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# SPDX-License-Identifier: GPL-2.0
menu "Firmware loader"
config FW_LOADER
tristate "Firmware loading facility" if EXPERT
default y
help
This enables the firmware loading facility in the kernel. The kernel
will first look for built-in firmware, if it has any. Next, it will
look for the requested firmware in a series of filesystem paths:
o firmware_class path module parameter or kernel boot param
o /lib/firmware/updates/UTS_RELEASE
o /lib/firmware/updates
o /lib/firmware/UTS_RELEASE
o /lib/firmware
Enabling this feature only increases your kernel image by about
828 bytes, enable this option unless you are certain you don't
need firmware.
You typically want this built-in (=y) but you can also enable this
as a module, in which case the firmware_class module will be built.
You also want to be sure to enable this built-in if you are going to
enable built-in firmware (CONFIG_EXTRA_FIRMWARE).
if FW_LOADER
config EXTRA_FIRMWARE
string "Build named firmware blobs into the kernel binary"
help
Device drivers which require firmware can typically deal with
having the kernel load firmware from the various supported
/lib/firmware/ paths. This option enables you to build into the
kernel firmware files. Built-in firmware searches are preceded
over firmware lookups using your filesystem over the supported
/lib/firmware paths documented on CONFIG_FW_LOADER.
This may be useful for testing or if the firmware is required early on
in boot and cannot rely on the firmware being placed in an initrd or
initramfs.
This option is a string and takes the (space-separated) names of the
firmware files -- the same names that appear in MODULE_FIRMWARE()
and request_firmware() in the source. These files should exist under
the directory specified by the EXTRA_FIRMWARE_DIR option, which is
/lib/firmware by default.
For example, you might set CONFIG_EXTRA_FIRMWARE="usb8388.bin", copy
the usb8388.bin file into /lib/firmware, and build the kernel. Then
any request_firmware("usb8388.bin") will be satisfied internally
inside the kernel without ever looking at your filesystem at runtime.
WARNING: If you include additional firmware files into your binary
kernel image that are not available under the terms of the GPL,
then it may be a violation of the GPL to distribute the resulting
image since it combines both GPL and non-GPL work. You should
consult a lawyer of your own before distributing such an image.
config EXTRA_FIRMWARE_DIR
string "Firmware blobs root directory"
depends on EXTRA_FIRMWARE != ""
default "/lib/firmware"
help
This option controls the directory in which the kernel build system
looks for the firmware files listed in the EXTRA_FIRMWARE option.
config FW_LOADER_USER_HELPER
bool "Enable the firmware sysfs fallback mechanism"
help
This option enables a sysfs loading facility to enable firmware
loading to the kernel through userspace as a fallback mechanism
if and only if the kernel's direct filesystem lookup for the
firmware failed using the different /lib/firmware/ paths, or the
path specified in the firmware_class path module parameter, or the
firmware_class path kernel boot parameter if the firmware_class is
built-in. For details on how to work with the sysfs fallback mechanism
refer to Documentation/driver-api/firmware/fallback-mechanisms.rst.
The direct filesystem lookup for firmware is always used first now.
If the kernel's direct filesystem lookup for firmware fails to find
the requested firmware a sysfs fallback loading facility is made
available and userspace is informed about this through uevents.
The uevent can be suppressed if the driver explicitly requested it,
this is known as the driver using the custom fallback mechanism.
If the custom fallback mechanism is used userspace must always
acknowledge failure to find firmware as the timeout for the fallback
mechanism is disabled, and failed requests will linger forever.
This used to be the default firmware loading facility, and udev used
to listen for uvents to load firmware for the kernel. The firmware
loading facility functionality in udev has been removed, as such it
can no longer be relied upon as a fallback mechanism. Linux no longer
relies on or uses a fallback mechanism in userspace. If you need to
rely on one refer to the permissively licensed firmwared:
https://github.com/teg/firmwared
Since this was the default firmware loading facility at one point,
old userspace may exist which relies upon it, and as such this
mechanism can never be removed from the kernel.
You should only enable this functionality if you are certain you
require a fallback mechanism and have a userspace mechanism ready to
load firmware in case it is not found. One main reason for this may
be if you have drivers which require firmware built-in and for
whatever reason cannot place the required firmware in initramfs.
Another reason kernels may have this feature enabled is to support a
driver which explicitly relies on this fallback mechanism. Only two
drivers need this today:
o CONFIG_LEDS_LP55XX_COMMON
o CONFIG_DELL_RBU
Outside of supporting the above drivers, another reason for needing
this may be that your firmware resides outside of the paths the kernel
looks for and cannot possibly be specified using the firmware_class
path module parameter or kernel firmware_class path boot parameter
if firmware_class is built-in.
A modern use case may be to temporarily mount a custom partition
during provisioning which is only accessible to userspace, and then
to use it to look for and fetch the required firmware. Such type of
driver functionality may not even ever be desirable upstream by
vendors, and as such is only required to be supported as an interface
for provisioning. Since udev's firmware loading facility has been
removed you can use firmwared or a fork of it to customize how you
want to load firmware based on uevents issued.
Enabling this option will increase your kernel image size by about
13436 bytes.
If you are unsure about this, say N here, unless you are Linux
distribution and need to support the above two drivers, or you are
certain you need to support some really custom firmware loading
facility in userspace.
config FW_LOADER_USER_HELPER_FALLBACK
bool "Force the firmware sysfs fallback mechanism when possible"
depends on FW_LOADER_USER_HELPER
help
Enabling this option forces a sysfs userspace fallback mechanism
to be used for all firmware requests which explicitly do not disable a
a fallback mechanism. Firmware calls which do prohibit a fallback
mechanism is request_firmware_direct(). This option is kept for
backward compatibility purposes given this precise mechanism can also
be enabled by setting the proc sysctl value to true:
/proc/sys/kernel/firmware_config/force_sysfs_fallback
If you are unsure about this, say N here.
endif # FW_LOADER
endmenu
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