[RFC] mm/ioremap: Probe platform for p4d huge map support

[Anshuman Khandual <anshuman.khandual@arm.com>]

Finishing up what the commit c2febafc67734a ("mm: convert generic code to
5-level paging") started out while levelling up P4D huge mapping support
at par with PUD and PMD. A new arch call back arch_ioremap_p4d_supported()
is being added which just maintains status quo (P4D huge map not supported)
on x86 and arm64.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: x86@kernel.org

Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
---

- Detected this from code audit while reviewing Nicholas Piggin's ioremap
  changes https://patchwork.kernel.org/project/linux-mm/list/?series=129479

- Build and boot tested on x86 and arm64 platforms
- Build tested on some others

 arch/arm64/mm/mmu.c   | 5 +++++
 arch/x86/mm/ioremap.c | 5 +++++
 include/linux/io.h    | 1 +
 lib/ioremap.c         | 2 ++
 4 files changed, 13 insertions(+)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index a1bfc4413982..646c82922d77 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -953,6 +953,11 @@ void *__init fixmap_remap_fdt(phys_addr_t dt_phys)
 	return dt_virt;
 }
 
+int __init arch_ioremap_p4d_supported(void)
+{
+	return 0;
+}
+
 int __init arch_ioremap_pud_supported(void)
 {
 	/*
diff --git a/arch/x86/mm/ioremap.c b/arch/x86/mm/ioremap.c
index 4b6423e7bd21..6cbbec83991d 100644
--- a/arch/x86/mm/ioremap.c
+++ b/arch/x86/mm/ioremap.c
@@ -440,6 +440,11 @@ void iounmap(volatile void __iomem *addr)
 }
 EXPORT_SYMBOL(iounmap);
 
+int __init arch_ioremap_p4d_supported(void)
+{
+	return 0;
+}
+
 int __init arch_ioremap_pud_supported(void)
 {
 #ifdef CONFIG_X86_64
diff --git a/include/linux/io.h b/include/linux/io.h
index 32e30e8fb9db..58514cebfce6 100644
--- a/include/linux/io.h
+++ b/include/linux/io.h
@@ -45,6 +45,7 @@ static inline int ioremap_page_range(unsigned long addr, unsigned long end,
 
 #ifdef CONFIG_HAVE_ARCH_HUGE_VMAP
 void __init ioremap_huge_init(void);
+int arch_ioremap_p4d_supported(void);
 int arch_ioremap_pud_supported(void);
 int arch_ioremap_pmd_supported(void);
 #else
diff --git a/lib/ioremap.c b/lib/ioremap.c
index 063213685563..c3dc213b6980 100644
--- a/lib/ioremap.c
+++ b/lib/ioremap.c
@@ -30,6 +30,8 @@ early_param("nohugeiomap", set_nohugeiomap);
 void __init ioremap_huge_init(void)
 {
 	if (!ioremap_huge_disabled) {
+		if (arch_ioremap_p4d_supported())
+			ioremap_p4d_capable = 1;
 		if (arch_ioremap_pud_supported())
 			ioremap_pud_capable = 1;
 		if (arch_ioremap_pmd_supported())
-- 
2.20.1