SchOS/kernel/drivers/gpu/vga/vga_pci.c

// Display controller -- the first driver to find its hardware by asking.
//
// It does not draw anything: vga_fb.c in this directory owns the framebuffer,
// and it learned where that is from the bootloader's VBE call. What this adds
// is a second, independent answer to the same question, from the bus rather
// than from the BIOS -- and a complaint when the two disagree.
//
// No `attached` hook and no per-instance state, unlike the e1000 and
// virtio-gpu: there is exactly one framebuffer the loader programmed, however
// many VGA-class controllers the bus turns up. A second one binding here
// publishes the same scanout, which is the truth -- it is the same screen.
//
// That is worth more than it sounds. Three places in the kernel used to
// hardcode 0xfd000000 as the framebuffer address, and the only reason nobody
// noticed was that QEMU's stdvga happens to put it there. A disagreement
// between VBE and the card's own BAR is exactly the symptom that would have
// shown up first on hardware where the guess was wrong, and until now there
// was nothing in the kernel capable of noticing it.

#include "types.h"
#include "defs.h"
#include "param.h"
#include "x86.h"
#include "spinlock.h"
#include "sleeplock.h"
#include "fs.h"
#include "file.h"
#include "pci.h"
#include "display.h"

// The framebuffer lives in the first memory BAR big enough to hold a screen.
// Display controllers commonly expose two apertures -- the framebuffer and a
// much smaller register block -- and which index they land on varies, so the
// choice is made by size rather than by position.
static struct pci_bar *
framebuffer_bar(struct pci_dev *d)
{
  uint64 need = vga_fb_size();

  for (int i = 0; i < 6; i++) {
    struct pci_bar *b = &d->bar[i];
    if (b->base == 0 || b->is_io)
      continue;
    if (b->size >= need)
      return b;
  }
  return 0;
}

static int
vga_pci_probe(struct pci_dev *d)
{
  struct pci_bar *bar = framebuffer_bar(d);
  uint64 vbe = vga_fb_phys();

  // Memory decoding has to be on for the aperture to respond at all. The BIOS
  // will have done this already to put up a boot splash, but a device we only
  // reached through enumeration cannot be assumed to have been touched.
  pci_enable(d, PCI_CMD_MEMORY);

  if (bar == 0) {
    cprintf("vga: display controller at %x:%x.%x has no framebuffer aperture\n",
            d->bus, d->slot, d->func);
    return 0;                           // still ours; nothing more to do
  }

  if (vbe != 0 && bar->base != vbe) {
    // Trust VBE: it describes the mode that is actually programmed and being
    // displayed, which is the thing vga.c is writing into. The BAR is the
    // card's whole aperture and the mode may sit at an offset within it. Say
    // so rather than silently preferring one.
    cprintf("vga: framebuffer disagreement -- VBE says %x, BAR says %x\n",
            (uint32)vbe, (uint32)bar->base);
  }

  cprintf("vga: display controller %x:%x, aperture %x (%d MB)\n",
          d->vendor, d->device, (uint32)bar->base,
          (uint32)(bar->size / (1024 * 1024)));
  return 0;
}

// Matched by class, not by model: every VGA-compatible controller answers
// class 0x03, and this driver cares about none of the differences between
// them.
static const struct pci_match vga_pci_match[] = {
    { PCI_ANY, PCI_ANY, PCI_CLASS_DISPLAY, 0xFF },
    { 0, 0, 0, 0 },
};

// The display vtable lives in kernel/dev/vga.c, which owns the framebuffer;
// this file only finds the card on the bus. Publishing it here is what makes
// device_find_class(DEVCLASS_DISPLAY) return something a compositor can use.
extern const struct display_ops vga_display_ops;

const struct pci_driver vga_pci_driver = {
    .name  = "vga",
    .match = vga_pci_match,
    .ops   = &vga_display_ops,
    .probe = vga_pci_probe,
};