Date:	Wed, 10 Apr 2013 13:03:12 -0700
From:	Greg KH <gregkh@...uxfoundation.org>
To:	Samuel Ortiz <sameo@...ux.intel.com>
Cc:	Tomas Winkler <tomas.winkler@...el.com>, arnd@...db.de,
	linux-kernel@...r.kernel.org
Subject: Re: [char-misc-next 1/3] mei: nfc: Initial nfc implementation

On Wed, Apr 10, 2013 at 09:19:45PM +0200, Samuel Ortiz wrote:
> Hi Greg,
> 
> On Wed, Apr 10, 2013 at 10:25:51AM -0700, Greg KH wrote:
> > On Tue, Apr 09, 2013 at 02:41:33AM +0300, Tomas Winkler wrote:
> > > From: Samuel Ortiz <sameo@...ux.intel.com>
> > > 
> > > NFC ME device is exported through the MEI bus to be consumed by the
> > > NFC subsystem.
> > > 
> > > NFC is represented by two mei clients: An info one and the actual
> > > NFC one. In order to properly build the ME id we first need to retrieve
> > > the firmware information from the info client and then disconnect from it.
> > 
> > I still don't understand why you aren't tieing this into the existing
> > nfc kernel api.  What am I missing?
> We can't tie this into the existing NFC kernel API because mei/nfc.c is not an
> NFC driver, it's an ME specific shim layer that does not do anything NFC (as
> in NFC Forum specified) related.

Then why call it "NFC" if you don't want to confuse everyone :)

> At that point in the code the only thing we know is that there is an NFC
> device behind the ME. As I said, it can be any NFC device, but it's not an
> Intel specific or ME specific NFC hardware, it's just a regular NFC one
> (pn544, microread, etc...) that is only accessible through the ME. The typical
> setup is where you access the NFC hardware through i2c, SPI or USB, but here
> it's "hidden" behind the ME.

Ok, but once you "unhide" it, it needs to talk through the in-kernel NFC
api, not some other random interface.

> The patch below sends the needed commands to the ME in order to fetch some
> information (vendor ID, radio ID) and understand which exact chipset
> is physically hooked to the ME (0x0, 0x0 -> microread, 0x1, 0x1 -> pn544).
> Once that's done, we can add the right device on the ME bus (This is done on
> patch #2), the same way you would add an NFC device to the USB bus when your
> USB host controller tells you about the vendor and product IDs.
> So after patch #1 and #2, we have an NFC device on the ME and this could be
> e.g. a pn544. We do have a driver for pn544 under drivers/nfc/ which will hook
> into the NFC kernel APIs once probed (The pn544 is missing an mei.c file that
> will register itself as an mei_cl driver for "pn544", as described in
> Documentation/misc-devices/mei/mei-client-bus.txt). So the NFC kernel APIs are
> called from the actual NFC driver, because mei/nfc.c is not an NFC driver,
> it's a NFC specific ME layer that is needed to register the right device (as
> in the kernel device/driver model) on the MEI bus.

So when will you send the patch for the pn544 driver?

> > >  include/uapi/linux/mei/nfc.h  | 135 +++++++++++++++++++++++
> > 
> > Based on our previous off-list emails, you said that this .h file is
> > needed for the userspace api.  But the nfc userspace api is through the
> > nfc API, which doesn't use any of the structures defined here.  So who
> > exactly uses this file?
> You're right, an NFC application that would use the kernel NFC APIs (AF_NFC
> and the NFC generic netlink one) will not need this header.
> But the Android stack does not use any of the NFC kernel APIs, it implements
> the whole NFC stack in userspace and typically sends raw HCI frames to a dumb
> kernel driver (not upstreamed) through /dev/pn544 for example.

That's horrible, use the in-kernel apis, don't create your own, and
Android should be using the correct apis as well.  If it isn't, we don't
have to support it here, in fact, I would strongly suggest that we not,
so as it gets properly changed.

In fact, why don't you change it?

> That works fine with the typical case where your pn544 is directly accessible
> through i2c. But if it's sitting behind the ME, you will need to send
> commands exported through this file to fetch the vendor and radio IDs, but
> also to send those HCI frames that the vanilla Android stack builds after
> encapsulating them into a struct mei_nfc_cmd. And this is all done through the
> /dev/mei interface.

No NFC data should be going through /dev/mei, use the proper kernel apis
please.

greg k-h
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