linux-kernel - Re: [PATCH] NVMe: Add a character device for each nvme device

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Date:	Fri, 27 Jul 2012 15:08:21 -0400
From:	Matthew Wilcox <willy@...ux.intel.com>
To:	Greg KH <greg@...ah.com>
Cc:	Keith Busch <keith.busch@...el.com>,
	linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH] NVMe: Add a character device for each nvme device

On Fri, Jul 27, 2012 at 11:25:46AM -0700, Greg KH wrote:
> On Fri, Jul 27, 2012 at 02:12:12PM -0400, Matthew Wilcox wrote:
> > I don't see a problem here, but I'm no expert at sysfs / character devices.
> > Alan, Greg, anyone else see any problems with how this character device is
> > created / destroyed?
> 
> Yes, see below:

Thanks!

> > > +	device_create(nvme_char_cl, NULL, MKDEV(nvme_char_major, dev->instance),
> > > +						NULL, "nvme%d", dev->instance);
> 
> You just created a device at the "root" of sysfs, which is wrong,
> especially when you do have a parent device here.  Please use it.

OK, that makes sense; this device should be the child of the pci_dev that
it belongs to.

> Also, why are you creating your own class?  Can't this just be a misc
> device?  And if you want to create your own class, please don't, use a
> bus, as that is what is really happening here, right?  We are trying to
> move away from using 'struct class' wherever possible (one of these days
> we'll just remove it...)

What we're trying to achieve here is to create one character device
per NVMe controller that gets plugged in.  Each NVMe controller is-a
PCI function.  The reason we're trying to do this is so that we can send
commands to the NVMe controller, even when there is no storage present
(eg a drive is shipped from the factory with no configured storage).

So we have no particular desire to create a new struct class, or struct
bus.  If we can create a misc device per PCIe function that's bound to our
driver, that's great!  Can you recommend a driver that does this already?
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