Capture the panel by reading the displayed drm buffer instead of fbdev
This commit is contained in:
flamingcow
+250
@@ -0,0 +1,250 @@
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package main
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import (
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"bytes"
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"fmt"
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"image"
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"image/color"
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"path/filepath"
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"unsafe"
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"golang.org/x/sys/unix"
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)
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// cabletest drives the panel through drm and page flips between two scanout
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// buffers of its own, so /dev/fb0 holds the kernel console and there is nothing
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// in it worth reading. The buffer being displayed is reachable from out here
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// though: GETCRTC names it, and root is allowed a handle to it without being
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// drm master, so it can be mapped and read without the program that owns it
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// having to cooperate or even notice.
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const (
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drmCardGlob = "/dev/dri/card*"
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drmVblankRelative = 0x1
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drmVblankHighCrtcMask = 0x3e
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drmVblankHighCrtcShft = 1
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xrgbRedShift = 16
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xrgbGreenShift = 8
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xrgbBlueShift = 0
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)
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func drmIOWR(nr, size uintptr) uintptr { return 3<<30 | size<<16 | 0x64<<8 | nr }
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type drmModeInfo struct {
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clock uint32
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hdisplay, hsyncStart, hsyncEnd, htotal, hskew uint16
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vdisplay, vsyncStart, vsyncEnd, vtotal, vscan uint16
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vrefresh, flags, typ uint32
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name [32]byte
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}
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type drmModeCardRes struct {
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fbIDPtr, crtcIDPtr, connIDPtr, encIDPtr uint64
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countFBs, countCRTCs, countConns, countEncs uint32
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minWidth, maxWidth, minHeight, maxHeight uint32
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}
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type drmModeCrtc struct {
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setConnectorsPtr uint64
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countConnectors uint32
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crtcID uint32
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fbID uint32
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x, y uint32
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gammaSize uint32
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modeValid uint32
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mode drmModeInfo
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}
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type drmModeFBCmd struct {
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fbID, width, height, pitch, bpp, depth uint32
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handle uint32
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}
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type drmModeMapDumb struct {
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handle, pad uint32
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offset uint64
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}
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type drmWaitVblank struct {
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typ uint32
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sequence uint32
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signal uint64
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tvSec int64
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tvUsec int64
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}
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var (
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drmGetResources = drmIOWR(0xa0, unsafe.Sizeof(drmModeCardRes{}))
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drmGetCrtc = drmIOWR(0xa1, unsafe.Sizeof(drmModeCrtc{}))
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drmGetFB = drmIOWR(0xad, unsafe.Sizeof(drmModeFBCmd{}))
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drmMapDumb = drmIOWR(0xb3, unsafe.Sizeof(drmModeMapDumb{}))
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drmWaitVblankIO = drmIOWR(0x3a, 24)
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)
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func drmIoctl(fd int, req uintptr, arg unsafe.Pointer) error {
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if _, _, errno := unix.Syscall(unix.SYS_IOCTL, uintptr(fd), req, uintptr(arg)); errno != 0 {
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return errno
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}
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return nil
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}
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type grabber struct {
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fd int
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crtc uint32
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index int
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}
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// The card and crtc actually putting something on a display, which is whichever
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// one cabletest chose when it set its mode.
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func openGrabber() (*grabber, error) {
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paths, err := filepath.Glob(drmCardGlob)
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if err != nil {
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return nil, err
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}
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for _, p := range paths {
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fd, err := unix.Open(p, unix.O_RDWR|unix.O_CLOEXEC, 0)
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if err != nil {
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continue
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}
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crtc, index, err := activeCrtc(fd)
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if err == nil {
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return &grabber{fd: fd, crtc: crtc, index: index}, nil
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}
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unix.Close(fd)
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}
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return nil, fmt.Errorf("no drm crtc is scanning out a framebuffer")
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}
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func activeCrtc(fd int) (uint32, int, error) {
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var res drmModeCardRes
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if err := drmIoctl(fd, drmGetResources, unsafe.Pointer(&res)); err != nil {
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return 0, 0, err
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}
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if res.countCRTCs == 0 {
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return 0, 0, fmt.Errorf("card has no crtcs")
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}
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crtcs := make([]uint32, res.countCRTCs)
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res.countFBs, res.countConns, res.countEncs = 0, 0, 0
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res.fbIDPtr, res.connIDPtr, res.encIDPtr = 0, 0, 0
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res.crtcIDPtr = uint64(uintptr(unsafe.Pointer(&crtcs[0])))
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if err := drmIoctl(fd, drmGetResources, unsafe.Pointer(&res)); err != nil {
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return 0, 0, err
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}
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for i, id := range crtcs {
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c := drmModeCrtc{crtcID: id}
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if err := drmIoctl(fd, drmGetCrtc, unsafe.Pointer(&c)); err != nil {
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continue
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}
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if c.fbID != 0 && c.modeValid != 0 {
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return id, i, nil
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}
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}
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return 0, 0, fmt.Errorf("no crtc is scanning out a framebuffer")
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}
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func (g *grabber) close() { unix.Close(g.fd) }
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// Reading the buffer takes about as long as a frame, so where the read starts
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// in the flip cycle is what decides whether it stays ahead of the writer. Woken
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// at a blank, the buffer named next has just gone on screen, which leaves a
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// whole frame plus however long cabletest spends drawing before anything
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// touches it again.
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func (g *grabber) waitVblank() error {
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v := drmWaitVblank{
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typ: drmVblankRelative | uint32(g.index<<drmVblankHighCrtcShft)&drmVblankHighCrtcMask,
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sequence: 1,
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}
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for {
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err := drmIoctl(g.fd, drmWaitVblankIO, unsafe.Pointer(&v))
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if err == unix.EINTR {
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continue
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}
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return err
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}
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}
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// Maps whichever buffer is on screen right now. The caller is expected to have
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// just woken at a blank, so this is the one with the longest life ahead of it.
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func (g *grabber) mapFront() (mem []byte, pitch, pw, ph int, err error) {
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c := drmModeCrtc{crtcID: g.crtc}
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if err := drmIoctl(g.fd, drmGetCrtc, unsafe.Pointer(&c)); err != nil {
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return nil, 0, 0, 0, fmt.Errorf("get crtc: %w", err)
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}
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if c.fbID == 0 {
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return nil, 0, 0, 0, fmt.Errorf("crtc %d is not scanning out anything", g.crtc)
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}
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// Only a drm master or root is given a handle to someone else's framebuffer.
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fb := drmModeFBCmd{fbID: c.fbID}
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if err := drmIoctl(g.fd, drmGetFB, unsafe.Pointer(&fb)); err != nil {
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return nil, 0, 0, 0, fmt.Errorf("get fb %d: %w", c.fbID, err)
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}
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if fb.handle == 0 {
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return nil, 0, 0, 0, fmt.Errorf("kernel gave no handle for fb %d", c.fbID)
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}
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if fb.bpp != 32 {
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return nil, 0, 0, 0, fmt.Errorf("expected 32bpp, got %d", fb.bpp)
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}
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m := drmModeMapDumb{handle: fb.handle}
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if err := drmIoctl(g.fd, drmMapDumb, unsafe.Pointer(&m)); err != nil {
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return nil, 0, 0, 0, fmt.Errorf("map fb %d: %w", c.fbID, err)
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}
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pitch, pw, ph = int(fb.pitch), int(fb.width), int(fb.height)
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mem, err = unix.Mmap(g.fd, int64(m.offset), pitch*ph,
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unix.PROT_READ, unix.MAP_SHARED)
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if err != nil {
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return nil, 0, 0, 0, fmt.Errorf("mmap fb %d: %w", c.fbID, err)
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}
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return mem, pitch, pw, ph, nil
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}
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// Scanout memory is uncached, so reading a frame out of it costs a good
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// fraction of a frame's time and could in principle be overtaken by the next
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// redraw. Rather than trust that it was not, the buffer is read twice and the
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// pair only accepted if they agree: the writer cycles through its buffers in
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// order and leaves this one alone for several frames after putting it on
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// screen, which is comfortably long enough for both reads. Disagreement means
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// that reasoning is wrong somewhere, which is worth hearing about rather than
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// papering over with another go.
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func (g *grabber) frame() (*image.NRGBA, error) {
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if err := g.waitVblank(); err != nil {
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return nil, fmt.Errorf("wait for vblank: %w", err)
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}
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mem, pitch, pw, ph, err := g.mapFront()
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if err != nil {
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return nil, err
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}
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a := make([]byte, len(mem))
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copy(a, mem)
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b := make([]byte, len(mem))
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copy(b, mem)
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unix.Munmap(mem)
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if !bytes.Equal(a, b) {
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return nil, fmt.Errorf("scanout buffer was redrawn while being read")
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}
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return decode(a, pitch, pw, ph), nil
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}
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// The panel is landscape and every draw is turned a quarter turn on its way
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// into it, so the turn is undone here to get back what a person standing in
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// front of it sees.
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func decode(buf []byte, pitch, pw, ph int) *image.NRGBA {
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img := image.NewNRGBA(image.Rect(0, 0, ph, pw))
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for x := 0; x < ph; x++ {
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for y := 0; y < pw; y++ {
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o := x*pitch + (pw-1-y)*4
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v := uint32(buf[o]) | uint32(buf[o+1])<<8 |
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uint32(buf[o+2])<<16 | uint32(buf[o+3])<<24
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img.SetNRGBA(x, y, color.NRGBA{
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R: uint8(v >> xrgbRedShift),
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G: uint8(v >> xrgbGreenShift),
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B: uint8(v >> xrgbBlueShift),
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A: 255,
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})
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}
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}
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return img
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}
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