Commit 02c39930 authored by Luis R. Rodriguez's avatar Luis R. Rodriguez Committed by Greg Kroah-Hartman
Browse files

firmware_loader: enhance Kconfig documentation over FW_LOADER 

If you try to read FW_LOADER today it speaks of old riddles and
unless you have been following development closely you will lose
track of what is what. Even the documentation for PREVENT_FIRMWARE_BUILD
is a bit fuzzy and how it fits into this big picture.

Give the FW_LOADER kconfig documentation some love with more up to
date developments and recommendations. While at it, wrap the FW_LOADER
code into its own menu to compartmentalize and make it clearer which
components really are part of the FW_LOADER. This should also make
it easier to later move these kconfig entries into the firmware_loader/
directory later.

This also now recommends using firmwared [0] for folks left needing a
uevent handler in userspace for the sysfs firmware fallback mechanis
given udev's uevent firmware mechanism was ripped out a while ago.

[0] https://github.com/teg/firmwared



Reviewed-by: default avatarKees Cook <keescook@chromium.org>
Signed-off-by: default avatarLuis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 84b5c4fe

drivers/base/Kconfig

+131 −34
Original line number Diff line number Diff line
@@ -70,39 +70,64 @@ config STANDALONE 
	  If unsure, say Y.



config PREVENT_FIRMWARE_BUILD

	bool "Prevent firmware from being built"

	bool "Disable drivers features which enable custom firmware building"

	default y

	help

	  Say yes to avoid building firmware. Firmware is usually shipped

	  with the driver and only when updating the firmware should a

	  rebuild be made.

	  If unsure, say Y here.

	  Say yes to disable driver features which enable building a custom

	  driver firmware at kernel build time. These drivers do not use the

	  kernel firmware API to load firmware (CONFIG_FW_LOADER), instead they

	  use their own custom loading mechanism. The required firmware is

	  usually shipped with the driver, building the driver firmware

	  should only be needed if you have an updated firmware source.



	  Firmware should not be being built as part of kernel, these days

	  you should always prevent this and say Y here. There are only two

	  old drivers which enable building of its firmware at kernel build

	  time:



	    o CONFIG_WANXL through CONFIG_WANXL_BUILD_FIRMWARE

	    o CONFIG_SCSI_AIC79XX through CONFIG_AIC79XX_BUILD_FIRMWARE



menu "Firmware loader"



config FW_LOADER

	tristate "Userspace firmware loading support" if EXPERT

	tristate "Firmware loading facility" if EXPERT

	default y

	---help---

	  This option is provided for the case where none of the in-tree modules

	  require userspace firmware loading support, but a module built

	  out-of-tree does.

	  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 "External firmware blobs to build into the kernel binary"

	depends on FW_LOADER

	string "Build named firmware blobs into the kernel binary"

	help

	  Various drivers in the kernel source tree may require firmware,

	  which is generally available in your distribution's linux-firmware

	  package.

	  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.



	  The linux-firmware package should install firmware into

	  /lib/firmware/ on your system, so they can be loaded by userspace

	  helpers on request.



	  This option allows firmware to be built into the kernel for the case

	  where the user either cannot or doesn't want to provide it from

	  userspace at runtime (for example, when the firmware in question is

	  required for accessing the boot device, and the user doesn't want to

	  use an initrd).

	  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()
@@ -113,7 +138,7 @@ config EXTRA_FIRMWARE 
	  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

	  without needing to call out to userspace.

	  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,
@@ -130,22 +155,94 @@ config EXTRA_FIRMWARE_DIR 
	  looks for the firmware files listed in the EXTRA_FIRMWARE option.



config FW_LOADER_USER_HELPER

	bool

	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 "Fallback user-helper invocation for firmware loading"

	depends on FW_LOADER

	select FW_LOADER_USER_HELPER

	bool "Force the firmware sysfs fallback mechanism when possible"

	depends on FW_LOADER_USER_HELPER

	help

	  This option enables / disables the invocation of user-helper

	  (e.g. udev) for loading firmware files as a fallback after the

	  direct file loading in kernel fails.  The user-mode helper is

	  no longer required unless you have a special firmware file that

	  resides in a non-standard path. Moreover, the udev support has

	  been deprecated upstream.

	  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



config WANT_DEV_COREDUMP

	bool

	help

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