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