risc-v/bl808: Configure MMU to cache User Text, Data and Heap

[lupyuen]

Summary

This PR configures the BL808 MMU to cache the the User Text, Data and Heap. We enable the T-Head MMU Flags for Shareable, Bufferable and Cacheable, as explained in the previous PR: apache#13199

This PR fixes the Slow Memory Access for NuttX Apps on Ox64 BL808 SBC: apache#12696. With this fix, Ox64 NuttX CoreMark jumps from 19 to 1,104. (Close to Buildroot Linux CoreMark)

Modified Files

arch/risc-v/Kconfig: Enabled ARCH_MMU_EXT_THEAD for BL808 SoC.

Impact

This PR affects only the BL808 SoC and the Ox64 SBC.

Testing

We tested on Pine64 Ox64 SBC with BL808 SoC:

Before the PR: CoreMark is 19 (NuttX Log)

NuttShell (NSH) NuttX-12.6.0-RC1
nsh> uname -a
NuttX 12.6.0-RC1 d59fbfdbdcd Aug 28 2024 10:19:58 risc-v ox64
nsh> coremark
CoreMark 1.0 : 19.093686 / GCC13.2.0 -Os -fno-strict-aliasing -fomit-frame-pointer -ffunction-sections -fdata-sections -nostdlib -g / HEAP

After the PR: CoreMark increases to 802 (NuttX Log)

NuttShell (NSH) NuttX-12.6.0-RC1
nsh> uname -a
NuttX 12.6.0-RC1 c398049866a Aug 28 2024 10:16:16 risc-v ox64
nsh> coremark
CoreMark 1.0 : 801.632415 / GCC13.2.0 -Os -fno-strict-aliasing -fomit-frame-pointer -ffunction-sections -fdata-sections -nosg / HEAP

And OSTest completes successfully:

nsh> ostest
ostest_main: Exiting with status 0

Note that Ox64 CoreMark will increase further to 1,104 when we compile CoreMark optimised with -O2

nsh> coremark
CoreMark 1.0 : 1103.509159 / GCC13.2.0 -O2 -fno-strtion-sections -fdata-sections -nostdlib  / HEAP