After discussion, Linus Torvalds today announced the incorporation of Godson architecture code into the Linux 5.19 kernel mainline However, because some codes have not passed the review, the CPU architecture code has been in place, and some key drivers have not been logged in, Linux 5.19 cannot be started on the hardware.
Loongarch is a CPU architecture based on MIPS64. However, as the upstream MIPS64 architecture has actually disappeared, loongarch started to develop its own Isa. Loongarch is described as inspired by MIPS64 and risc-v, and some loongarch kernel code is actually reusing or closely copying existing MIPS code.
Earlier this year, loongarch was added to GCC 12 as the main system compiler. Compared with other arm or risc-v designs, the performance of the current generation of Godson 3a5000 CPU is not very strong at this stage.
Even though Godson's system cannot be started in Linux kernel 5.19, it is meaningful to introduce loongarch into Linux 5.19. The kernel porting of loongarch CPU architecture has passed more than 10 rounds of review to obtain the support of Linux kernel.
Developers want to mainstream the CPU architecture code so that they can submit loongarch support code for the GNU C library (glibc). Mainstreaming their glibc goals first requires the use of a reliable user space ABI to determine kernel support. However, since glibc 2.38 is expected to be released in August, loongarch needs to be merged for Linux 5.19 to have enough time to release this version in July, and let glibc loongarch code complete the release of the next version.
Merging loongarch now also reduces the maintenance burden of any tree range changes that may occur in Linux 5.20, etc.
Therefore, most loongarch codes are merged for Linux 5.19, but some EFI codes required for startup are missing. The IRQ driver is shared with the MIPs code of Linux, but the complexity means that it is not ready for use, and the PCI Loongson code needs to be modified and merged through the PCI subsystem region.
At present, loongarch in Linux 5.19 contains 21k lines of new code, not including the drivers required for logging in. Probably by the Linux 5.20 kernel cycle later this summer, the rest of the required driver support will be reviewed to produce a bootable loongarch system.