Hi, I’m Asami. My project “adding QEMU/AArch64 support to coreboot” is making good progress. I’ve almost done to write porting code and I’m now writing a new tool to make a FIT payload for QEMU/AArch64. Here is my CL. In this article, I’m going to talk about how to run C code in the bootblock stage.
The way to run C code before DRAM has been initialized is various in the coreboot project. The most famous and used in the x86 system is known as the Cache-As-Ram (CAR). CAR can be set up by 1. enable CPU cache 2. enable the ‘no eviction’ mode 3. change cache mode from write-through to write-back. ‘No-eviction’ means that the CPU doesn’t write any data to DRAM as long as the size of data is less than the CPU cache. Then, you can get all the data from the CPU cache. The implementation for CAR is src/cpu/intel/car/non-evict/cache_as_ram.S.
Another way to run C code especially implemented in ARM system is relocating the bootblock code to SRAM. System on a Chip (SoC) has an ARM CPU and often includes SRAM as a cache. For example, QEMU VExpress machine has SRAM is located at 0x48000000 that we can know in qemu/hw/arm/vexpress.c#L120.
However, what should the system that doesn’t have SRAM do? My target machine, QEMU virt machine, doesn’t have SRAM according to the implementation of qemu/hw/arm/virt.c. In this case, should we initialize DRAM earlier?
The answer is No but it’s only for my project. Because QEMU is not actual hardware so DRAM already works. I just need to relocate the bootblock code to DRAM directly. I believe that other ARM systems that have no SRAM should initialize DRAM earlier.
You can see the relocation code in bootblock_custom.S under review. https://review.coreboot.org/c/coreboot/+/33387