// Find out how much memory this machine actually has.
//
// PHYSTOP used to be a constant: 0xE000000, 224 MB, inherited from xv6 and
// never revisited. That is not a limit on how much memory the kernel *uses* --
// it is an assertion that exactly that much exists. Boot a machine with less
// and kinit1() walks off the end of RAM into nothing; boot one with more and
// every byte above 224 MB is invisible. Measured before this file existed:
// SchOS booted at 224 MB and triple faulted at 192 MB, for no reason at all.
//
// The firmware has known the answer the whole time. boot/stage2.S asks it with
// INT 0x15 / E820 in real mode -- the same interface Linux's boot stub and GRUB
// use -- and leaves the result in low memory. stage2main.c already parses it to
// print detected RAM on the splash. The kernel simply never read it.

#include "types.h"
#include "defs.h"
#include "memlayout.h"
#include "mmu.h"

// Set before kinit1() and never again. Starts at the old constant so that a
// crash between entry and e820_init() behaves the way it always did rather
// than in some new way.
uint64 phystop = PHYSTOP_FALLBACK;

// Where stage2 left the map. Must match E820_COUNT_ADDR / E820_MAP_ADDR in
// boot/stage2main.c, which are themselves relative to VBE_INFO_ADDR.
#define E820_COUNT_ADDR (0x30000 + 0x200)
#define E820_MAP_ADDR   (0x30000 + 0x208)

#define E820_USABLE     1
#define E820_MAXENTRIES 32

struct e820entry {
  uint64 base;
  uint64 length;
  uint32 type;
  uint32 acpi_ext;
} __attribute__((packed));

void
e820_init(void)
{
  uint32 count = *(uint32 *)P2V(E820_COUNT_ADDR);
  struct e820entry *e = (struct e820entry *)P2V(E820_MAP_ADDR);

  // A count of zero means the firmware declined; anything absurd means we are
  // reading something that is not a memory map. Either way the constant is the
  // only honest answer left.
  if (count == 0 || count > E820_MAXENTRIES) {
    cprintf("e820: no usable map (count %d), assuming %d MB\n",
            count, (int)(PHYSTOP_FALLBACK / (1024 * 1024)));
    return;
  }

  // The end of the usable region that *contains the kernel*, not the highest
  // usable address anywhere.
  //
  // This distinction matters and is easy to get wrong. A memory map is not one
  // run of RAM: there are reserved holes in it -- ACPI tables, firmware
  // scratch, the legacy hole below 1 MB -- and kalloc's freerange() walks
  // straight from the end of the kernel image to phystop handing out every page
  // between. Taking the global maximum would step over a reserved region and
  // put firmware tables on the free list, which fails later, elsewhere, and
  // looks like anything but a memory map problem.
  uint64 top = 0;
  for (uint32 i = 0; i < count; i++) {
    if (e[i].type != E820_USABLE)
      continue;
    uint64 base = e[i].base, end = base + e[i].length;
    if (base <= EXTMEM && EXTMEM < end) {
      top = end;
      break;
    }
  }

  if (top == 0) {
    cprintf("e820: no usable region covers 0x%x, assuming %d MB\n",
            EXTMEM, (int)(PHYSTOP_FALLBACK / (1024 * 1024)));
    return;
  }

  // kvmalloc() maps physical 0-4 GB with four 1 GB pages, and marks the top one
  // uncached because that is where PCI BARs and the framebuffer live. So 3 GB
  // is the ceiling for anything the kernel will treat as RAM: above it the
  // direct map either does not exist or is device memory that would be
  // catastrophically slow to allocate from and is not ours to hand out.
  if (top > PHYSTOP_MAX)
    top = PHYSTOP_MAX;

  top &= ~(uint64)(PGSIZE - 1);

  // Below this the kernel image, its page tables and the DMA arena do not fit,
  // and continuing produces a failure much harder to read than this line.
  if (top < PHYSTOP_MIN) {
    cprintf("e820: only %d MB usable; SchOS needs at least %d MB\n",
            (int)(top / (1024 * 1024)), (int)(PHYSTOP_MIN / (1024 * 1024)));
    // Left at the fallback deliberately rather than lowered: a machine this
    // small is going to fail, and it should fail having said why.
    return;
  }

  phystop = top;
  cprintf("e820: %d MB usable (%d entries)\n",
          (int)(phystop / (1024 * 1024)), count);
}