Kernel configuration/Specific options

This article has additional information on lots of kernel config options, such as why we require them to be enabled or disabled in postmarketOS. See Kernel configuration for an overview.
pmbootstrap 2.x redirects to this page when your kernel is not passing the kconfig check. Not all options have a description here, and it is probably not a good effort of time to copy all of them here - look at kconfigcheck.toml or search the web for more information on these.
== Required to be enabled/disabled ==
Enable or disable the following options to get postmarketOS working. Use pmbootstrap kconfig check to check which options need to be modified.
=== CONFIG_DEVTMPFS ===
About
This adds support for the devtmpfs filesystem which should be mounted to /dev at startup. The kernel driver core creates device nodes for all registered devices in that filesystem. All device nodes are owned by root and have the default mode of 0600. Userspace can add and delete the nodes as needed.
Read more...
Location in menu
Device Drivers -->
 Generic Driver Options -->
 Maintain a devtmpfs filesystem to mount at /dev
pmOS specific
This should always be set, otherwise you get this when you try SSH:
PTY allocation request failed on channel 0
-ash: a: unknown operand
=== CONFIG_TMPFS_POSIX_ACL ===
 About
Support POSIX Access Control Lists (ACLs) for tmpfs-based filesystems.
Read more...
Location in menu
File Systems -->
 Pseudo Filesystems -->
 Tmpfs virtual memory file system support (former shm fs) -->
 Tmpfs POSIX Access Control Lists
pmOS specific
This is needed for some UIs (phosh) to be able to control bluetooth.
=== CONFIG_VT ===
About
If you say Y here, you will get support for terminal devices with display and keyboard devices. These are called "virtual" because you can run several virtual terminals (also called virtual consoles) on one physical terminal. This is rather useful, for example one virtual terminal can collect system messages and warnings, another one can be used for a text-mode user session, and a third could run an X session, all in parallel. Switching between virtual terminals is done with certain key combinations, usually Alt-<function key>.
Read more...
Location in menu
It depends on CONFIG_EXPERT, so enable it first:
General setup -->
 Configure standard kernel features (expert users)
Then:
Device Drivers -->
 Character Devices -->
 Enable TTY -->
 Virtual Terminal
pmOS specific
This should always be set, otherwise you cannot get virtual terminal over USB.
=== CONFIG_DM_CRYPT ===
About
This device-mapper target allows you to create a device that transparently encrypts the data on it.
Read more...
Location in menu
Device Drivers -->
 Multiple devices driver support (RAID and LVM) -->
 Device mapper support -->
 Crypt target support
pmOS specific
This should always be set, required for full disk encyption.
=== CONFIG_EXT4_FS ===
About
This is the next generation of the ext3 filesystem.
Read more...
Location in menu
File systems -->
 The Extended 4 (ext4) filesystem
pmOS specific
This should always be set since pmbootstrap creates an ext4 rootfs.
=== CONFIG_UEVENT_HELPER ===
About
The uevent helper program is forked by the kernel for every uevent. Before the switch to the netlink-based uevent source, this was used to hook hotplug scripts into kernel device events. It usually pointed to a shell script at /sbin/hotplug. This should not be used today, because usual systems create many events at bootup or device discovery in a very short time frame. One forked process per event can create so many processes that it creates a high system load, or on smaller systems it is known to create out-of-memory situations during bootup.
Read more...
Location in menu
Device Drivers -->
 Generic Driver Options -->
 Support for uevent helper
pmOS specific
This option should always be enabled on pmos kernels that are 3.16+, it's the config option that makes /proc/sys/kernel/hotplug appear.
=== CONFIG_KERNEL_GZIP, CONFIG_KERNEL_XZ, CONFIG_KERNEL_LZO and CONFIG_KERNEL_LZMA ===
About
;GZIP
: The old and tried gzip compression. It provides a good balance between compression ratio and decompression speed.
: Read more...
;XZ
: XZ uses the LZMA2 algorithm and instruction set specific BCJ filters which can improve compression ratio of executable code. The size of the kernel is about 30% smaller with XZ in comparison to gzip. The speed is about the same as with LZMA: The decompression speed of XZ is better than that of bzip2 but worse than gzip and LZO. Compression is slow.
: Read more...
;LZO
: Its compression ratio is the poorest among the choices. The kernel size is about 10% bigger than gzip; however its speed (both compression and decompression) is the fastest.
: Read more...
;LZMA
: This compression algorithm's ratio is best. Decompression speed is between gzip and bzip2. Compression is slowest. The kernel size is about 33% smaller with LZMA in comparison to gzip.
: Read more...
Location in menu
General Setup -->
 Kernel compression mode -->
 Gzip
 LZMA
 XZ
 LZO
pmOS specific
You should set one of these to Y.
* Gzip: Works without specifying anything else.
* LZMA: If kernel compilation fails with this compression type, read Troubleshooting:kernel#Kernel_compilation:_BusyBox_lzma_compression_fails.
* XZ: Make sure to add xz to makedepends in the APKBUILD of your kernel package.
* LZO: If you get lzop: high compression not compiled in during compilation of the kernel you probably need this patch or this other patch.
=== CONFIG_ANDROID_PARANOID_NETWORK ===
About
Android adds a "paranoid network" option to the Linux kernel, which restricts access to some networking features depending on the group of the calling process. The list of groups that are allowed access to networking features is in the kernel source file: /include/linux/android_aids.h 
Setting this config will enable the paranoid network security feature''
Read more...
Location in menu
Networking Support -->
 Networking Options -->
 Only allow certain groups to create sockets
Android specific
This option should not be set in pmOS or any other OS not using Android security model.
=== CONFIG_PFT===
About
This caused builds to fail with a modern GCC at least once, and while it originally was meant as security feature by Qualcomm, "it is unsupported by Qualcomm, and opens up to a wide range of potential attack surfaces that has not been audited by anyone." .
Location in menu
Device Drivers -->
 Qualcomm MSM specific device drivers -->
 Per-File-Tagger driver
pmOS specific
Please disable it, so we have one less source for failing builds and enabling it has no benefit anyway.
=== CONFIG_SYSVIPC ===
Location in menu
General Setup -->
 System V IPC
pmOS specific
Please enable it for postmarketOS, so busybox' syslogd can log to a ringbuffer in the kernel (fixes ).
=== CONFIG_USE_VFB===
About
Weston and Plasma Mobile might choose to draw on this virtual frame buffer, just like it happened for the samsung-jflte (). Disabling the USE_VFB option fixed this, so let's make sure we don't have that option enabled for any kernel.
Location in menu
Device Drivers -->
 Graphics supports -->
 MSM Framebuffer support -->
 MSM Virtual Framebuffer support
pmOS specific
Please disable it, so we have one less source of failure.
=== CONFIG_KINETO_GAN ===
About
Related to a proprietary "T-Mobile's WiFi-Calling" feature. It should not affect normal calls, so it should be safe to disable it.
Location in menu
Device Drivers -->
 Network device support -->
 Kineto GAN virtual network interface
pmOS specific
Causes sudo, ip and other commands to hang (). Please disable it.
=== CONFIG_BLK_DEV_INITRD ===
About
Initial RAM filesystem and RAM disk (initramfs/initrd) support
Location in menu
General setup -->
 Initial RAM filesystem and RAM disk (initramfs/initrd) support
pmOS specific
Required. See for notes about changing the initramfs compression.
=== CONFIG_BLK_DEV_RAM ===
About
Enable this would make the kernel initial rootfs image as initramfs
pmOS specific
Need to Disable on some device because it is preventing the kernel to load cpio based inirid when the kernel is booted with uboot on some device.
=== CONFIG_CGROUPS ===
About
This option adds support for grouping sets of processes together, for use with process control subsystems such as Cpusets, CFS, memory controls or device isolation.
Read more...
Location in menu
General setup -->
 Control Group support
pmOS specific
Required for elogind, and therefore necessary to be able to run most desktop environments.
=== CONFIG_CRYPTO_XTS ===
About
XTS: IEEE1619/D16 narrow block cipher
Read more...
Location in menu
Cryptographic API -->
 Length-preserving ciphers and modes -->
 XTS support
pmOS specific
Needed for the default cryptsetup cipher in pmbootstrap (aes-xts-plain64) ()
=== CONFIG_SAMSUNG_TUI ===
About
Provide methods for TUI such as acquisition of buffer to be used as secure frame buffer in Secure World and synchronization of display and touch device between Normal World and Secure World.
Location in menu
Device Drivers -->
 Misc devices -->
 TUI HW Handler
pmOS specific
Needs to be disabled. Prevents device from booting.
=== CONFIG_SEC_RESTRICT_ROOTING ===
About
Restrict unauthorized executions with root permission.
Location in menu
Kernel hacking -->
 Samsung Rooting Restriction Feature
pmOS specific
Needs to be disabled. Blocks user from gaining root permissions.
=== CONFIG_TZDEV ===
About
Samsung TZ Based Secure OS interface driver
Location in menu
Device Drivers -->
 Misc devices -->
 Samsung TZ Based Secure OS Support
pmOS specific
Needs to be disabled. Prevents device from booting.
=== CONFIG_INPUT_EVDEV ===
About
makes input device events accessible under char device 13:64+ - /dev/input/eventX in a generic way
Location in menu
Device Drivers --> 
 Input device support -->
 Generic input layer (needed for keyboard, mouse, ...) (INPUT [=y]) -->
 Event Interface
pmOS specific
Required by `pmbootstrap kconfig check`
== Optional ==
These kernel options are optional and for some devices required to fix some issues.
=== CONFIG_INPUT_UINPUT ===
About
User level driver support
fbkeyboard needs this to work
=== CONFIG_LBDAF ===
About
Support for large (2TB+) block devices and files
The ext4 filesystem requires that this feature be enabled in order to support filesystems that have the huge_file feature enabled. Otherwise, it will refuse to mount in the read-write mode any filesystems that use the huge_file feature, which is enabled by default by mke2fs.ext4.
Location in menu
-*- Enable the block layer --->
 [*] Support for large (2TB+) block devices and files
pmOS specific
This is recommended for all 32bit (armhf, armv7, x86) devices. The option is not present on 64bit (aarch64, x86_64) devices.
Without this option, it is not possible to mount ext4 partitions that have been created with the default settings. The postmarketOS SD card installation uses these default parameters. There doesn't seem to be a point in changing that to work around badly configured kernels: even if the postmarketOS installation did not require this option, you might come across an SD card with ext4, that does not work on your device.
=== CONFIG_USB_G_ANDROID ===
About
The Android Composite Gadget supports multiple USB functions: adb, acm, mass storage, mtp, accessory and rndis.
Each function can be configured and enabled/disabled dynamically from userspace through a sysfs interface.
Location in menu
Device Drivers -->
 USB Support -->
 USB Gadget Support -->
 USB Gadget Drivers -->
 Android Composite Gadget
pmOS specific
Enabling this option is needed for usb networking on some of the android kernels.
=== CONFIG_USB_ETH ===
About
This driver implements Ethernet style communication, in one of several ways:
* The "Communication Device Class" (CDC) Ethernet Control Model. That protocol is often avoided with pure Ethernet adapters, in favor of simpler vendor-specific hardware, but is widely supported by firmware for smart network devices.
* On hardware can't implement that protocol, a simple CDC subset is used, placing fewer demands on USB.
* CDC Ethernet Emulation Model (EEM) is a newer standard that has a simpler interface that can be used by more USB hardware.
RNDIS support is an additional option, more demanding than than subset.
Read more...
Location in menu
Device Drivers -->
 USB Support -->
 USB Gadget Support -->
 USB Gadget Drivers -->
 Ethernet Gadget
pmOS specific
Most devices do not need this to be set. If telnet/SSH does not work you can try and set this option to Y.
=== CONFIG_USB_ETH_RNDIS ===
About
Microsoft Windows XP bundles the "Remote NDIS" (RNDIS) protocol, and Microsoft provides redistributable binary RNDIS drivers for older versions of Windows.
If you say "y" here, the Ethernet gadget driver will try to provide a second device configuration, supporting RNDIS to talk to such Microsoft USB hosts.
Read more...
Location in menu
Device Drivers -->
 USB Support -->
 USB Gadget Support -->
 USB Gadget Drivers -->
 RNDIS Support
pmOS specific
Required if you set CONFIG_USB_ETH to 'Y'.
=== CONFIG_MSM_KGSL ===
About
3D graphics driver. Required to use hardware accelerated OpenGL ES 2.0 and 1.1.
Read more...
Location in menu
Device Drivers -->
 Graphics Support -->
 MSM 3D graphics Driver
pmOS specific
If framebuffer does not work set this to 'N'
=== CONFIG_CMDLINE ===
About
Enter arguments here that should be compiled into the kernel image and used at boot time. If the boot loader provides a command line at boot time, it is appended to this string to form the full kernel command line, when the system boots.
Read more...
Location in menu
General Setup -->
 Boot options -->
 Default kernel command string
or
 Boot options -->
 Default kernel command string
pmOS specific
No
=== CONFIG_FHANDLE ===
About
open by fhandle syscalls
Location in menu
General setup -->
 open by fhandle syscalls
pmOS specific
If USB OTG isn't working for you, try enable this config.
=== CONFIG_BUILD_ARM_APPENDED_DTB_IMAGE ===
About
Build a concatenated zImage/dtb by default
Enabling this option will cause a concatenated zImage and list of DTBs to be built by default (instead of a standalone zImage.)
The image will built in arch/arm/boot/zImage-dtb
Location in menu
General Setup -->
 Boot options -->
 appended device tree blob to zImage
pmOS specific
Set this if your device needs an appended device tree. If you are unsure, use extract-dtb on an existing kernel image. Not to be confused with CONFIG_ARM_APPENDED_DTB!
=== CONFIG_FW_LOADER_USER_HELPER ===
About
Enable the firmware sysfs fallback mechanism found in drivers/base/firmware_loader/Kconfig
Location in menu
Device Drivers -->
 Generic Driver Options -->
 Firmware loader -->
 Firmware loading facility (FW_LOADER [=y]) -->
 Enable the firmware sysfs fallback mechanism
pmOS specific
Prior to kernel version 3.7, the kernel had no capability of finding firmware itself so it had to call out to userspace to find the firmware for it. This changed with kernel 3.7 where the kernel gained "direct loading support" and where the userspace helper was only called as a fallback. In kernel 3.9 this functionality was put behind the kconfig option FW_LOADER_USER_HELPER which is what this patch checks to be enabled.
Without this option, the udev rule present in postmarketos-base won't be notified that firmware needs to be loaded and as such can't call firmwareload.sh which loads firmware from /lib/firmware/postmarketos.
=== CONFIG_KEY_DH_OPERATIONS ===
↓ fallthrough
=== CONFIG_CRYPTO_DH ===
↓ fallthrough
=== CONFIG_CRYPTO_DES ===
↓ fallthrough
=== CONFIG_CRYPTO_USER_API_SKCIPHER ===
↓ fallthrough
=== CONFIG_CRYPTO_USER_API_HASH ===
↓ fallthrough
=== CONFIG_CRYPTO_ECB ===
↓ fallthrough
=== CONFIG_CRYPTO_MD4 ===
↓ fallthrough
=== CONFIG_CRYPTO_MD5 ===
↓ fallthrough
=== CONFIG_CRYPTO_CBC ===
↓ fallthrough
=== CONFIG_CRYPTO_SHA1 ===
↓ fallthrough
=== CONFIG_CRYPTO_SHA256 ===
↓ fallthrough
=== CONFIG_CRYPTO_SHA512 ===
↓ fallthrough
=== CONFIG_CRYPTO_AES ===
↓ fallthrough
=== CONFIG_CRYPTO_DES ===
↓ fallthrough
=== CONFIG_CRYPTO_CMAC ===
↓ fallthrough
=== CONFIG_CRYPTO_HMAC ===
↓ fallthrough
About
These options enables ability to use iwd daemon (wpa_supplicant replacement)
Location in menu
Security Options --->
	[*] Enable access key retention support
		[*] Diffie-Hellman operations on retained keys
Cryptographic API --->
	[*] Diffie-Hellman algorithm
	[*] DES and Triple DES EDE cipher algorithms
	[*] User-space interface for symmetric key cipher algorithms
pmOS specific
Enable only on recent mainline kernels. After enabling these options, you can use iwd daemon (better performance) instead of wpa_supplicant.
=== CONFIG_USER_NS ===
About
User namespace
Required for BubbleWrap sandboxing used by The GNOME Web (Epiphany) and other software.
pmOS specific
Phosh environment must have it enabled for Web browser (Epiphany) being functional.
Location in menu
General setup --->
 Namespaces support --->
 [*] User namespace
=== CONFIG_DEVPTS_MULTIPLE_INSTANCES ===
About
Support multiple instances of devpts
Required for BubbleWrap sandboxing used by The GNOME Web (Epiphany) and other software.
pmOS specific
Phosh environment must have it enabled for Web browser (Epiphany) being functional.
Location in menu
Device Drivers -->
 Character Devices -->
 Enable TTY -->
 Unix98 PTY support -->
 [*] Support multiple instances of devpts
=== DRM_UDL ===
See Displaylink/UDL
=== DRM_GUD ===
See GUD
=== SECCOMP ===
About
Enable seccomp to safely compute untrusted bytecode
Required for BubbleWrap sandboxing used by The GNOME Web (Epiphany) and other software.
pmOS specific
Phosh environment must have it enabled for Web browser (Epiphany) being functional.
== Anbox ==
To enable Anbox support, some options are required
=== CONFIG_SQUASHFS ===
About
Squashfs is a highly compressed read-only filesystem for Linux. It uses zlib, lzo or xz compression to compress both files, inodes and directories. squashfs is intended for general read-only filesystem use, for archival use (i.e. in cases where a .tar.gz file may be used), and in embedded systems where low overhead is needed.
Anbox specific
Anbox uses a squashfs to store the Android rootfs.
=== CONFIG_SQUASHFS_XZ ===
About
Squashfs support for reading squashfs file systems compressed with XZ compression. XZ gives better compression than the default zlib compression, at the expense of greater CPU and memory overhead.
Anbox specific
Anbox uses a xz-compressed squashfs.
=== CONFIG_SQUASHFS_XATTR ===
About
Squashfs support for extended attributes (xattrs). Xattrs are name:value pairs associated with inodes by the kernel or by users (see the attr(5) manual page).
Anbox specific
For some reason, even if the squashfs is created with -no-xattrs,
Anbox seems to fail badly without.
=== CONFIG_ASHMEM ===
About
The ashmem subsystem is a new shared memory allocator, similar to POSIX SHM but with different behavior and sporting a simpler file-based API.
It is, in theory, a good memory allocator for low-memory devices, because it can discard shared memory units when under memory pressure.
Anbox specific
Android requires it, so you need it to run Anbox.
=== CONFIG_BINDER_IPC ===
About
Binder is used in Android for both communication between processes and remote method invocation. This means one Android process can call a method/routine in another Android process, using Binder to identify, invoke and pass arguments between said processes.
Anbox specific
Android uses it to communicate between process. It must be present for Anbox to work.
=== CONFIG_BINDERFS ===
About
Binderfs is a pseudo-filesystem for the Android Binder IPC driver which can be mounted per-ipc namespace allowing to run multiple instances of Android. Each binderfs mount initially only contains a binder-control device. It can be used to dynamically allocate new binder IPC devices via ioctls.
Anbox-on-postmarketOS specific
It should not be set, because binderfs support in Anbox is broken in postmarketOS. For now, we just use regular binder instead because we don't need several binders.
It could be an incompatibility with musl. In sys/ioctl.h, the ioctl function is prototyped as such: int ioctl(int, int, ...);, in accordance to POSIX. On the hand, in the linux programmer manual, ioctl is of type int ioctl(int, unsigned long, ...);. GCC warns about it: "Warning: overflow in conversion from 'long unsigned int' to 'int' change value from 3238552065 to -1056415231", but according to dalias on #musl it should be harmless.
=== CONFIG_ANDROID_BINDER_DEVICES ===
About
Default value for the binder.devices parameter. The binder.devices parameter is a comma-separated list of strings that specifies the names of the binder device nodes that will be created. Each binder device has its own context manager, and is therefore logically separated from the other devices.
Anbox specific
This must contain binder, as Anbox expects /dev/binder.
It should also contain hwbinder to be future-proof (as it will be required for Android 10).
=== CONFIG_TMPFS_XATTR ===
About
Xattr support for tmpfs.
Anbox specific
Android requires it because it sets xattr on files in /dev, which is a tmpfs. Otherwise you get errors such as this in /var/lib/anbox/console.log :
 libc: fsetxattr failed to set context (u:object_r:dalvik_prop:s0) for "/dev/__properties__/u:object_r:dalvik_prop:s0"
 libc: fsetxattr failed to set context (u:object_r:config_prop:s0) for "/dev/__properties__/u:object_r:config_prop:s0"
=== NETFILTER_XT_MATCH_COMMENT ===
About
It allows putting comments in iptables rules. It's a dummy module that ignore its --comment= argument.
depends on NETFILTER, NETFILTER_XTABLES and NETFILTER_ADVANCED.
pmOS specific
It is a requirement of anbox-bridge.sh, which we use in the openRC init script. Anbox won't start without that. 
= See also =
* Linux Kernel Driver Database
* How to find configuration options quickly in make menuconfig
* Compiling kernels with envkernel.sh
* make it easy to change kernel config options with pmbootstrap
Category:Technical Reference