powerpc/mm: Allocate static page tables for fixmap

Allocate static page tables for the fixmap area. This allows
setting mappings through page tables before memblock is ready.
That's needed to use early_ioremap() early and to use standard
page mappings with fixmap.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/4f4b1412d34de6801b8e925cb88fc69d056ff536.1589866984.git.christophe.leroy@csgroup.eu

arch/powerpc/include/asm/fixmap.h

@@ -86,6 +86,10 @@ enum fixed_addresses {
 #define __FIXADDR_SIZE	(__end_of_fixed_addresses << PAGE_SHIFT)
 #define FIXADDR_START		(FIXADDR_TOP - __FIXADDR_SIZE)
 
+#define FIXMAP_ALIGNED_SIZE	(ALIGN(FIXADDR_TOP, PGDIR_SIZE) - \
+				 ALIGN_DOWN(FIXADDR_START, PGDIR_SIZE))
+#define FIXMAP_PTE_SIZE	(FIXMAP_ALIGNED_SIZE / PGDIR_SIZE * PTE_TABLE_SIZE)
+
 #define FIXMAP_PAGE_NOCACHE PAGE_KERNEL_NCG
 #define FIXMAP_PAGE_IO	PAGE_KERNEL_NCG
 

arch/powerpc/kernel/setup_32.c

@@ -80,7 +80,7 @@ notrace void __init machine_init(u64 dt_ptr)
 	/* Configure static keys first, now that we're relocated. */
 	setup_feature_keys();
 
-	early_ioremap_setup();
+	early_ioremap_init();
 
 	/* Enable early debugging if any specified (see udbg.h) */
 	udbg_early_init();

arch/powerpc/mm/pgtable_32.c

@@ -29,11 +29,27 @@
 #include <asm/fixmap.h>
 #include <asm/setup.h>
 #include <asm/sections.h>
+#include <asm/early_ioremap.h>
 
 #include <mm/mmu_decl.h>
 
 extern char etext[], _stext[], _sinittext[], _einittext[];
 
+static u8 early_fixmap_pagetable[FIXMAP_PTE_SIZE] __page_aligned_data;
+
+notrace void __init early_ioremap_init(void)
+{
+	unsigned long addr = ALIGN_DOWN(FIXADDR_START, PGDIR_SIZE);
+	pte_t *ptep = (pte_t *)early_fixmap_pagetable;
+	pmd_t *pmdp = pmd_ptr_k(addr);
+
+	for (; (s32)(FIXADDR_TOP - addr) > 0;
+	     addr += PGDIR_SIZE, ptep += PTRS_PER_PTE, pmdp++)
+		pmd_populate_kernel(&init_mm, pmdp, ptep);
+
+	early_ioremap_setup();
+}
+
 static void __init *early_alloc_pgtable(unsigned long size)
 {
 	void *ptr = memblock_alloc(size, size);