risc-v/litex: Update docs for vexriscv invalidate and no-op flush.

[jerenkrantz]

Per further feedback from Litex community on Discord, the vexriscv cache is write-through so no flush is required - therefore, make it a nop.

And add a pointer to vexriscv invalidation magic instruction docs:

https://github.com/SpinalHDL/VexRiscv?tab=readme-ov-file#dbuscachedplugin

Summary

A prior PR #17696 issued an invalidation instruction when asked to flush; but, that's not strictly necessary as the cache is write-through. Therefore, make flush a nop. This matches Zephyr's behavior which implements vexriscv caching:

https://github.com/zephyrproject-rtos/zephyr/pull/97925/files

I believe a nop is slightly better than trying to make flush function conditional upstream, but I'm open to modifying the PR accordingly.

Impact

When asked to flush, NuttX won't invalidate the entire cache.

Testing

I tested with arty_a7:nsh build which ran successfully locally.

[jerenkrantz]

I see that the CI run failed, but in reviewing the build logs, I'm not sure how this PR could have triggered that failure. Could this be a transient failure or some other CI failure in master?

[acassis]

@jerenkrantz the issue is happening in BL602 board:

riscv-none-elf-ld: cannot find /github/workspace/sources/nuttx/arch/risc-v/src/crt0.o: No such file or directory
make[4]: *** [Makefile:57: errno] Error 1
make[4]: Target 'install' not remade because of errors.
make[3]: *** [Makefile:88: errno_install] Error 2
riscv-none-elf-ld: cannot find /github/workspace/sources/nuttx/arch/risc-v/src/crt0.o: No such file or directory
make[4]: *** [Makefile:57: hello] Error 1
make[4]: Target 'install' not remade because of errors.
make[3]: *** [Makefile:88: hello_install] Error 2
riscv-none-elf-ld: cannot find /github/workspace/sources/nuttx/arch/risc-v/src/crt0.o: No such file or directory
make[4]: *** [Makefile:56: struct] Error 1
make[4]: Target 'install' not remade because of errors.
make[3]: *** [Makefile:88: struct_install] Error 2
riscv-none-elf-ld: cannot find /github/workspace/sources/nuttx/arch/risc-v/src/crt0.o: No such file or directory
make[4]: *** [Makefile:57: signal] Error 1
make[4]: Target 'install' not remade because of errors.
make[3]: *** [Makefile:88: signal_install] Error 2
riscv-none-elf-ld: cannot find /github/workspace/sources/nuttx/arch/risc-v/src/crt0.o: No such file or directory
make[4]: *** [Makefile:57: mutex] Error 1
make[4]: Target 'install' not remade because of errors.
make[3]: *** [Makefile:88: mutex_install] Error 2
riscv-none-elf-ld: cannot find /github/workspace/sources/nuttx/arch/risc-v/src/crt0.o: No such file or directory
make[4]: *** [Makefile:101: hello++1] Error 1
riscv-none-elf-ld: cannot find /github/workspace/sources/nuttx/arch/risc-v/src/crt0.o: No such file or directory
make[4]: *** [Makefile:105: hello++2] Error 1
riscv-none-elf-ld: cannot find /github/workspace/sources/nuttx/arch/risc-v/src/crt0.o: No such file or directory
make[4]: *** [Makefile:112: hello++3] Error 1
make[4]: Target 'install' not remade because of errors.
make[3]: *** [Makefile:88: helloxx_install] Error 2
riscv-none-elf-ld: cannot find /github/workspace/sources/nuttx/arch/risc-v/src/crt0.o: No such file or directory
make[4]: *** [Makefile:57: pthread] Error 1
make[4]: Target 'install' not remade because of errors.
make[3]: *** [Makefile:88: pthread_install] Error 2
riscv-none-elf-ld: cannot find /github/workspace/sources/nuttx/arch/risc-v/src/crt0.o: No such file or directory
make[4]: *** [Makefile:57: task] Error 1
make[4]: Target 'install' not remade because of errors.
make[3]: *** [Makefile:88: task_install] Error 2
make[3]: Target 'all' not remade because of errors.
make[2]: *** [Makefile:61: build] Error 2
make[2]: Target 'all' not remade because of errors.
make[1]: *** [Makefile:54: /github/workspace/sources/apps/examples/elf_all] Error 2
make[1]: Target 'all' not remade because of errors.
make: *** [tools/LibTargets.mk:248: /github/workspace/sources/apps/libapps.a] Error 2
make: Target 'all' not remade because of errors.
/github/workspace/sources/nuttx/tools/testbuild.sh: line 385: /github/workspace/sources/nuttx/../nuttx/nuttx.manifest: No such file or directory
  [1/1] Normalize bl602evb/elf

Maybe @lupyuen could know what is happening anyway I will restart the CI to check whether it is a transient issue or not.

[lupyuen]

Sorry @simbit18: Any idea why bl602evb/elf fails sometimes? Should we disable it? Thanks!

https://github.com/apache/nuttx/actions/runs/20786276050/job/59697000727?pr=17792#step:11:156

riscv-none-elf-ld: cannot find /github/workspace/sources/nuttx/arch/risc-v/src/crt0.o: No such file or directory

[simbit18]

Sorry @simbit18: Any idea why bl602evb/elf fails sometimes? Should we disable it? Thanks!

https://github.com/apache/nuttx/actions/runs/20786276050/job/59697000727?pr=17792#step:11:156

riscv-none-elf-ld: cannot find /github/workspace/sources/nuttx/arch/risc-v/src/crt0.o: No such file or directory

Hi @lupyuen, I have no idea why this is happening, sorry.