This PR configures the T-Head MMU to cache the the User Text, Data and Heap. We enable the MMU Flags for Shareable, Bufferable and Cacheable, as explained in this article: https://lupyuen.github.io/articles/plic3#appendix-mmu-caching-for-t-head-c906
This PR fixes the Slow Memory Access for NuttX Apps on BL808 and SG2000 SoCs: https://github.com/apache/nuttx/issues/12696. With this fix, SG2000 NuttX CoreMark jumps from 21 to 2,423. (Close to SG2000 Debian CoreMark)
We introduce a Kconfig Option: `ARCH_MMU_EXT_THEAD` ("System Type > Enable T-Head MMU extension support"). Enabling this Kconfig Option will configure the T-Head MMU to cache the User Text, Data and Heap.
This PR enables the MMU cache for only SG2000 SoC (Milk-V Duo S SBC). The next PR will apply the same settings to BL808 SoC (Pine64 Ox64 SBC).
Modified Files:
`arch/risc-v/Kconfig`: Added Kconfig Option `ARCH_MMU_EXT_THEAD` that will configure the T-Head MMU. Enabled `ARCH_MMU_EXT_THEAD` for SG2000 SoC.
`arch/risc-v/src/common/riscv_mmu.h`: Set the T-Head MMU Flags (Shareable, Bufferable and Cacheable) for User Text, Data and Heap, if `ARCH_MMU_EXT_THEAD` is enabled
`arch/risc-v/src/common/riscv_addrenv.c`: Extended the MMU Flags from 32 bits to 64 bits, to accommodate the T-Head MMU Flags
`arch/risc-v/src/common/riscv_exception.c`: Extended the MMU Flags from 32 bits to 64 bit, to accommodate the T-Head MMU Flags. This code is enabled only for MMU Paging (`CONFIG_PAGING`).
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