arm64: Add simple earlyprintk support

This patch adds support for "earlyprintk=" parameter on the kernel
command line. The format is:

  earlyprintk=<name>[,<addr>][,<options>]

where <name> is the name of the (UART) device, e.g. "pl011", <addr> is
the I/O address. The <options> aren't currently used.

The mapping of the earlyprintk device is done very early during kernel
boot and there are restrictions on which functions it can call. A
special early_io_map() function is added which creates the mapping from
the pre-defined EARLY_IOBASE to the device I/O address passed via the
kernel parameter. The pgd entry corresponding to EARLY_IOBASE is
pre-populated in head.S during kernel boot.

Only PL011 is currently supported and it is assumed that the interface
is already initialised by the boot loader before the kernel is started.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>

1 files changed, 42 insertions, 0 deletions

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index a6885d896ab6..f4dd585898c5 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -25,6 +25,7 @@
 #include <linux/nodemask.h>
 #include <linux/memblock.h>
 #include <linux/fs.h>
+#include <linux/io.h>
 
 #include <asm/cputype.h>
 #include <asm/sections.h>
@@ -251,6 +252,47 @@ static void __init create_mapping(phys_addr_t phys, unsigned long virt,
         } while (pgd++, addr = next, addr != end);
 }
 
+#ifdef CONFIG_EARLY_PRINTK
+/*
+ * Create an early I/O mapping using the pgd/pmd entries already populated
+ * in head.S as this function is called too early to allocated any memory. The
+ * mapping size is 2MB with 4KB pages or 64KB or 64KB pages.
+ */
+void __iomem * __init early_io_map(phys_addr_t phys, unsigned long virt)
+{
+        unsigned long size, mask;
+        bool page64k = IS_ENABLED(ARM64_64K_PAGES);
+        pgd_t *pgd;
+        pud_t *pud;
+        pmd_t *pmd;
+        pte_t *pte;
+
+        /*
+         * No early pte entries with !ARM64_64K_PAGES configuration, so using
+         * sections (pmd).
+         */
+        size = page64k ? PAGE_SIZE : SECTION_SIZE;
+        mask = ~(size - 1);
+
+        pgd = pgd_offset_k(virt);
+        pud = pud_offset(pgd, virt);
+        if (pud_none(*pud))
+                return NULL;
+        pmd = pmd_offset(pud, virt);
+
+        if (page64k) {
+                if (pmd_none(*pmd))
+                        return NULL;
+                pte = pte_offset_kernel(pmd, virt);
+                set_pte(pte, __pte((phys & mask) | PROT_DEVICE_nGnRE));
+        } else {
+                set_pmd(pmd, __pmd((phys & mask) | PROT_SECT_DEVICE_nGnRE));
+        }
+
+        return (void __iomem *)((virt & mask) + (phys & ~mask));
+}
+#endif
+
 static void __init map_mem(void)
 {
         struct memblock_region *reg;