[PATCH] riscv: deprecate CONFIG_MMU=n

[Clément Léger]

Deprecation of NOMMU support for riscv was discussed during LPC 2023
[1]. Reasons for this involves lack of users as well as maintenance
efforts to support this mode. psABI FDPIC specification also never
made it upstream and last public messages of this development seems to
date back from 2020 [2]. Plan the deprecation to be done in 2 years from
now. Mark the Kconfig option as deprecated by adding a new dummy option
which explicitly displays the deprecation in case of CONFIG_MMU=n. This option
is selected indirectly by CONFIG_RISCV_M_MODE since an option can not
select another one directly with a "select" in case of such CONFIG=n.
Additionally, display a pr_err() message at boot time in case of NOMMU
build to warn about upcoming deprecation.

Link: https://lpc.events/event/17/contributions/1478/ [1]
Link: https://groups.google.com/a/groups.riscv.org/g/sw-dev/c/ZjYUJswknQ4/m/WYRRylTwAAAJ [2]
Signed-off-by: Clément Léger <cleger at rivosinc.com>

---
 arch/riscv/Kconfig        | 8 ++++++++
 arch/riscv/kernel/setup.c | 4 ++++
 2 files changed, 12 insertions(+)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index bffbd869a068..8da58c102d3f 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -221,6 +221,7 @@ config ARCH_MMAP_RND_COMPAT_BITS_MAX
 # set if we run in machine mode, cleared if we run in supervisor mode
 config RISCV_M_MODE
 	bool
+	select NOMMU
 	default !MMU
 
 # set if we are running in S-mode and can use SBI calls
@@ -236,6 +237,13 @@ config MMU
 	  Select if you want MMU-based virtualised addressing space
 	  support by paged memory management. If unsure, say 'Y'.
 
+config NOMMU
+	depends on !MMU
+	bool "NOMMU kernel (DEPRECATED)"
+	help
+	  NOMMU kernel is deprecated and is scheduled for removal by
+	  the beginning of 2027.
+
 config PAGE_OFFSET
 	hex
 	default 0xC0000000 if 32BIT && MMU
diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
index 4f73c0ae44b2..8799816ef0a6 100644
--- a/arch/riscv/kernel/setup.c
+++ b/arch/riscv/kernel/setup.c
@@ -295,6 +295,10 @@ void __init setup_arch(char **cmdline_p)
 	riscv_set_dma_cache_alignment();
 
 	riscv_user_isa_enable();
+
+#if !defined(CONFIG_MMU)
+	pr_err("RISC-V NOMMU support is deprecated and scheduled for removal by the beginning of 2027\n");
+#endif
 }
 
 bool arch_cpu_is_hotpluggable(int cpu)
-- 
2.43.0