linux-kernel - Re: [PATCH v2] arch/tile: add hypervisor-based character driver for SPI flash ROM

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Message-Id: <201105291745.26468.arnd@arndb.de>
Date:	Sun, 29 May 2011 17:45:26 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Chris Metcalf <cmetcalf@...era.com>
Cc:	Greg KH <gregkh@...e.de>, linux-kernel@...r.kernel.org,
	Eric Biederman <ebiederm@...stanetworks.com>,
	Chris Wright <chrisw@...s-sol.org>,
	Benjamin Thery <benjamin.thery@...l.net>,
	Phil Carmody <ext-phil.2.carmody@...ia.com>
Subject: Re: [PATCH v2] arch/tile: add hypervisor-based character driver for SPI flash ROM

On Sunday 29 May 2011 14:18:36 Chris Metcalf wrote:
> On 5/29/2011 7:45 AM, Greg KH wrote:
> > On Sat, May 28, 2011 at 08:32:07PM -0400, Chris Metcalf wrote:
> >>> As you are only using 1 minor device, why not just use a misc device
> >>> instead?  It's simpler, and you get the sysfs code for free, which you
> >>> forgot to do, so your device node will never show up in userspace :(
> >> Interesting; this appears to be a bug.  We use 4 minors (see "srom_devs =
> >> 4" higher up).  I'll fix this.  We may have some other devices that would
> >> benefit from being recast as misc devices, so I'll look at our set of
> >> internal devices.
> > This kind of implies that you didn't test this code, right?  You might
> > want to do that next time :)
> 
> No, this bug has been in the code since day one (I just double-checked our
> SCM), and it has always worked fine.  I'm looking into how this is possible
> now, but trust me, we've tested this aspect of the driver the whole time. :-)
> 

AFAICT, the driver just calls cdev_add in a loop, adding one device
add a time. This is actually a correct way to register multiple
character devices, but simply passing the number of devices you
want to add as the third argument would be simpler.

On a related note, the number of devices you add is a module parameter,
which seems a bit backwards, since the hypervisor should actually
know how many devices there are. Can't you just ask the hypervisor?

	Arnd
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