spi: misaligned payload on stack

[bradfitz]

Sorry for lack of proper fix and pull request, but so I don't forget this:

diff --git a/spi/spi.go b/spi/spi.go
index 3eca73e..69d165d 100644
--- a/spi/spi.go
+++ b/spi/spi.go
@@ -109,10 +109,12 @@ func (d *Device) Sync() error {
        return d.f.Sync()
 }

+var used *payload
+
 // Do does a duplex transmission to write to the SPI device and read
 // len(buf) numbers of bytes.
 func (d *Device) Do(buf []byte, delay time.Duration) error {
-       p := payload{
+       p := &payload{
                tx:          uint64(uintptr(unsafe.Pointer(&buf[0]))),
                rx:          uint64(uintptr(unsafe.Pointer(&buf[0]))),
                length:      uint32(len(buf)),
@@ -120,7 +122,8 @@ func (d *Device) Do(buf []byte, delay time.Duration) error {
                delay:       uint16(delay.Nanoseconds() / 1000),
                bitsPerWord: d.bitsPerWord,
        }
-       return d.ioctl(msgArg(1), uintptr(unsafe.Pointer(&p)))
+       used = p // force escape
+       return d.ioctl(msgArg(1), uintptr(unsafe.Pointer(p)))
 }

 // Close closes the SPI device and releases related resources.

Totally the wrong way to fix it. I was lazy and impatient.