[RFC PATCH v1 46/57] arm64: Generalize fixmap for boot-time page size

[Ryan Roberts]

Some fixmap fixed address slots previously depended on PAGE_SIZE (i.e.
to determine how many slots were required to cover a given size). Since
we require the fixed address slots to be compile-time constant, let's
work out the worst case number of required slots when page size is
PAGE_SIZE_MIN instead.

Additionally, let's determine the worst-case number of PTE tables we
require and statically allocate enough memory.

For compile-time page size builds, the end result is the same as it was
previously.

Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
---

***NOTE***
Any confused maintainers may want to read the cover note here for context:
https://lore.kernel.org/all/20241014105514.3206191-1-ryan.roberts@xxxxxxx/

 arch/arm64/include/asm/fixmap.h | 12 ++++++++----
 arch/arm64/mm/fixmap.c          | 34 ++++++++++++++++++++++-----------
 2 files changed, 31 insertions(+), 15 deletions(-)

diff --git a/arch/arm64/include/asm/fixmap.h b/arch/arm64/include/asm/fixmap.h
index 9a496d54dfe6e..c73fd3c1334ff 100644
--- a/arch/arm64/include/asm/fixmap.h
+++ b/arch/arm64/include/asm/fixmap.h
@@ -43,7 +43,7 @@ enum fixed_addresses {
 	 * whether it crosses any page boundary.
 	 */
 	FIX_FDT_END,
-	FIX_FDT = FIX_FDT_END + DIV_ROUND_UP(MAX_FDT_SIZE, PAGE_SIZE) + 1,
+	FIX_FDT = FIX_FDT_END + DIV_ROUND_UP(MAX_FDT_SIZE, PAGE_SIZE_MIN) + 1,
 
 	FIX_EARLYCON_MEM_BASE,
 	FIX_TEXT_POKE0,
@@ -79,7 +79,7 @@ enum fixed_addresses {
 	 * Temporary boot-time mappings, used by early_ioremap(),
 	 * before ioremap() is functional.
 	 */
-#define NR_FIX_BTMAPS		(SZ_256K / PAGE_SIZE)
+#define NR_FIX_BTMAPS		(SZ_256K / PAGE_SIZE_MIN)
 #define FIX_BTMAPS_SLOTS	7
 #define TOTAL_FIX_BTMAPS	(NR_FIX_BTMAPS * FIX_BTMAPS_SLOTS)
 
@@ -101,8 +101,12 @@ enum fixed_addresses {
 
 #define FIXADDR_SIZE		(__end_of_permanent_fixed_addresses << PAGE_SHIFT)
 #define FIXADDR_START		(FIXADDR_TOP - FIXADDR_SIZE)
-#define FIXADDR_TOT_SIZE	(__end_of_fixed_addresses << PAGE_SHIFT)
-#define FIXADDR_TOT_START	(FIXADDR_TOP - FIXADDR_TOT_SIZE)
+#define __FIXADDR_TOT_SIZE(page_shift)					\
+	(__end_of_fixed_addresses << (page_shift))
+#define __FIXADDR_TOT_START(page_shift)					\
+	(FIXADDR_TOP - __FIXADDR_TOT_SIZE(page_shift))
+#define FIXADDR_TOT_SIZE	__FIXADDR_TOT_SIZE(PAGE_SHIFT)
+#define FIXADDR_TOT_START	__FIXADDR_TOT_START(PAGE_SHIFT)
 
 #define FIXMAP_PAGE_IO     __pgprot(PROT_DEVICE_nGnRE)
 
diff --git a/arch/arm64/mm/fixmap.c b/arch/arm64/mm/fixmap.c
index 15ce3253ad359..a0dcf2375ccb4 100644
--- a/arch/arm64/mm/fixmap.c
+++ b/arch/arm64/mm/fixmap.c
@@ -17,27 +17,39 @@
 #include <asm/tlbflush.h>
 
 /* ensure that the fixmap region does not grow down into the PCI I/O region */
-static_assert(FIXADDR_TOT_START > PCI_IO_END);
+static_assert(__FIXADDR_TOT_START(PAGE_SHIFT_MAX) > PCI_IO_END);
 
-#define NR_BM_PTE_TABLES \
-	SPAN_NR_ENTRIES(FIXADDR_TOT_START, FIXADDR_TOP, PMD_SHIFT)
-#define NR_BM_PMD_TABLES \
-	SPAN_NR_ENTRIES(FIXADDR_TOT_START, FIXADDR_TOP, PUD_SHIFT)
+#define FIXMAP_LEVEL(page_shift, lvl, vstart, vend)			\
+	SPAN_NR_ENTRIES(vstart, vend, PGTABLE_LEVEL_SHIFT(page_shift, lvl))
 
-static_assert(NR_BM_PMD_TABLES == 1);
+#define FIXMAP_PAGES(page_shift, level)					\
+	FIXMAP_LEVEL(page_shift, level,					\
+		     __FIXADDR_TOT_START(page_shift), FIXADDR_TOP)
+
+#define FIXMAP_SIZE(page_shift, level)					\
+	(FIXMAP_PAGES(page_shift, level) * (UL(1) << (page_shift)))
+
+#define FIXMAP_PTE_SIZE_MAX						\
+	MAX_IF_HAVE_PGSZ(FIXMAP_SIZE(ARM64_PAGE_SHIFT_4K, 2),		\
+			 FIXMAP_SIZE(ARM64_PAGE_SHIFT_16K, 2),		\
+			 FIXMAP_SIZE(ARM64_PAGE_SHIFT_64K, 2))
+
+static_assert(FIXMAP_PAGES(ARM64_PAGE_SHIFT_4K, 1) == 1);
+static_assert(FIXMAP_PAGES(ARM64_PAGE_SHIFT_16K, 1) == 1);
+static_assert(FIXMAP_PAGES(ARM64_PAGE_SHIFT_64K, 1) == 1);
 
 #define __BM_TABLE_IDX(addr, shift) \
 	(((addr) >> (shift)) - (FIXADDR_TOT_START >> (shift)))
 
 #define BM_PTE_TABLE_IDX(addr)	__BM_TABLE_IDX(addr, PMD_SHIFT)
 
-static pte_t bm_pte[NR_BM_PTE_TABLES][PTRS_PER_PTE] __page_aligned_bss;
-static pmd_t bm_pmd[PTRS_PER_PMD] __page_aligned_bss __maybe_unused;
-static pud_t bm_pud[PTRS_PER_PUD] __page_aligned_bss __maybe_unused;
+static pte_t bm_pte[FIXMAP_PTE_SIZE_MAX / sizeof(pte_t)] __page_aligned_bss;
+static pmd_t bm_pmd[MAX_PTRS_PER_PMD] __page_aligned_bss __maybe_unused;
+static pud_t bm_pud[MAX_PTRS_PER_PUD] __page_aligned_bss __maybe_unused;
 
 static inline pte_t *fixmap_pte(unsigned long addr)
 {
-	return &bm_pte[BM_PTE_TABLE_IDX(addr)][pte_index(addr)];
+	return &bm_pte[BM_PTE_TABLE_IDX(addr) * PTRS_PER_PTE + pte_index(addr)];
 }
 
 static void __init early_fixmap_init_pte(pmd_t *pmdp, unsigned long addr)
@@ -46,7 +58,7 @@ static void __init early_fixmap_init_pte(pmd_t *pmdp, unsigned long addr)
 	pte_t *ptep;
 
 	if (pmd_none(pmd)) {
-		ptep = bm_pte[BM_PTE_TABLE_IDX(addr)];
+		ptep = &bm_pte[BM_PTE_TABLE_IDX(addr) * PTRS_PER_PTE];
 		__pmd_populate(pmdp, __pa_symbol(ptep), PMD_TYPE_TABLE);
 	}
 }
-- 
2.43.0