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

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Message-ID: <CAJK669bUXHqaDUZDNZ9i85pRKENUMS_RXw4z7BsSq7Hiw=OOfg@mail.gmail.com>
Date:	Wed, 5 Sep 2012 15:00:20 +0200
From:	Sjur Brændeland <sjurbren@...il.com>
To:	Rusty Russell <rusty@...tcorp.com.au>,
	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Amit Shah <amit.shah@...hat.com>, linux-kernel@...r.kernel.org,
	Ohad Ben-Cohen <ohad@...ery.com>,
	Linus Walleij <linus.walleij@...aro.org>,
	virtualization@...ts.linux-foundation.org,
	Sjur Brændeland <sjur.brandeland@...ricsson.com>
Subject: 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
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