galahad2lcd/internal/device/usb.go

package device

import (
	"context"
	"fmt"
	"time"

	"github.com/google/gousb"

	"galahad2lcd/internal/protocol"
)

// Lian Li Galahad II LCD USB layout (from the reference driver and
// protocol reversals).
const (
	interfaceNumber = 1
	endpointNumber  = 2
	configNumber    = 1
	writeTimeout    = time.Second
)

// USBDevice is the gousb-backed implementation of Device.
type USBDevice struct {
	gctx *gousb.Context
	dev  *gousb.Device
	intf *gousb.Interface
	ep   *gousb.OutEndpoint
}

// Open locates the display by vendor/product ID and claims its video
// interface. The kernel driver is detached automatically and reattached on
// Close.
func Open(ctx context.Context, vendorID, productID int) (*USBDevice, error) {
	gctx := gousb.NewContext()

	dev, err := gctx.OpenDeviceWithVIDPID(gousb.ID(vendorID), gousb.ID(productID))
	if err != nil {
		gctx.Close()
		return nil, fmt.Errorf("open USB device %04x:%04x: %w", vendorID, productID, err)
	}
	if dev == nil {
		gctx.Close()
		return nil, fmt.Errorf("USB device %04x:%04x: %w", vendorID, productID, ErrNotFound)
	}

	// Auto-detach releases any kernel driver (e.g. usbhid) so we can claim
	// the interface, and reattaches it when we are done.
	if err := dev.SetAutoDetach(true); err != nil {
		_ = dev.Close()
		gctx.Close()
		return nil, fmt.Errorf("enable auto-detach: %w", err)
	}

	cfg, err := dev.Config(configNumber)
	if err != nil {
		_ = dev.Close()
		gctx.Close()
		return nil, fmt.Errorf("select USB config %d: %w", configNumber, err)
	}

	intf, err := cfg.Interface(interfaceNumber, 0)
	if err != nil {
		_ = cfg.Close()
		_ = dev.Close()
		gctx.Close()
		return nil, fmt.Errorf("claim interface %d: %w", interfaceNumber, err)
	}

	ep, err := intf.OutEndpoint(endpointNumber)
	if err != nil {
		intf.Close()
		_ = dev.Close()
		gctx.Close()
		return nil, fmt.Errorf("open out endpoint 0x0%x: %w", endpointNumber, err)
	}

	return &USBDevice{gctx: gctx, dev: dev, intf: intf, ep: ep}, nil
}

// OpenWithRetry keeps trying to open the device until ctx is done or
// timeout elapses, with exponential backoff. This lets the service start
// before the USB device has finished enumerating at boot.
func OpenWithRetry(ctx context.Context, vendorID, productID int, timeout time.Duration) (*USBDevice, error) {
	deadline := time.Now().Add(timeout)
	backoff := 500 * time.Millisecond

	var lastErr error
	for {
		d, err := Open(ctx, vendorID, productID)
		if err == nil {
			return d, nil
		}
		if ctx.Err() != nil {
			return nil, ctx.Err()
		}
		lastErr = err
		if time.Now().After(deadline) {
			return nil, fmt.Errorf("device %04x:%04x unavailable for %s: %w",
				vendorID, productID, timeout, lastErr)
		}

		select {
		case <-ctx.Done():
			return nil, ctx.Err()
		case <-time.After(backoff):
			if backoff < 5*time.Second {
				backoff *= 2
			}
		}
	}
}

// WriteFrame encodes the frame into USB packets and transmits them with a
// bounded timeout per packet.
func (d *USBDevice) WriteFrame(ctx context.Context, frame []byte) error {
	packets, err := protocol.EncodePackets(frame)
	if err != nil {
		return fmt.Errorf("encode frame: %w", err)
	}

	for _, pkt := range packets {
		writeCtx, cancel := context.WithTimeout(ctx, writeTimeout)
		n, err := d.ep.WriteContext(writeCtx, pkt)
		cancel()

		if err != nil {
			return fmt.Errorf("USB write: %w", err)
		}
		if n != len(pkt) {
			return fmt.Errorf("USB short write: %d of %d bytes", n, len(pkt))
		}
	}
	return nil
}

// Close releases the interface, device and libusb context.
func (d *USBDevice) Close() error {
	var firstErr error
	if d.intf != nil {
		d.intf.Close()
	}
	if d.dev != nil {
		if err := d.dev.Close(); err != nil && firstErr == nil {
			firstErr = err
		}
	}
	if d.gctx != nil {
		if err := d.gctx.Close(); err != nil && firstErr == nil {
			firstErr = err
		}
	}
	return firstErr
}