[PATCH v6 05/13] firmware_loader: enhance Kconfig documentation over FW_LOADER

Luis R. Rodriguez
2018-05-08T18:12:39Z

If you try to read FW_LOADER today it speaks of old riddles and
unless you have been following development closely you will loose
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 compartamentalize 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

Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
---
drivers/base/Kconfig | 165 ++++++++++++++++++++++++++++++++++---------
1 file changed, 131 insertions(+), 34 deletions(-)

diff --git a/drivers/base/Kconfig b/drivers/base/Kconfig
index 29b0eb452b3a..a4fe86caecca 100644
--- a/drivers/base/Kconfig
+++ b/drivers/base/Kconfig
@@ -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 firmwar 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 these 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 preceeded
+ 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 firwmare is always used first now.
+
+ If the kernel's direct filesystem lookup for firware 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 supressed 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 possibily 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
--
2.17.0