linux-kernel - Re: [PATCH 3/7] xen-blkfront: handle Xen major numbers other than XENVBD

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Date:	Fri, 25 Feb 2011 15:38:35 +0000
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Jeremy Fitzhardinge <Jeremy.Fitzhardinge@...rix.com>
Subject: Re: [PATCH 3/7] xen-blkfront: handle Xen major numbers other than
 XENVBD

On Tue, 22 Feb 2011, Konrad Rzeszutek Wilk wrote:
> > @@ -120,6 +120,10 @@ static DEFINE_SPINLOCK(minor_lock);
> >  #define EXTENDED (1<<EXT_SHIFT)
> >  #define VDEV_IS_EXTENDED(dev) ((dev)&(EXTENDED))
> >  #define BLKIF_MINOR_EXT(dev) ((dev)&(~EXTENDED))
> > +#define EMULATED_HD_DISK_MINOR_OFFSET (0)
> > +#define EMULATED_HD_DISK_NAME_OFFSET (EMULATED_HD_DISK_MINOR_OFFSET / 256)
> 
> 0 / 256 ? Why not just 0?

Because if we set EMULATED_HD_DISK_MINOR_OFFSET to something different
from 0, then the divison by 256 would matter.
If you look at the code below you'll see that:

offset = minor / nr_parts;

where nr_parts is 256 and offset is the the device name offset.
EMULATED_HD_DISK_MINOR_OFFSET is the offset of the variable "minor" and
EMULATED_HD_DISK_NAME_OFFSET is the offset of the variable "offset".
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