SchOS/kernel/dev/fb.c

// /dev/fb0 -- the linear framebuffer as a file.
//
// Until now this node existed but was wired to major 2, whose devsw entry was
// { nullread, nullwrite }: reads returned end-of-file and writes were thrown
// away, silently. The only thing that actually worked was mmap, and only
// because sys_mmap special-cased the major number.
//
// Reads and writes here address the framebuffer by byte offset, so lseek
// positions the "cursor" and a whole frame can be written with one write().
// That is slower than mmap and always will be -- it is a copy -- but it is
// what makes the device inspectable from a shell, which is worth a great deal
// while there is no GUI yet to inspect it with.

#include "types.h"
#include "defs.h"
#include "param.h"
#include "mmu.h"
#include "proc.h"
#include "spinlock.h"
#include "sleeplock.h"
#include "fs.h"
#include "file.h"
#include "fb.h"
#include "uaccess.h"
#include "errno.h"

// Describe one colour channel. A channel that does not exist gets length 0,
// which is how callers are supposed to detect "no alpha" rather than by
// assuming from bits_per_pixel.
static void
chan(struct fb_bitfield *f, unsigned int offset, unsigned int length)
{
  f->offset = offset;
  f->length = length;
  f->msb_right = 0;
}

// Pixel layout for the depths vga_draw_pixel knows how to write. Keeping the
// two in step matters: this is the kernel telling userspace how to build a
// pixel, and if it disagrees with how the kernel builds one, every colour
// drawn by a program is wrong in a way that looks like a hardware fault.
static void
pixel_format(struct fb_var_screeninfo *v, unsigned int bpp)
{
  switch (bpp) {
  case 32:
    chan(&v->red, 16, 8); chan(&v->green, 8, 8); chan(&v->blue, 0, 8);
    chan(&v->transp, 24, 0);   // the byte is there; nothing honours it
    break;
  case 24:
    chan(&v->red, 16, 8); chan(&v->green, 8, 8); chan(&v->blue, 0, 8);
    chan(&v->transp, 0, 0);
    break;
  case 16:
    // RGB565, matching the packing vga_draw_pixel does for this depth.
    chan(&v->red, 11, 5); chan(&v->green, 5, 6); chan(&v->blue, 0, 5);
    chan(&v->transp, 0, 0);
    break;
  default:
    chan(&v->red, 0, 0); chan(&v->green, 0, 0); chan(&v->blue, 0, 0);
    chan(&v->transp, 0, 0);
    break;
  }
}

int
fbread(struct inode *ip, uint off, char *dst, int n)
{
  volatile uchar *fb = vga_fb_ptr();
  uint64 size = vga_fb_size();

  if (!fb)
    return -ENODEV;
  if (n < 0)
    return -EINVAL;
  if ((uint64)off >= size)
    return 0;                       // past the end of the screen: EOF
  if ((uint64)off + (uint64)n > size)
    n = (int)(size - (uint64)off);

  memmove(dst, (void*)(fb + off), n);
  return n;
}

// Set once something has drawn through this device, so that fbclose knows
// whether the screen still belongs to the text console. Reading the
// framebuffer, or asking it its geometry, does not count -- `fbinfo` with no
// arguments must not wipe its own output off the screen.
static int fb_dirty;

// Called from sys_mmap when /dev/fb0 is mapped. A mapping is a licence to
// draw, and the drawing happens with no further syscalls, so this is the last
// point at which the kernel hears about it at all.
void
fb_mapped(void)
{
  fb_dirty = 1;
}

int
fbwrite(struct inode *ip, uint off, char *src, int n)
{
  volatile uchar *fb = vga_fb_ptr();
  uint64 size = vga_fb_size();

  if (!fb)
    return -ENODEV;
  if (n < 0)
    return -EINVAL;
  if ((uint64)off >= size)
    return -ENOSPC;                 // a screen cannot be extended
  if ((uint64)off + (uint64)n > size)
    n = (int)(size - (uint64)off);

  memmove((void*)(fb + off), src, n);
  fb_dirty = 1;
  return n;
}

// The last descriptor on /dev/fb0 is going away.
//
// Whatever was drawing has finished, and the screen still holds its final
// frame -- so the shell prompt underneath is invisible and the machine looks
// hung. It is not: the process exited cleanly and the console is writing
// happily into a framebuffer nobody can read any more. There is no character
// backing store to repaint from (the console draws glyphs straight into the
// framebuffer and scrolls with a memmove), so the honest recovery is to clear
// and go home rather than to pretend the old text can be restored.
void
fbclose(struct inode *ip)
{
  if (!fb_dirty)
    return;
  fb_dirty = 0;

  // Said out loud, because a screen clearing on its own is otherwise
  // indistinguishable from a bug -- and this is the only place in the system
  // that wipes the display without being asked to.
  cprintf("fb0: released by pid %d, console restored\n", myproc() ? myproc()->pid : -1);
  vga_screen_reset();
}

// FBIO* handling, called from sys_ioctl once it has established that the fd
// refers to this device. `arg` is the raw user address; each request
// validates it for the size it is actually going to touch, rather than for
// the largest struct here -- fb_fix_screeninfo is half the size of
// fb_var_screeninfo, and a buffer sitting near the end of a mapping is a
// legitimate place to put the smaller one.
int
fb_ioctl(int request, addr_t arg)
{
  int w, h;

  if (!vga_fb_ptr())
    return -ENODEV;

  vga_dims(&w, &h);

  switch (request) {
  case FBIOGET_VSCREENINFO: {
    struct fb_var_screeninfo *v =
        (struct fb_var_screeninfo *)user_ptr(arg, sizeof(*v));
    if (!v)
      return -EFAULT;
    memset(v, 0, sizeof(*v));
    v->xres = v->xres_virtual = (unsigned int)w;
    v->yres = v->yres_virtual = (unsigned int)h;
    v->bits_per_pixel = vga_fb_bpp();
    v->activate = FB_ACTIVATE_NOW;
    pixel_format(v, v->bits_per_pixel);
    // height/width (physical millimetres) and every timing field stay zero:
    // VBE does not report them and a guess would be worse than an admission.
    return 0;
  }

  case FBIOGET_FSCREENINFO: {
    struct fb_fix_screeninfo *f =
        (struct fb_fix_screeninfo *)user_ptr(arg, sizeof(*f));
    if (!f)
      return -EFAULT;
    memset(f, 0, sizeof(*f));
    safestrcpy(f->id, "schosfb", sizeof(f->id));
    f->smem_start  = (unsigned long)vga_fb_phys();
    f->smem_len    = (unsigned int)vga_fb_size();
    f->type        = FB_TYPE_PACKED_PIXELS;
    f->visual      = FB_VISUAL_TRUECOLOR;
    f->line_length = vga_fb_stride();
    return 0;
  }

  // The mode came from the bootloader's VBE call and we have no way to
  // change it. Saying so is more useful than accepting the request and
  // leaving the caller to discover, by drawing, that nothing happened.
  case FBIOPUT_VSCREENINFO:
    return -EINVAL;

  case FBIOBLANK:
    return -EINVAL;
  }

  return -ENOTTY;
}