spi: virtio: mark device ready before registering the controller

virtio_spi_probe() registers the SPI controller with
devm_spi_register_controller(). spi_register_controller() binds a child
inline unless its driver has asked for asynchronous probing, so a
peripheral that performs a transfer during its own probe reaches
virtio_spi_transfer_one(), which kicks the virtqueue before probe has
returned.

The driver never calls virtio_device_ready(), so DRIVER_OK is set on its
behalf by virtio_dev_probe(), only once probe has returned. The virtio
spec is explicit about that ordering in 3.1 Device Initialization:
  |  The driver MUST NOT send any buffer available notifications to the
  |  device before setting DRIVER_OK.

A device that waits for DRIVER_OK before servicing the queue therefore
leaves the transfer unanswered, and virtio_spi_transfer_one() waits for its
completion with no timeout, so probe never returns.

Mark the device ready before registering the controller, as done for the
same reason in commit f5866db64f ("virtio_console: enable VQs early") and
commit 1d774589f9 ("i2c: virtio: mark device ready before registering the
adapter").

Fixes: f98cabe3f6 ("SPI: Add virtio SPI driver")
Signed-off-by: Jasper Wise <jaspwise@amazon.co.uk>
Link: https://patch.msgid.link/20260813084618.613172-1-jaspwise@amazon.co.uk
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-virtio.c

@@ -358,6 +358,8 @@ static int virtio_spi_probe(struct virtio_device *vdev)
 	if (ret)
 		return dev_err_probe(&vdev->dev, ret, "Cannot register virtqueue cleanup\n");
 
+	virtio_device_ready(vdev);
+
 	/* Use devm version to register controller */
 	ret = devm_spi_register_controller(&vdev->dev, ctrl);
 	if (ret)