Linux Kernel changes in 5.x for i.MX users and developers

[#1]

Overview

In this doc, I will try to give you a brief of the Linux kernel changes between 4.x and 5.4 (5.0/5.1/5.2/5.3/5.4), which related to i.MX users/developers:

• Important bug fix and improvements according to my experiences
• New features you should keep an eye on
• Interfaces changes may impact applications
• i.MX up-streaming

I cannot make sure every single change's description is 100% correct. Either, I cannot list all of the impacts to the i.MX platform due to my limited knowledge and experiences. I hope this doc can help you on developing kernel drivers, user applications or debugging issues of 5.x i.MX kernel release.

Changes

Subsystem/Modules	Detail changes	Comments	References
Kernel Cores	binder: new binderfs, a pseudo-filesystem for the Android Binder IPC driver which can be mounted per-ipc namespace allowing to run multiple instances of Android	Hmm... that may imply we can run multi-instance Android without Hypervisor?
	sysctl: add panic_print sysctl to configure which information to print at panic time, w/ replay option	So we can define the panic information for better debugging the crash issue. Also option for users to configure the "panic_print" to replay all dmesg in buffer, some of which they may have never seen due to the loglevel setting, which will help panic debugging	How to use
	boot: Add boot option ( driver_async_probe=... ) to specify drivers to be async probed	Asynchronous driver probing can help much on kernel fastboot, and this option can provide a flexible way to optimize and quickly verify async driver probe.	Asynchronous device/driver probing support [LWN.net]
	swiotlb: add debugfs to track swiotlb buffer usage	The device driver will not be able to do dma operations once swiotlb buffer is full, either because the driver is using so many IO TLB blocks inflight, or because there is memory leak issue in device driver. Export buffer usage in debugfs can help. Please note, almost all of the dma buffers on i.MX8 platform comes from swiotlb.	aarch64 Linux Kernel Memory Management
	New mount syscall API	The kernel supports a wide variety of filesystem types, and each has its own, often extensive set of options. As a result, the mount() system call is complex. This makes application code who need mount FS more clear.	Six (or seven) new system calls for filesystem mounting [LWN.net]
	New APIs to support pidfs	clone return pidfs, new syscall pidfd_send_signal(2), which uses file descriptors from /proc/<pid> as stable handles on struct pid.	Toward race-free process signaling [LWN.net]
	locking: rwsem improvement	unification and simpler micro-optimizations, performance improvement of this locking primitive.
	Preparations for PREEMPT_RT	It's excited that community decided to merge the PREEMPT_RT kernel changes into mainline. It's well known as rt-patch for hard real-time use cases like industry.	Index of /pub/linux/kernel/projects/rt/
Memory management	KASAN: Improved the KASAN performance for arm64	KernelAddressSANitizer (KASAN) is a dynamic memory error detector. It provides a fast and comprehensive solution for finding use-after-free and out-of-bounds bugs. It's useful when debugging kernel drivers and modules memory issue.	The Kernel Address Sanitizer (KASAN) — The Linux Kernel documentation
	Fragmentation avoidance improvements, reducing fragmentation events by over 90%. With this change, the page allocator would spread allocations across zones before introducing fragmentation.	We can always found customer's product memory fragment in some use cases in old kernel. Which would cause kmalloc or cma alloc failure and system stop. Balance between zones (i.MX8 we have DMA/NORMAL zones) to avoid fragment is very helpful.	patch intro One of customer case(fixed by not using kmalloc): https://jira.sw.nxp.com/browse/MLK-23220
	Increase success rates and reduce latency of compaction (physical memory defragmentation)	Memory compaction is the way to avoid memory fragment.	Memory compaction [LWN.net]
	Improve Out Of Memory (OOM) reports, include victim's memcg	More clear kernel OOM report may help lot
	Remove the ancient OOM killer heuristic that preferred to kill children of the "worst" process rather than the process itself	Improve the OOM efficiency
	zram improvements, which can help estimate wasted memory, and perform writeback that will free it	zram is actually used very widely in the Android system as backup swap for physical memory. Swap performance is very important.	Low RAM Configuration  |  Android Open Source Project
	Simplify some of the early memory allocations by replacing usage of older memblock APIs with newer and shinier ones	memblock API changes. If you used memblock in your drivers, then need to check.	Boot time memory management — The Linux Kernel documentation
	psi: Improves the Pressure Stall Information resource monitoring	With this mechanism, Android can monitor for, and ward off, mounting memory shortages before they cause problems for the user. For example, using memory stall, monitors in userspace like the low memory killer daemon (lmkd) can detect mounting pressure and kill less important processes. Supported in Android10, replace the vmpressure.	Low Memory Killer Daemon (lmkd)  |  Android Open Source Project  Pressure stall monitors [LWN.net]
	Improve vmap allocation	Speed up the vmalloc	https://lkml.org/lkml/2018/10/19/786
	Introduce madvise() flags MADV_COLD, MADV_FREE, MADV_PAGEOUT	MADV_COLD marks pages as inactive (thus more easily reclaimed under memory pressure), but doesn't discard the contents like MADV_FREE does, and MADV_PAGEOUT , which reclaims pages immediately. Android would use this flags for better memory management	kernel/git/torvalds/linux.git - Linux kernel source tree
	memcg: from v1->v2	shrink all memcg caches for the slab cache Throttle allocators when reclaim cannot keep up with v2 memory.high limit Introduce gradual reclaim pressure.....	Control Group v2 — The Linux Kernel documentation
Block layer	Boot to a device-mapper device without initramfs	DM is widely used, you can not bootup to DM rootfs directly. Android 10 would use dynamic partition by creating super partition for /system, /vendor of dm-linear	Implementing Dynamic Partitions  |  Android Open Source Project
Tracing and Perf	Perf: lots of improvement	TBD
Security	security: Create "kernel hardening" config area	Help mitigate kernel vulnerabilities and find bugs in kernel drivers, like Stack buffer overflow mitigation, Hardened usercopy	Kernel Hardening  |  Android Open Source Project
	LSM: Add kernel lockdown functionality	When enabled, the new "lockdown" feature will restrict some kernel functionality, even for the root user, making it harder for compromised root accounts to compromise the rest of the OS	Kernel lockdown in 4.17? [LWN.net]
Networking	Enable MSG_ZEROCOPY for udp sockets	Improve the UDP sending performance. Pay attention that the zero copy UDP sockets only limit to the send operations, not receive. Also require application code change, not efficient at small MTU.	Zero-copy networking [LWN.net]
ARM/ARM64	New SoCs: i.MX7ULP with EVK i.MX8MQ with EVK i.MX8MM  i.MX8QXP
	perf vendor events: Add Cortex-A57 and Cortex-A72 events	support all ARMv8 recommended events
	Make CONFIG_ZONE_DMA32 configurable commit	Which means you can remove the DMA zone, and use only one single normal zone in kernel.
Drivers	drm: Initial merge of timeline sync objects	Timeline syncobj gives user more flexibility and convenience to do sychronization. Android does not used.
	Staging driver: i.MX7 MIPI CSI subdev
	NXP QuadSPI driver
	Introduce Sound Open Firmware (SOF) for audio DSP devices	Big changes to the current HiFi4 DSP (in i.MX8QM/QXP) infrastructure. The SOF is an open source audio DSP firmware and SDK that provides audio firmware infrastructure and development tools. Also integrated with current kernel alsa subsystem.	Home - Sound Open Firmware  Firmware - AlsaProject  How to build i.MX8 HiFi4 firmware
	Add NXP SJA1105 DSA network driver with ptp support	Used for networking switch
	Add TJA11xx PHY driver
	Add lpspi driver support
	fsl_lpuart: add imx8qxp support
	Add SCU watchdog/RTC support
	regmap: add i3c bus support commit	Means we can have i3c bus support in regmap now.

References

• Linux_Kernel_Newbies - Linux Kernel Newbies 
• Welcome to LWN.net [LWN.net] 
• kernel/git/torvalds/linux.git - Linux kernel source tree