> 1. The AArch64 architecture is significantly different from AArch32 (the > official name of the 32-bit ARM architecture), it is not an extension. > It has a new exception model, new instruction set (even the register > names are different), new C ABI, PCS. It has a hardware compat mode but > that's about it, there is no interworking between the two (not allowed > by hardware, nor that the ABIs would make this possible). > > 2. Easier code maintenance. While I started the kernel port from the > arch/arm/ code base (it was at a time when Linux didn't even have > generic unistd.h and the AArch64 specs kept changing) the code diverged > significantly both because of architecture differences and software > requirements like new ABI, generic headers (unistd.h, stat.h etc). The > port undergone significant rewrite following the rules for new > architecture ports and some of the few similarities will be moved to a > more generic place (like the mmu_gather patches from peterz). AArch64 > SoCs are going into a more standardised direction and the SoC code will > be kept to a minimum and moved under drivers/. So merging AArch32 and > AArch64 together would be rather detrimental to code maintenance for > both architectures (#ifdef's or duplicate 32/64 files). > > 3. Flexibility. It's early days for AArch64, we don't have a hardware > platform supported yet and there are ongoing debates around ACPI, > firmware standardisation for CPU booting, hotplug, power management. We > need the flexibility to improve the code base without worrying about > backwards compatibility (apart from the compat layer which isn't at all > affected by the hardware developments).
These are the same reasons the x86_64 people gave and regretted later. Alan -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
