arch/arc/mm/highmem.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  * Copyright (C) 2015 Synopsys, Inc. (www.synopsys.com)
   4  */
   5
   6 #include <linux/memblock.h>
   7 #include <linux/export.h>
   8 #include <linux/highmem.h>
   9 #include <linux/pgtable.h>
  10 #include <asm/processor.h>
  11 #include <asm/pgalloc.h>
  12 #include <asm/tlbflush.h>
  13
  14 /*
  15  * HIGHMEM API:
  16  *
  17  * kmap() API provides sleep semantics hence referred to as "permanent maps"
  18  * It allows mapping LAST_PKMAP pages, using @last_pkmap_nr as the cursor
  19  * for book-keeping
  20  *
  21  * kmap_atomic() can't sleep (calls pagefault_disable()), thus it provides
  22  * shortlived ala "temporary mappings" which historically were implemented as
  23  * fixmaps (compile time addr etc). Their book-keeping is done per cpu.
  24  *
  25  *      Both these facts combined (preemption disabled and per-cpu allocation)
  26  *      means the total number of concurrent fixmaps will be limited to max
  27  *      such allocations in a single control path. Thus KM_TYPE_NR (another
  28  *      historic relic) is a small'ish number which caps max percpu fixmaps
  29  *
  30  * ARC HIGHMEM Details
  31  *
  32  * - the kernel vaddr space from 0x7z to 0x8z (currently used by vmalloc/module)
  33  *   is now shared between vmalloc and kmap (non overlapping though)
  34  *
  35  * - Both fixmap/pkmap use a dedicated page table each, hooked up to swapper PGD
  36  *   This means each only has 1 PGDIR_SIZE worth of kvaddr mappings, which means
  37  *   2M of kvaddr space for typical config (8K page and 11:8:13 traversal split)
  38  *
  39  * - The fixed KMAP slots for kmap_local/atomic() require KM_MAX_IDX slots per
  40  *   CPU. So the number of CPUs sharing a single PTE page is limited.
  41  *
  42  * - pkmap being preemptible, in theory could do with more than 256 concurrent
  43  *   mappings. However, generic pkmap code: map_new_virtual(), doesn't traverse
  44  *   the PGD and only works with a single page table @pkmap_page_table, hence
  45  *   sets the limit
  46  */
  47
  48 extern pte_t * pkmap_page_table;
  49
  50 static noinline pte_t * __init alloc_kmap_pgtable(unsigned long kvaddr)
  51 {
  52         pmd_t *pmd_k = pmd_off_k(kvaddr);
  53         pte_t *pte_k;
  54
  55         pte_k = (pte_t *)memblock_alloc_low(PAGE_SIZE, PAGE_SIZE);
  56         if (!pte_k)
  57                 panic("%s: Failed to allocate %lu bytes align=0x%lx\n",
  58                       __func__, PAGE_SIZE, PAGE_SIZE);
  59
  60         pmd_populate_kernel(&init_mm, pmd_k, pte_k);
  61         return pte_k;
  62 }
  63
  64 void __init kmap_init(void)
  65 {
  66         /* Due to recursive include hell, we can't do this in processor.h */
  67         BUILD_BUG_ON(PAGE_OFFSET < (VMALLOC_END + FIXMAP_SIZE + PKMAP_SIZE));
  68         BUILD_BUG_ON(LAST_PKMAP > PTRS_PER_PTE);
  69         BUILD_BUG_ON(FIX_KMAP_SLOTS > PTRS_PER_PTE);
  70
  71         pkmap_page_table = alloc_kmap_pgtable(PKMAP_BASE);
  72         alloc_kmap_pgtable(FIXMAP_BASE);
  73 }