linux-kernel - Re: [PATCH, RFC] virtio_blk: add cache control support

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Date:	Wed, 16 Mar 2011 15:09:58 +0100
From:	Christoph Hellwig <hch@....de>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Christoph Hellwig <hch@....de>, qemu-devel@...gnu.org,
	linux-kernel@...r.kernel.org, stefanha@...il.com, kwolf@...hat.com,
	prerna@...ux.vnet.ibm.com, aliguori@...ibm.com
Subject: Re: [PATCH, RFC] virtio_blk: add cache control support

On Wed, Mar 16, 2011 at 02:39:39PM +1030, Rusty Russell wrote:
> > +	if (strncmp(buf, "write through", sizeof("write through") - 1) == 0) {
> > +		;
> > +	} else if (strncmp(buf, "write back", sizeof("write back") - 1) == 0) {
> 
>    Is there a reason we're not letting gcc and/or strcmp do the
> optimization work here?

I'm happ to switch strcmp.

> > +	vdev->config->set(vdev, offsetof(struct virtio_blk_config, features),
> > +	                  &features, sizeof(features));
> > +
> > +	vdev->config->get(vdev, offsetof(struct virtio_blk_config, features),
> > +			  &features2, sizeof(features2));
> > +
> > +	if ((features & VIRTIO_BLK_RT_WCE) !=
> > +	    (features2 & VIRTIO_BLK_RT_WCE))
> > +		return -EIO;
> 
>    This seems like a debugging check you left in.  Or do you suspect
> some issues?

No, it's intentional.  config space writes can't return errors, so we need
to check that the value has really changed.  I'll add a comment explaining it.

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