cabletest/fb.go

package main

import (
	"encoding/binary"
	"fmt"
	"path/filepath"
	"unsafe"

	"golang.org/x/sys/unix"
)

// Drawing lands in a plain memory buffer and is blitted to a scanout buffer the
// display is not reading, which is then swapped in whole at a vertical blank.
// Writing into the live scanout buffer instead, as the fbdev interface invites,
// races the beam: the blit takes a few hundred microseconds, and whatever the
// display reads during it is part of one frame and part of the next.
const (
	drmIoctlBase = 0x64

	drmModeConnected     = 1
	drmModeTypePreferred = 1 << 3
	drmModePageFlipEvent = 0x01
	drmEventFlipComplete = 0x02

	// What ADDFB means by 32 bits per pixel and 24 bits of colour.
	xrgbRedShift   = 16
	xrgbGreenShift = 8
	xrgbBlueShift  = 0

	// Two would be enough to stop the panel tearing, since one buffer being
	// displayed while the other is drawn into is all double buffering means.
	// More than two is for anything reading a frame back out: they are cycled
	// in order, so a buffer is left alone for the three frames between going on
	// screen and coming round again, and reading one out of uncached scanout
	// memory takes a good fraction of a frame.
	scanoutBuffers = 4
)

func drmIO(nr uintptr) uintptr         { return drmIoctlBase<<8 | nr }
func drmIOWR(nr, size uintptr) uintptr { return 3<<30 | size<<16 | drmIoctlBase<<8 | nr }

type drmModeInfo struct {
	clock                                         uint32
	hdisplay, hsyncStart, hsyncEnd, htotal, hskew uint16
	vdisplay, vsyncStart, vsyncEnd, vtotal, vscan uint16
	vrefresh, flags, typ                          uint32
	name                                          [32]byte
}

type drmModeCardRes struct {
	fbIDPtr, crtcIDPtr, connIDPtr, encIDPtr     uint64
	countFBs, countCRTCs, countConns, countEncs uint32
	minWidth, maxWidth, minHeight, maxHeight    uint32
}

type drmModeGetConnector struct {
	encodersPtr, modesPtr, propsPtr, propValuesPtr uint64
	countModes, countProps, countEncoders          uint32
	encoderID, connectorID, connectorType          uint32
	connectorTypeID, connection                    uint32
	mmWidth, mmHeight, subpixel, pad               uint32
}

type drmModeGetEncoder struct {
	encoderID, encoderType        uint32
	crtcID                        uint32
	possibleCRTCs, possibleClones uint32
}

type drmModeCrtc struct {
	setConnectorsPtr uint64
	countConnectors  uint32
	crtcID           uint32
	fbID             uint32
	x, y             uint32
	gammaSize        uint32
	modeValid        uint32
	mode             drmModeInfo
}

type drmModeFBCmd struct {
	fbID, width, height, pitch, bpp, depth uint32
	handle                                 uint32
}

type drmModeCreateDumb struct {
	height, width, bpp, flags uint32
	handle, pitch             uint32
	size                      uint64
}

type drmModeMapDumb struct {
	handle, pad uint32
	offset      uint64
}

type drmModeCrtcPageFlip struct {
	crtcID, fbID, flags, reserved uint32
	userData                      uint64
}

var (
	drmSetMaster    = drmIO(0x1e)
	drmDropMaster   = drmIO(0x1f)
	drmGetResources = drmIOWR(0xa0, unsafe.Sizeof(drmModeCardRes{}))
	drmSetCrtc      = drmIOWR(0xa2, unsafe.Sizeof(drmModeCrtc{}))
	drmGetEncoder   = drmIOWR(0xa6, unsafe.Sizeof(drmModeGetEncoder{}))
	drmGetConnector = drmIOWR(0xa7, unsafe.Sizeof(drmModeGetConnector{}))
	drmAddFB        = drmIOWR(0xae, unsafe.Sizeof(drmModeFBCmd{}))
	drmPageFlip     = drmIOWR(0xb0, unsafe.Sizeof(drmModeCrtcPageFlip{}))
	drmCreateDumb   = drmIOWR(0xb2, unsafe.Sizeof(drmModeCreateDumb{}))
	drmMapDumb      = drmIOWR(0xb3, unsafe.Sizeof(drmModeMapDumb{}))
)

func drmIoctl(fd int, req uintptr, arg unsafe.Pointer) error {
	if _, _, errno := unix.Syscall(unix.SYS_IOCTL, uintptr(fd), req, uintptr(arg)); errno != 0 {
		return errno
	}
	return nil
}

type scanout struct {
	fbID uint32
	mem  []byte
}

// w and h are the logical canvas, which is portrait; pw and ph are the panel,
// which is landscape. Every draw is turned a quarter turn on its way to memory,
// so logical top lands on the panel's right edge.
type framebuffer struct {
	fd     int
	back   []byte
	w      int
	h      int
	pw     int
	ph     int
	stride int

	bufs   [scanoutBuffers]scanout
	front  int
	crtcID uint32
	connID uint32

	// One token per completed flip. The render loop waits on this rather than on
	// a timer, so drawing is paced by the panel instead of by a guess at its rate.
	flips chan struct{}
}

func (fb *framebuffer) offset(x, y int) int {
	return x*fb.stride + (fb.pw-1-y)*4
}

func (fb *framebuffer) fromPanel(x, y int) (int, int) {
	return y, fb.pw - 1 - x
}

func cardResources(fd int) (crtcs, conns []uint32, err error) {
	var res drmModeCardRes
	if err := drmIoctl(fd, drmGetResources, unsafe.Pointer(&res)); err != nil {
		return nil, nil, fmt.Errorf("get resources: %w", err)
	}
	if res.countCRTCs == 0 || res.countConns == 0 {
		return nil, nil, fmt.Errorf("card has no crtcs or connectors")
	}
	crtcs = make([]uint32, res.countCRTCs)
	conns = make([]uint32, res.countConns)
	res.countFBs, res.countEncs = 0, 0
	res.fbIDPtr, res.encIDPtr = 0, 0
	res.crtcIDPtr = uint64(uintptr(unsafe.Pointer(&crtcs[0])))
	res.connIDPtr = uint64(uintptr(unsafe.Pointer(&conns[0])))
	if err := drmIoctl(fd, drmGetResources, unsafe.Pointer(&res)); err != nil {
		return nil, nil, fmt.Errorf("get resources: %w", err)
	}
	return crtcs, conns, nil
}

// The preferred mode is the panel's native one; anything else would be the
// driver scaling a wrong-sized image onto it.
func preferredMode(fd int, connID uint32) (drmModeInfo, error) {
	c := drmModeGetConnector{connectorID: connID}
	if err := drmIoctl(fd, drmGetConnector, unsafe.Pointer(&c)); err != nil {
		return drmModeInfo{}, err
	}
	if c.countModes == 0 {
		return drmModeInfo{}, fmt.Errorf("connector %d reported no modes", connID)
	}
	modes := make([]drmModeInfo, c.countModes)
	q := drmModeGetConnector{
		connectorID: connID,
		countModes:  c.countModes,
		modesPtr:    uint64(uintptr(unsafe.Pointer(&modes[0]))),
	}
	if err := drmIoctl(fd, drmGetConnector, unsafe.Pointer(&q)); err != nil {
		return drmModeInfo{}, err
	}
	if q.countModes == 0 {
		return drmModeInfo{}, fmt.Errorf("connector %d reported no modes", connID)
	}
	for _, m := range modes[:q.countModes] {
		if m.typ&drmModeTypePreferred != 0 {
			return m, nil
		}
	}
	return modes[0], nil
}

func crtcFor(fd int, c drmModeGetConnector, crtcs []uint32) (uint32, error) {
	encoders := []uint32{c.encoderID}
	if c.countEncoders > 0 {
		list := make([]uint32, c.countEncoders)
		q := drmModeGetConnector{
			connectorID:   c.connectorID,
			countEncoders: c.countEncoders,
			encodersPtr:   uint64(uintptr(unsafe.Pointer(&list[0]))),
		}
		if err := drmIoctl(fd, drmGetConnector, unsafe.Pointer(&q)); err == nil {
			encoders = append(encoders, list[:q.countEncoders]...)
		}
	}
	for _, id := range encoders {
		if id == 0 {
			continue
		}
		e := drmModeGetEncoder{encoderID: id}
		if err := drmIoctl(fd, drmGetEncoder, unsafe.Pointer(&e)); err != nil {
			continue
		}
		// Already driving this connector, otherwise anything it can be wired to.
		if e.crtcID != 0 {
			return e.crtcID, nil
		}
		for i, crtc := range crtcs {
			if e.possibleCRTCs&(1<<uint(i)) != 0 {
				return crtc, nil
			}
		}
	}
	return 0, fmt.Errorf("connector %d has no usable crtc", c.connectorID)
}

// The connector to drive and a crtc that can drive it.
func findDisplay(fd int) (connID, crtcID uint32, err error) {
	crtcs, conns, err := cardResources(fd)
	if err != nil {
		return 0, 0, err
	}
	for _, id := range conns {
		c := drmModeGetConnector{connectorID: id}
		if err := drmIoctl(fd, drmGetConnector, unsafe.Pointer(&c)); err != nil {
			continue
		}
		if c.connection != drmModeConnected || c.countModes == 0 {
			continue
		}
		crtc, err := crtcFor(fd, c, crtcs)
		if err != nil {
			continue
		}
		return id, crtc, nil
	}
	return 0, 0, fmt.Errorf("no connected connector with a mode")
}

// The card number is not stable across machines, so the card driving a
// connected display is the one we want.
func openCard() (int, error) {
	paths, err := filepath.Glob("/dev/dri/card*")
	if err != nil {
		return -1, err
	}
	for _, p := range paths {
		fd, err := unix.Open(p, unix.O_RDWR|unix.O_CLOEXEC, 0)
		if err != nil {
			continue
		}
		if _, _, err := findDisplay(fd); err == nil {
			return fd, nil
		}
		unix.Close(fd)
	}
	return -1, fmt.Errorf("no drm device with a connected display")
}

func (fb *framebuffer) addScanout(i int) error {
	create := drmModeCreateDumb{width: uint32(fb.pw), height: uint32(fb.ph), bpp: 32}
	if err := drmIoctl(fb.fd, drmCreateDumb, unsafe.Pointer(&create)); err != nil {
		return fmt.Errorf("create dumb buffer: %w", err)
	}
	fb.stride = int(create.pitch)

	add := drmModeFBCmd{
		width:  uint32(fb.pw),
		height: uint32(fb.ph),
		pitch:  create.pitch,
		bpp:    32,
		depth:  24,
		handle: create.handle,
	}
	if err := drmIoctl(fb.fd, drmAddFB, unsafe.Pointer(&add)); err != nil {
		return fmt.Errorf("add fb: %w", err)
	}
	fb.bufs[i].fbID = add.fbID

	m := drmModeMapDumb{handle: create.handle}
	if err := drmIoctl(fb.fd, drmMapDumb, unsafe.Pointer(&m)); err != nil {
		return fmt.Errorf("map dumb buffer: %w", err)
	}
	mem, err := unix.Mmap(fb.fd, int64(m.offset), int(create.size),
		unix.PROT_READ|unix.PROT_WRITE, unix.MAP_SHARED)
	if err != nil {
		return fmt.Errorf("mmap scanout: %w", err)
	}
	fb.bufs[i].mem = mem
	return nil
}

func openFramebuffer() (*framebuffer, error) {
	fd, err := openCard()
	if err != nil {
		return nil, err
	}
	fb := &framebuffer{fd: fd, flips: make(chan struct{}, 1)}

	// Without master the modeset below is refused, and taking it is also what
	// stops the kernel console drawing into the display behind us.
	if err := drmIoctl(fd, drmSetMaster, nil); err != nil {
		unix.Close(fd)
		return nil, fmt.Errorf("take drm master: %w", err)
	}

	connID, crtcID, err := findDisplay(fd)
	if err != nil {
		fb.close()
		return nil, err
	}
	mode, err := preferredMode(fd, connID)
	if err != nil {
		fb.close()
		return nil, err
	}
	fb.connID, fb.crtcID = connID, crtcID
	fb.pw, fb.ph = int(mode.hdisplay), int(mode.vdisplay)
	fb.w, fb.h = fb.ph, fb.pw

	for i := range fb.bufs {
		if err := fb.addScanout(i); err != nil {
			fb.close()
			return nil, err
		}
	}
	fb.back = make([]byte, fb.stride*fb.ph)

	set := drmModeCrtc{
		setConnectorsPtr: uint64(uintptr(unsafe.Pointer(&fb.connID))),
		countConnectors:  1,
		crtcID:           crtcID,
		fbID:             fb.bufs[0].fbID,
		modeValid:        1,
		mode:             mode,
	}
	if err := drmIoctl(fd, drmSetCrtc, unsafe.Pointer(&set)); err != nil {
		fb.close()
		return nil, fmt.Errorf("set crtc: %w", err)
	}

	go fb.readEvents()
	// Nothing has been flipped yet, so the first frame is owed its turn.
	fb.flips <- struct{}{}
	return fb, nil
}

// Flip completions arrive on the drm fd as a stream of length-prefixed events.
func (fb *framebuffer) readEvents() {
	buf := make([]byte, 4096)
	for {
		n, err := unix.Read(fb.fd, buf)
		if n <= 0 || err != nil {
			return
		}
		for off := 0; off+8 <= n; {
			typ := binary.LittleEndian.Uint32(buf[off:])
			length := int(binary.LittleEndian.Uint32(buf[off+4:]))
			if length < 8 || off+length > n {
				return
			}
			if typ == drmEventFlipComplete {
				select {
				case fb.flips <- struct{}{}:
				default:
				}
			}
			off += length
		}
	}
}

func (fb *framebuffer) close() {
	for i := range fb.bufs {
		if fb.bufs[i].mem != nil {
			unix.Munmap(fb.bufs[i].mem)
		}
	}
	// Dropping master hands the display back to the kernel console, which
	// restores its own mode. The framebuffers and dumb buffers are reclaimed
	// when the last reference to the fd goes.
	drmIoctl(fb.fd, drmDropMaster, nil)
	unix.Close(fb.fd)
}

func (fb *framebuffer) pixel(c rgb) uint32 {
	return uint32(c.r)<<xrgbRedShift | uint32(c.g)<<xrgbGreenShift | uint32(c.b)<<xrgbBlueShift
}

type rgb struct {
	r, g, b uint8
}

func (fb *framebuffer) fill(c rgb) {
	v := fb.pixel(c)
	row := make([]byte, fb.stride)
	for x := 0; x+4 <= fb.stride; x += 4 {
		row[x+0] = byte(v)
		row[x+1] = byte(v >> 8)
		row[x+2] = byte(v >> 16)
		row[x+3] = byte(v >> 24)
	}
	for y := 0; y < fb.ph; y++ {
		copy(fb.back[y*fb.stride:], row)
	}
}

// One logical column is contiguous after the turn, so it fills a span at a time.
func (fb *framebuffer) rect(x0, y0, w, h int, c rgb) {
	x1, y1 := min(x0+w, fb.w), min(y0+h, fb.h)
	x0, y0 = max(x0, 0), max(y0, 0)
	if x0 >= x1 || y0 >= y1 {
		return
	}
	v := fb.pixel(c)
	span := make([]byte, (y1-y0)*4)
	for i := 0; i+4 <= len(span); i += 4 {
		span[i+0] = byte(v)
		span[i+1] = byte(v >> 8)
		span[i+2] = byte(v >> 16)
		span[i+3] = byte(v >> 24)
	}
	for x := x0; x < x1; x++ {
		copy(fb.back[fb.offset(x, y1-1):], span)
	}
}

// Blends src over the existing pixel, with cov as 0-255 coverage.
func (fb *framebuffer) blend(x, y int, c rgb, cov uint8) {
	if x < 0 || y < 0 || x >= fb.w || y >= fb.h || cov == 0 {
		return
	}
	o := fb.offset(x, y)
	if cov == 255 {
		v := fb.pixel(c)
		fb.back[o+0] = byte(v)
		fb.back[o+1] = byte(v >> 8)
		fb.back[o+2] = byte(v >> 16)
		fb.back[o+3] = byte(v >> 24)
		return
	}
	a := uint32(cov)
	old := uint32(fb.back[o+0]) | uint32(fb.back[o+1])<<8 |
		uint32(fb.back[o+2])<<16 | uint32(fb.back[o+3])<<24
	orr := uint8(old >> xrgbRedShift)
	og := uint8(old >> xrgbGreenShift)
	ob := uint8(old >> xrgbBlueShift)
	mix := rgb{
		r: uint8((uint32(c.r)*a + uint32(orr)*(255-a)) / 255),
		g: uint8((uint32(c.g)*a + uint32(og)*(255-a)) / 255),
		b: uint8((uint32(c.b)*a + uint32(ob)*(255-a)) / 255),
	}
	v := fb.pixel(mix)
	fb.back[o+0] = byte(v)
	fb.back[o+1] = byte(v >> 8)
	fb.back[o+2] = byte(v >> 16)
	fb.back[o+3] = byte(v >> 24)
}

// Copies into the scanout buffer furthest from being displayed and asks for it
// at the next blank. The copy cannot tear because nothing is displaying that
// buffer, and the swap cannot tear because the hardware does it between frames.
func (fb *framebuffer) flush() error {
	next := (fb.front + 1) % scanoutBuffers
	copy(fb.bufs[next].mem, fb.back)

	flip := drmModeCrtcPageFlip{
		crtcID: fb.crtcID,
		fbID:   fb.bufs[next].fbID,
		flags:  drmModePageFlipEvent,
	}
	if err := drmIoctl(fb.fd, drmPageFlip, unsafe.Pointer(&flip)); err != nil {
		return fmt.Errorf("page flip: %w", err)
	}
	fb.front = next
	return nil
}