linux-kernel - [PATCH 2/6] riscv: Allow NOMMU kernels to access all of RAM

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Message-ID: <20241026171441.3047904-3-samuel.holland@sifive.com>
Date: Sat, 26 Oct 2024 10:13:54 -0700
From: Samuel Holland <samuel.holland@...ive.com>
To: Palmer Dabbelt <palmer@...belt.com>,
	linux-kernel@...r.kernel.org,
	linux-riscv@...ts.infradead.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Arnd Bergmann <arnd@...db.de>,
	Alexandre Ghiti <alexghiti@...osinc.com>,
	Samuel Holland <samuel.holland@...ive.com>
Subject: [PATCH 2/6] riscv: Allow NOMMU kernels to access all of RAM

NOMMU kernels currently cannot access memory below the kernel link
address. Remove this restriction by setting PAGE_OFFSET to the actual
start of RAM, as determined from the devicetree. The kernel link address
must be a constant, so keep using CONFIG_PAGE_OFFSET for that purpose.

Signed-off-by: Samuel Holland <samuel.holland@...ive.com>
---

 arch/riscv/include/asm/page.h    | 12 ++++--------
 arch/riscv/include/asm/pgtable.h |  2 +-
 2 files changed, 5 insertions(+), 9 deletions(-)

diff --git a/arch/riscv/include/asm/page.h b/arch/riscv/include/asm/page.h
index 32d308a3355f..24d1ac052609 100644
--- a/arch/riscv/include/asm/page.h
+++ b/arch/riscv/include/asm/page.h
@@ -26,12 +26,9 @@
  * When not using MMU this corresponds to the first free page in
  * physical memory (aligned on a page boundary).
  */
-#ifdef CONFIG_64BIT
 #ifdef CONFIG_MMU
+#ifdef CONFIG_64BIT
 #define PAGE_OFFSET		kernel_map.page_offset
-#else
-#define PAGE_OFFSET		_AC(CONFIG_PAGE_OFFSET, UL)
-#endif
 /*
  * By default, CONFIG_PAGE_OFFSET value corresponds to SV57 address space so
  * define the PAGE_OFFSET value for SV48 and SV39.
@@ -41,6 +38,9 @@
 #else
 #define PAGE_OFFSET		_AC(CONFIG_PAGE_OFFSET, UL)
 #endif /* CONFIG_64BIT */
+#else
+#define PAGE_OFFSET		((unsigned long)phys_ram_base)
+#endif /* CONFIG_MMU */
 
 #ifndef __ASSEMBLY__
 
@@ -97,11 +97,7 @@ typedef struct page *pgtable_t;
 #define MIN_MEMBLOCK_ADDR      0
 #endif
 
-#ifdef CONFIG_MMU
 #define ARCH_PFN_OFFSET		(PFN_DOWN((unsigned long)phys_ram_base))
-#else
-#define ARCH_PFN_OFFSET		(PAGE_OFFSET >> PAGE_SHIFT)
-#endif /* CONFIG_MMU */
 
 struct kernel_mapping {
 	unsigned long page_offset;
diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
index e79f15293492..e224ac66e635 100644
--- a/arch/riscv/include/asm/pgtable.h
+++ b/arch/riscv/include/asm/pgtable.h
@@ -12,7 +12,7 @@
 #include <asm/pgtable-bits.h>
 
 #ifndef CONFIG_MMU
-#define KERNEL_LINK_ADDR	PAGE_OFFSET
+#define KERNEL_LINK_ADDR	_AC(CONFIG_PAGE_OFFSET, UL)
 #define KERN_VIRT_SIZE		(UL(-1))
 #else
 
-- 
2.45.1