Re: [RFC 1/2] virtio_console: Add support for DMA memory allocation

The driver certainly shouldn't offer VIRTIO_CONSOLE_F_DMA_MEM if you
don't have DMA!

OK, so the feature table could be done like this:

 static unsigned int features[] = {
	VIRTIO_CONSOLE_F_SIZE,
	VIRTIO_CONSOLE_F_MULTIPORT,
#if VIRTIO_CONSOLE_HAS_DMA
	VIRTIO_CONSOLE_F_DMA_MEM,
#endif
}

If the device then asks for VIRTIO_CONSOLE_F_DMA_MEM
when DMA is not supported, virtio will do BUG_ON() from
virtio_check_driver_offered_feature().

Is this acceptable or should we add a check in virtcons_probe()
and let the probing fail instead?

E.g:
	/* Refuse to bind if F_DMA_MEM request cannot be met */
	if (!VIRTIO_CONSOLE_HAS_DMA &&
	    (vdev->config->get_features(vdev) & (1 << VIRTIO_CONSOLE_F_DMA_MEM))){
		dev_err(&vdev->dev,
			"DMA_MEM requested, but arch does not support DMA\n");
		err = -EINVAL;
		goto fail;
	}

Regards,
Sjur