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FIRMWARE(9) Kernel Developer's Manual FIRMWARE(9)


firmware_register, firmware_unregister, firmware_get, firmware_putfirmware image loading and management


#include <sys/param.h>
#include <sys/systm.h>
#include <sys/linker.h>
#include <sys/firmware.h>

struct firmware {
	const char	*name;		/* system-wide name */
	const void	*data;		/* location of image */
	size_t		datasize;	/* size of image in bytes */
	unsigned int	version;	/* version of the image */

const struct firmware *
firmware_register(const char *imagename, const void *data, size_t datasize, unsigned int version, const struct firmware *parent);

firmware_unregister(const char *imagename);

const struct firmware *
firmware_get(const char *imagename);

firmware_put(const struct firmware *fp, int flags);


The firmware abstraction provides a convenient interface for loading firmware images into the kernel, and for accessing such images from kernel components.

A firmware image (or image for brevity) is an opaque block of data residing in kernel memory. It is associated to a unique imagename which constitutes a search key, and to an integer version number, which is also an opaque piece of information for the firmware subsystem.

An image is registered with the firmware subsystem by calling the function (), and unregistered by calling firmware_unregister(). These functions are usually (but not exclusively) called by specially crafted kernel modules that contain the firmware image. The modules can be statically compiled in the kernel, or loaded by /boot/loader, manually at runtime, or on demand by the firmware subsystem.

Clients of the firmware subsystem can request access to a given image by calling the function () with the imagename they want as an argument. If a matching image is not already registered, the firmware subsystem will try to load it using the mechanisms specified below (typically, a kernel module with firmware_register the same name as the image).


The kernel firmware_register firmware API is made of the following functions:

() registers with the kernel an image of size datasize located at address data, under the name imagename.

The function returns NULL on error (e.g. because an image with the same name already exists, or the image table is full), or a const struct firmware * pointer to the image requested.

() tries to unregister the firmware image imagename from the system. The function is successful and returns 0 if there are no pending references to the image, otherwise it does not unregister the image and returns EBUSY.

() returns the requested firmware image. If the image is not yet registered with the system, the function tries to load it. This involves the linker subsystem and disk access, so firmware_get() must not be called with any locks (except for Giant). Note also that if the firmware image is loaded from a filesystem it must already be mounted. In particular this means that it may be necessary to defer requests from a driver attach method unless it is known the root filesystem is already mounted.

On success, () returns a pointer to the image description and increases the reference count for this image. On failure, the function returns NULL.

() drops a reference to a firmware image. The flags argument may be set to FIRMWARE_UNLOAD to indicate that firmware_put is free to reclaim resources associated with the firmware image if this is the last reference. By default a firmware image will be deferred to a taskqueue(9) thread so the call may be done while holding a lock. In certain cases, such as on driver detach, this cannot be allowed.


As mentioned before, any component of the system can register firmware images at any time by simply calling firmware_register().

This is typically done when a module containing a firmware image is given control, whether compiled in, or preloaded by /boot/loader, or manually loaded with kldload(8). However, a system can implement additional mechanisms to bring these images in memory before calling ().

When () does not find the requested image, it tries to load it using one of the available loading mechanisms. At the moment, there is only one, namely Loadable kernel modules:

A firmware image named foo is looked up by trying to load the module named foo.ko, using the facilities described in kld(4). In particular, images are looked up in the directories specified by the sysctl variable kern.module_path which on most systems defaults to /boot/kernel;/boot/modules.

Note that in case a module contains multiple images, the caller should first request a () for the first image contained in the module, followed by requests for the other images.


A firmware module is built by embedding the firmware image into a suitable loadable kernel module that calls firmware_register() on loading, and firmware_unregister() on unloading.

Various system scripts and makefiles let you build a module by simply writing a Makefile with the following entries:

        KMOD=   imagename
        FIRMWS= image_file:imagename[:version]
        .include <>

where KMOD is the basename of the module; FIRMWS is a list of colon-separated tuples indicating the image_file's to be embedded in the module, the imagename and version of each firmware image.

If you need to embed firmware images into a system, you should write appropriate entries in the <files.arch> file, e.g. this example is from sys/arm/xscale/ixp425/files.ixp425:

ixp425_npe_fw.c                         optional npe_fw                 \
        compile-with    "${AWK} -f $S/tools/fw_stub.awk			\
			IxNpeMicrocode.dat:npe_fw -mnpe -c${.TARGET}"	\
        no-implicit-rule before-depend local                            \
        clean           "ixp425_npe_fw.c"
# NB: ld encodes the path in the binary symbols generated for the
#     firmware image so link the file to the object directory to
#     get known values for reference in the _fw.c file.
IxNpeMicrocode.fwo  optional npe_fw					\
        dependency      "IxNpeMicrocode.dat"				\
        compile-with    "${LD} -b binary -d -warn-common		\
			    -r -d -o ${.TARGET} IxNpeMicrocode.dat"	\
        no-implicit-rule                                                \
        clean           "IxNpeMicrocode.fwo"
IxNpeMicrocode.dat                      optional npe_fw                 \
        dependency      ".PHONY"                                        \
        compile-with    "uudecode < $S/contrib/dev/npe/IxNpeMicrocode.dat.uu" \
        no-obj no-implicit-rule                                         \
        clean           "IxNpeMicrocode.dat"

Note that generating the firmware modules in this way requires the availability of the following tools: awk(1), make(1), the compiler and the linker.


kld(4), module(9)



The firmware system was introduced in FreeBSD 6.1.


This manual page was written by Max Laier <>.

August 2, 2008 Debian