bytes of the selected fw_cfg blob, as a substring,
> +		in increasing address order, similar to memcpy(), zero-padded
> +		if necessary should the maximum data length of the selected
> +		item be reached, as described above.
> +
> +		--- Per-arch Register Details ---
> +
> +		-------------------------------------------------------------
> +		arch	access	       base	ctrl	ctrl	data	max.
> +			mode	    address	offset	endian	offset	data
> +						(bytes)			(bytes)
> +		-------------------------------------------------------------
> +		x86*	IOport	      0x510	0	LE	1	1
> +		arm	MMIO	  0x9020000	8	BE	0	8
> +		sun4u	IOport	      0x510	0	LE	1	1
> +		sun4m	MMIO	0xd00000510	0	BE	2	1
> +		ppc/mac	MMIO	 0xf0000510	0	BE	2	1
> +		-------------------------------------------------------------
> +
> +		NOTE 1. On platforms where the fw_cfg registers are exposed as
> +		IO ports, the data port number will always be one greater than
> +		the port number of the control register. I.e., the two ports
> +		are overlapping, and can not be mapped separately.
> +
> +		=== Firmware Configuration Items of Interest ===
> +
> +		Originally, the index key, size, and formatting of blobs in
> +		fw_cfg was hard coded by mutual agreement between QEMU on the
> +		host side, and the guest-side firmware. Later on, a file
> +		transfer interface was added: by reading a special blob, the
> +		fw_cfg consumer can retrieve a list of records containing the
> +		name, selector key, and size of further fw_cfg blobs made
> +		available by the host. Below we describe three fw_cfg blobs
> +		of interest to the sysfs driver.
> +
> +		--- Signature (Key 0x0000, FW_CFG_SIGNATURE) ---
> +
> +		The presence of the fw_cfg device can be verified by selecting
> +		the signature blob by writing 0x0000 to the control register,
> +		and reading four bytes from the data register. If the fw_cfg
> +		device is present, the four bytes read will match the ASCII
> +		characters "QEMU".
> +
> +		--- Revision (Key 0x0001, FW_CFG_ID) ---
> +
> +		A 32-bit little-endian unsigned integer, this item is used as
> +		an interface revision number.
> +
> +		--- File Directory (Key 0x0019, FW_CFG_FILE_DIR) ---
> +
> +		Any fw_cfg blobs stored at key 0x0020 FW_CFG_FILE_FIRST() or
> +		higher will have an associated entry in this "directory" blob,
> +		which facilitates the discovery of available items by software
> +		(e.g. BIOS) running on the guest. The format of the directory
> +		blob is shown below.
> +
> +		NOTE: All integers are stored in big-endian format!
> +
> +		/* the entire file directory "blob" */
> +		struct FWCfgFiles {
> +			u32 count;		/* total number of entries */
> +			struct FWCfgFile f[];	/* entry array, see below */
> +		};
> +
> +		/* an individual directory entry, 64 bytes total */
> +		struct FWCfgFile {
> +			u32 size;	/* size of referenced blob */
> +			u16 select;	/* selector key for referenced blob */
> +			u16 reserved;
> +			char name[56];	/* blob name, nul-terminated ASCII */
> +		};

I think the above is somewhat redundant with regard to
"Documentation/devicetree/bindings/arm/fw-cfg.txt". That may not
necessarily be a problem, I'd just like to make you aware of that file
too. Perhaps that file should be updated too.

... In any case, I wanted to call your attention to that file, because
it's been now raised that the driver should handle both ACPI and DT
guests. In DT, the device is already exposed -- that's how the UEFI
guest firmware for aarch64 guests learns about it.

So, if you'd like to follow the example of
"drivers/virtio/virtio_mmio.c" (which I think would be a good idea), in
order to recognize the device in both DT and ACPI guests, then on the
QEMU side, you won't have to do anything for DT, because
"qemu,fw-cfg-mmio" is already exposed in create_fw_cfg() [hw/arm/virt.c].

Thanks
Laszlo

> +
> +		=== SysFS fw_cfg Interface ===
> +
> +		The fw_cfg sysfs interface described in this document is only
> +		intended to display discoverable blobs (i.e., those registered
> +		with the file directory), as there is no way to determine the
> +		presence or size of "legacy" blobs (with selector keys between
> +		0x0002 and 0x0018) programmatically.
> +
> +		All fw_cfg information is shown under:
> +
> +			/sys/firmware/qemu_fw_cfg/
> +
> +		The only legacy blob displayed is the fw_cfg device revision:
> +
> +			/sys/firmware/qemu_fw_cfg/rev
> +
> +		--- Discoverable fw_cfg blobs by selector key ---
> +
> +		All discoverable blobs listed in the fw_cfg file directory are
> +		displayed as entries named after their unique selector key
> +		value, e.g.:
> +
> +			/sys/firmware/qemu_fw_cfg/by_key/32
> +			/sys/firmware/qemu_fw_cfg/by_key/33
> +			/sys/firmware/qemu_fw_cfg/by_key/34
> +			...
> +
> +		Each such fw_cfg sysfs entry has the following values exported
> +		as attributes:
> +
> +		name  	: The 56-byte nul-terminated ASCII string used as the
> +			  blob's 'file name' in the fw_cfg directory.
> +		size  	: The length of the blob, as given in the fw_cfg
> +			  directory.
> +		key	: The value of the blob's selector key as given in the
> +			  fw_cfg directory. This value is the same as used in
> +			  the parent directory name.
> +		raw	: The raw bytes of the blob, obtained by selecting the
> +			  entry via the control register, and reading a number
> +			  of bytes equal to the blob size from the data
> +			  register.
> diff --git a/drivers/firmware/Kconfig b/drivers/firmware/Kconfig
> index 665efca..0466e80 100644
> --- a/drivers/firmware/Kconfig
> +++ b/drivers/firmware/Kconfig
> @@ -135,6 +135,16 @@ config ISCSI_IBFT
>  	  detect iSCSI boot parameters dynamically during system boot, say Y.
>  	  Otherwise, say N.
>  
> +config FW_CFG_SYSFS
> +	tristate "QEMU fw_cfg device support in sysfs"
> +	depends on SYSFS
> +	default n
> +	help
> +	  Say Y or M here to enable the exporting of the QEMU firmware
> +	  configuration (fw_cfg) file entries via sysfs. Entries are
> +	  found under /sys/firmware/fw_cfg when this option is enabled
> +	  and loaded.
> +
>  config QCOM_SCM
>  	bool
>  	depends on ARM || ARM64
> diff --git a/drivers/firmware/Makefile b/drivers/firmware/Makefile
> index 2ee8347..efba22a 100644
> --- a/drivers/firmware/Makefile
> +++ b/drivers/firmware/Makefile
> @@ -12,6 +12,7 @@ obj-$(CONFIG_DMIID)		+= dmi-id.o
>  obj-$(CONFIG_ISCSI_IBFT_FIND)	+= iscsi_ibft_find.o
>  obj-$(CONFIG_ISCSI_IBFT)	+= iscsi_ibft.o
>  obj-$(CONFIG_FIRMWARE_MEMMAP)	+= memmap.o
> +obj-$(CONFIG_FW_CFG_SYSFS)	+= qemu_fw_cfg.o
>  obj-$(CONFIG_QCOM_SCM)		+= qcom_scm.o
>  obj-$(CONFIG_QCOM_SCM_64)	+= qcom_scm-64.o
>  obj-$(CONFIG_QCOM_SCM_32)	+= qcom_scm-32.o
> diff --git a/drivers/firmware/qemu_fw_cfg.c b/drivers/firmware/qemu_fw_cfg.c
> new file mode 100644
> index 0000000..3a67a16
> --- /dev/null
> +++ b/drivers/firmware/qemu_fw_cfg.c
> @@ -0,0 +1,456 @@
> +/*
> + * drivers/firmware/qemu_fw_cfg.c
> + *
> + * Expose entries from QEMU's firmware configuration (fw_cfg) device in
> + * sysfs (read-only, under "/sys/firmware/qemu_fw_cfg/...").
> + *
> + * Copyright 2015 Carnegie Mellon University
> + */
> +
> +#include <linux/module.h>
> +#include <linux/slab.h>
> +#include <linux/io.h>
> +#include <linux/ioport.h>
> +
> +MODULE_AUTHOR("Gabriel L. Somlo <somlo@cmu.edu>");
> +MODULE_DESCRIPTION("QEMU fw_cfg sysfs support");
> +MODULE_LICENSE("GPL");
> +
> +/* selector key values for "well-known" fw_cfg entries */
> +#define FW_CFG_SIGNATURE  0x00
> +#define FW_CFG_ID         0x01
> +#define FW_CFG_FILE_DIR   0x19
> +
> +/* size in bytes of fw_cfg signature */
> +#define FW_CFG_SIG_SIZE 4
> +
> +/* fw_cfg "file name" is up to 56 characters (including terminating nul) */
> +#define FW_CFG_MAX_FILE_PATH 56
> +
> +/* fw_cfg file directory entry type */
> +struct fw_cfg_file {
> +	u32 size;
> +	u16 select;
> +	u16 reserved;
> +	char name[FW_CFG_MAX_FILE_PATH];
> +};
> +
> +/* fw_cfg device i/o access options type */
> +struct fw_cfg_access {
> +	const char *name;
> +	phys_addr_t base;
> +	u8 size;
> +	u8 ctrl_offset;
> +	u8 data_offset;
> +	bool is_mmio;
> +};
> +
> +/* table of fw_cfg device i/o access options for known architectures */
> +static struct fw_cfg_access fw_cfg_modes[] = {
> +	{
> +		.name = "fw_cfg IOport on i386, sun4u",
> +		.base = 0x510,
> +		.size = 0x02,
> +		.ctrl_offset = 0x00,
> +		.data_offset = 0x01,
> +		.is_mmio = false,
> +	}, {
> +		.name = "fw_cfg MMIO on arm",
> +		.base = 0x9020000,
> +		.size = 0x0a,
> +		.ctrl_offset = 0x08,
> +		.data_offset = 0x00,
> +		.is_mmio = true,
> +	}, {
> +		.name = "fw_cfg MMIO on sun4m",
> +		.base = 0xd00000510,
> +		.size = 0x03,
> +		.ctrl_offset = 0x00,
> +		.data_offset = 0x02,
> +		.is_mmio = true,
> +	}, {
> +		.name = "fw_cfg MMIO on ppc/mac",
> +		.base = 0xf0000510,
> +		.size = 0x03,
> +		.ctrl_offset = 0x00,
> +		.data_offset = 0x02,
> +		.is_mmio = true,
> +	}, { } /* END */
> +};
> +
> +/* fw_cfg device i/o currently selected option set */
> +static struct fw_cfg_access *fw_cfg_mode;
> +
> +/* fw_cfg device i/o register addresses */
> +static void __iomem *fw_cfg_dev_base;
> +static void __iomem *fw_cfg_reg_ctrl;
> +static void __iomem *fw_cfg_reg_data;
> +
> +/* atomic access to fw_cfg device (potentially slow i/o, so using mutex) */
> +static DEFINE_MUTEX(fw_cfg_dev_lock);
> +
> +/* pick appropriate endianness for selector key */
> +static inline u16 fw_cfg_sel_endianness(u16 key)
> +{
> +	return fw_cfg_mode->is_mmio ? cpu_to_be16(key) : cpu_to_le16(key);
> +}
> +
> +/* type for fw_cfg "directory scan" visitor/callback function */
> +typedef int (*fw_cfg_file_callback)(const struct fw_cfg_file *f);
> +
> +/* run a given callback on each fw_cfg directory entry */
> +static int fw_cfg_scan_dir(fw_cfg_file_callback callback)
> +{
> +	int ret = 0;
> +	u32 count, i;
> +	struct fw_cfg_file f;
> +
> +	mutex_lock(&fw_cfg_dev_lock);
> +	iowrite16(fw_cfg_sel_endianness(FW_CFG_FILE_DIR), fw_cfg_reg_ctrl);
> +	ioread8_rep(fw_cfg_reg_data, &count, sizeof(count));
> +	for (i = 0; i < be32_to_cpu(count); i++) {
> +		ioread8_rep(fw_cfg_reg_data, &f, sizeof(f));
> +		ret = callback(&f);
> +		if (ret)
> +			break;
> +	}
> +	mutex_unlock(&fw_cfg_dev_lock);
> +	return ret;
> +}
> +
> +/* read chunk of given fw_cfg blob (caller responsible for sanity-check) */
> +static inline void fw_cfg_read_blob(u16 key,
> +				    void *buf, loff_t pos, size_t count)
> +{
> +	mutex_lock(&fw_cfg_dev_lock);
> +	iowrite16(fw_cfg_sel_endianness(key), fw_cfg_reg_ctrl);
> +	while (pos-- > 0)
> +		ioread8(fw_cfg_reg_data);
> +	ioread8_rep(fw_cfg_reg_data, buf, count);
> +	mutex_unlock(&fw_cfg_dev_lock);
> +}
> +
> +/* clean up fw_cfg device i/o */
> +static void fw_cfg_io_cleanup(void)
> +{
> +	if (fw_cfg_mode->is_mmio) {
> +		iounmap(fw_cfg_dev_base);
> +		release_mem_region(fw_cfg_mode->base, fw_cfg_mode->size);
> +	} else {
> +		ioport_unmap(fw_cfg_dev_base);
> +		release_region(fw_cfg_mode->base, fw_cfg_mode->size);
> +	}
> +}
> +
> +/* probe and map fw_cfg device */
> +static int __init fw_cfg_io_probe(void)
> +{
> +	char sig[FW_CFG_SIG_SIZE];
> +
> +	for (fw_cfg_mode = &fw_cfg_modes[0];
> +	     fw_cfg_mode->base; fw_cfg_mode++) {
> +
> +		phys_addr_t base = fw_cfg_mode->base;
> +		u8 size = fw_cfg_mode->size;
> +
> +		/* reserve and map mmio or ioport region */
> +		if (fw_cfg_mode->is_mmio) {
> +			if (!request_mem_region(base, size, fw_cfg_mode->name))
> +				continue;
> +			fw_cfg_dev_base = ioremap(base, size);
> +			if (!fw_cfg_dev_base) {
> +				release_mem_region(base, size);
> +				continue;
> +			}
> +		} else {
> +			if (!request_region(base, size, fw_cfg_mode->name))
> +				continue;
> +			fw_cfg_dev_base = ioport_map(base, size);
> +			if (!fw_cfg_dev_base) {
> +				release_region(base, size);
> +				continue;
> +			}
> +		}
> +
> +		/* set control and data register addresses */
> +		fw_cfg_reg_ctrl = fw_cfg_dev_base + fw_cfg_mode->ctrl_offset;
> +		fw_cfg_reg_data = fw_cfg_dev_base + fw_cfg_mode->data_offset;
> +
> +		/* verify fw_cfg device signature */
> +		fw_cfg_read_blob(FW_CFG_SIGNATURE, sig, 0, FW_CFG_SIG_SIZE);
> +		if (memcmp(sig, "QEMU", FW_CFG_SIG_SIZE) == 0)
> +			/* success, we're done */
> +			return 0;
> +
> +		/* clean up before probing next access mode */
> +		fw_cfg_io_cleanup();
> +	}
> +
> +	return -ENODEV;
> +}
> +
> +/* fw_cfg revision attribute, in /sys/firmware/qemu_fw_cfg top-level dir. */
> +static u32 fw_cfg_rev;
> +
> +static ssize_t fw_cfg_showrev(struct kobject *k, struct attribute *a, char *buf)
> +{
> +	return sprintf(buf, "%u\n", fw_cfg_rev);
> +}
> +
> +static const struct {
> +	struct attribute attr;
> +	ssize_t (*show)(struct kobject *k, struct attribute *a, char *buf);
> +} fw_cfg_rev_attr = {
> +	.attr = { .name = "rev", .mode = S_IRUSR },
> +	.show = fw_cfg_showrev,
> +};
> +
> +/* fw_cfg_sysfs_entry type */
> +struct fw_cfg_sysfs_entry {
> +	struct kobject kobj;
> +	struct fw_cfg_file f;
> +	struct list_head list;
> +};
> +
> +/* get fw_cfg_sysfs_entry from kobject member */
> +static inline struct fw_cfg_sysfs_entry *to_entry(struct kobject *kobj)
> +{
> +	return container_of(kobj, struct fw_cfg_sysfs_entry, kobj);
> +}
> +
> +/* fw_cfg_sysfs_attribute type */
> +struct fw_cfg_sysfs_attribute {
> +	struct attribute attr;
> +	ssize_t (*show)(struct fw_cfg_sysfs_entry *entry, char *buf);
> +};
> +
> +/* get fw_cfg_sysfs_attribute from attribute member */
> +static inline struct fw_cfg_sysfs_attribute *to_attr(struct attribute *attr)
> +{
> +	return container_of(attr, struct fw_cfg_sysfs_attribute, attr);
> +}
> +
> +/* global cache of fw_cfg_sysfs_entry objects */
> +static LIST_HEAD(fw_cfg_entry_cache);
> +
> +/* kobjects removed lazily by kernel, mutual exclusion needed */
> +static DEFINE_SPINLOCK(fw_cfg_cache_lock);
> +
> +static inline void fw_cfg_sysfs_cache_enlist(struct fw_cfg_sysfs_entry *entry)
> +{
> +	spin_lock(&fw_cfg_cache_lock);
> +	list_add_tail(&entry->list, &fw_cfg_entry_cache);
> +	spin_unlock(&fw_cfg_cache_lock);
> +}
> +
> +static inline void fw_cfg_sysfs_cache_delist(struct fw_cfg_sysfs_entry *entry)
> +{
> +	spin_lock(&fw_cfg_cache_lock);
> +	list_del(&entry->list);
> +	spin_unlock(&fw_cfg_cache_lock);
> +}
> +
> +static void fw_cfg_sysfs_cache_cleanup(void)
> +{
> +	struct fw_cfg_sysfs_entry *entry, *next;
> +
> +	list_for_each_entry_safe(entry, next, &fw_cfg_entry_cache, list) {
> +		/* will end up invoking fw_cfg_sysfs_cache_delist()
> +		 * via each object's release() method (i.e. destructor)
> +		 */
> +		kobject_put(&entry->kobj);
> +	}
> +}
> +
> +/* default_attrs: per-entry attributes and show methods */
> +
> +#define FW_CFG_SYSFS_ATTR(_attr) \
> +struct fw_cfg_sysfs_attribute fw_cfg_sysfs_attr_##_attr = { \
> +	.attr = { .name = __stringify(_attr), .mode = S_IRUSR }, \
> +	.show = fw_cfg_sysfs_show_##_attr, \
> +}
> +
> +static ssize_t fw_cfg_sysfs_show_size(struct fw_cfg_sysfs_entry *e, char *buf)
> +{
> +	return sprintf(buf, "%u\n", e->f.size);
> +}
> +
> +static ssize_t fw_cfg_sysfs_show_key(struct fw_cfg_sysfs_entry *e, char *buf)
> +{
> +	return sprintf(buf, "%u\n", e->f.select);
> +}
> +
> +static ssize_t fw_cfg_sysfs_show_name(struct fw_cfg_sysfs_entry *e, char *buf)
> +{
> +	return sprintf(buf, "%s\n", e->f.name);
> +}
> +
> +static FW_CFG_SYSFS_ATTR(size);
> +static FW_CFG_SYSFS_ATTR(key);
> +static FW_CFG_SYSFS_ATTR(name);
> +
> +static struct attribute *fw_cfg_sysfs_entry_attrs[] = {
> +	&fw_cfg_sysfs_attr_size.attr,
> +	&fw_cfg_sysfs_attr_key.attr,
> +	&fw_cfg_sysfs_attr_name.attr,
> +	NULL,
> +};
> +
> +/* sysfs_ops: find fw_cfg_[entry, attribute] and call appropriate show method */
> +static ssize_t fw_cfg_sysfs_attr_show(struct kobject *kobj, struct attribute *a,
> +				      char *buf)
> +{
> +	struct fw_cfg_sysfs_entry *entry = to_entry(kobj);
> +	struct fw_cfg_sysfs_attribute *attr = to_attr(a);
> +
> +	return attr->show(entry, buf);
> +}
> +
> +static const struct sysfs_ops fw_cfg_sysfs_attr_ops = {
> +	.show = fw_cfg_sysfs_attr_show,
> +};
> +
> +/* release: destructor, to be called via kobject_put() */
> +static void fw_cfg_sysfs_release_entry(struct kobject *kobj)
> +{
> +	struct fw_cfg_sysfs_entry *entry = to_entry(kobj);
> +
> +	fw_cfg_sysfs_cache_delist(entry);
> +	kfree(entry);
> +}
> +
> +/* kobj_type: ties together all properties required to register an entry */
> +static struct kobj_type fw_cfg_sysfs_entry_ktype = {
> +	.default_attrs = fw_cfg_sysfs_entry_attrs,
> +	.sysfs_ops = &fw_cfg_sysfs_attr_ops,
> +	.release = fw_cfg_sysfs_release_entry,
> +};
> +
> +/* raw-read method and attribute */
> +static ssize_t fw_cfg_sysfs_read_raw(struct file *filp, struct kobject *kobj,
> +				     struct bin_attribute *bin_attr,
> +				     char *buf, loff_t pos, size_t count)
> +{
> +	struct fw_cfg_sysfs_entry *entry = to_entry(kobj);
> +
> +	if (pos > entry->f.size)
> +		return -EINVAL;
> +
> +	if (count > entry->f.size - pos)
> +		count = entry->f.size - pos;
> +
> +	fw_cfg_read_blob(entry->f.select, buf, pos, count);
> +	return count;
> +}
> +
> +static struct bin_attribute fw_cfg_sysfs_attr_raw = {
> +	.attr = { .name = "raw", .mode = 0400 },
> +	.read = fw_cfg_sysfs_read_raw,
> +};
> +
> +/* kobjects & kset representing top-level, by_key, and by_name folders */
> +static struct kobject *fw_cfg_top_ko;
> +static struct kobject *fw_cfg_sel_ko;
> +
> +/* callback function to register an individual fw_cfg file */
> +static int __init fw_cfg_register_file(const struct fw_cfg_file *f)
> +{
> +	int err;
> +	struct fw_cfg_sysfs_entry *entry;
> +
> +	/* allocate new entry */
> +	entry = kzalloc(sizeof(*entry), GFP_KERNEL);
> +	if (!entry)
> +		return -ENOMEM;
> +
> +	/* set file entry information */
> +	entry->f.size = be32_to_cpu(f->size);
> +	entry->f.select = be16_to_cpu(f->select);
> +	strcpy(entry->f.name, f->name);
> +
> +	/* register entry under "/sys/firmware/qemu_fw_cfg/by_key/" */
> +	err = kobject_init_and_add(&entry->kobj, &fw_cfg_sysfs_entry_ktype,
> +				   fw_cfg_sel_ko, "%d", entry->f.select);
> +	if (err)
> +		goto err_register;
> +
> +	/* add raw binary content access */
> +	err = sysfs_create_bin_file(&entry->kobj, &fw_cfg_sysfs_attr_raw);
> +	if (err)
> +		goto err_add_raw;
> +
> +	/* success, add entry to global cache */
> +	fw_cfg_sysfs_cache_enlist(entry);
> +	return 0;
> +
> +err_add_raw:
> +	kobject_del(&entry->kobj);
> +err_register:
> +	kfree(entry);
> +	return err;
> +}
> +
> +/* unregister top-level or by_key folder */
> +static inline void fw_cfg_kobj_cleanup(struct kobject *kobj)
> +{
> +	kobject_del(kobj);
> +	kobject_put(kobj);
> +}
> +
> +static int __init fw_cfg_sysfs_init(void)
> +{
> +	int err;
> +
> +	/* probe for the fw_cfg "hardware" */
> +	err = fw_cfg_io_probe();
> +	if (err)
> +		return err;
> +
> +	/* create /sys/firmware/qemu_fw_cfg/ and its subdirectories */
> +	err = -ENOMEM;
> +	fw_cfg_top_ko = kobject_create_and_add("qemu_fw_cfg", firmware_kobj);
> +	if (!fw_cfg_top_ko)
> +		goto err_top;
> +	fw_cfg_sel_ko = kobject_create_and_add("by_key", fw_cfg_top_ko);
> +	if (!fw_cfg_sel_ko)
> +		goto err_sel;
> +
> +	/* get revision number, add matching top-level attribute */
> +	fw_cfg_read_blob(FW_CFG_ID, &fw_cfg_rev, 0, sizeof(fw_cfg_rev));
> +	fw_cfg_rev = le32_to_cpu(fw_cfg_rev);
> +	err = sysfs_create_file(fw_cfg_top_ko, &fw_cfg_rev_attr.attr);
> +	if (err)
> +		goto err_rev;
> +
> +	/* process fw_cfg file directory entry, registering each file */
> +	err = fw_cfg_scan_dir(fw_cfg_register_file);
> +	if (err)
> +		goto err_scan;
> +
> +	/* success */
> +	pr_debug("fw_cfg: loaded.\n");
> +	return 0;
> +
> +err_scan:
> +	fw_cfg_sysfs_cache_cleanup();
> +	sysfs_remove_file(fw_cfg_top_ko, &fw_cfg_rev_attr.attr);
> +err_rev:
> +	fw_cfg_kobj_cleanup(fw_cfg_sel_ko);
> +err_sel:
> +	fw_cfg_kobj_cleanup(fw_cfg_top_ko);
> +err_top:
> +	fw_cfg_io_cleanup();
> +	return err;
> +}
> +
> +static void __exit fw_cfg_sysfs_exit(void)
> +{
> +	pr_debug("fw_cfg: unloading.\n");
> +	fw_cfg_sysfs_cache_cleanup();
> +	fw_cfg_kobj_cleanup(fw_cfg_sel_ko);
> +	fw_cfg_kobj_cleanup(fw_cfg_top_ko);
> +	fw_cfg_io_cleanup();
> +}
> +
> +module_init(fw_cfg_sysfs_init);
> +module_exit(fw_cfg_sysfs_exit);
>