linux-kernel - Re: [PATCH v2 1/3] usb: gadget: ccid: add support for USB CCID Gadget Device

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Message-ID: <20180528093254.GC4651@gmail.com>
Date:   Mon, 28 May 2018 11:32:54 +0200
From:   Marcus Folkesson <marcus.folkesson@...il.com>
To:     Andrzej Pietrasiewicz <andrzej.p@...sung.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jonathan Corbet <corbet@....net>,
        Felipe Balbi <balbi@...nel.org>, davem@...emloft.net,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Ruslan Bilovol <ruslan.bilovol@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        linux-usb@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] usb: gadget: ccid: add support for USB CCID
 Gadget Device

Hi Andrzej,

Thank you for reviewing.

On Mon, May 28, 2018 at 11:12:27AM +0200, Andrzej Pietrasiewicz wrote:
> W dniu 28.05.2018 o 10:38, Marcus Folkesson pisze:
> > Hi Andrzej,
> > 
> > On Mon, May 28, 2018 at 09:04:51AM +0200, Andrzej Pietrasiewicz wrote:
> >> Mi Marcus,
> >>
> >> W dniu 26.05.2018 o 23:19, Marcus Folkesson pisze:
> >>> Chip Card Interface Device (CCID) protocol is a USB protocol that
> >>> allows a smartcard device to be connected to a computer via a card
> >>> reader using a standard USB interface, without the need for each manufacturer
> >>> of smartcards to provide its own reader or protocol.
> >>>
> >>> This gadget driver makes Linux show up as a CCID device to the host and let a
> >>> userspace daemon act as the smartcard.
> >>>
> >>> This is useful when the Linux gadget itself should act as a cryptographic
> >>> device or forward APDUs to an embedded smartcard device.
> >>>
> >>> Signed-off-by: Marcus Folkesson <marcus.folkesson@...il.com>
> >>> ---
> >>
> >>>    
> >>> +config USB_CONFIGFS_CCID
> >>> +	bool "Chip Card Interface Device (CCID)"
> >>> +	depends on USB_CONFIGFS
> >>> +	select USB_F_CCID
> >>> +	help
> >>> +	  The CCID function driver provides generic emulation of a
> >>> +	  Chip Card Interface Device (CCID).
> >>> +
> >>> +	  You will need a user space server talking to /dev/ccidg*,
> >>> +	  since the kernel itself does not implement CCID/TPDU/APDU
> >>> +	  protocol.
> >>
> >> Your function needs a userspace daemon to work.
> >> It seems you want to use FunctionFS for such a purpose
> >> instead of creating a new function.
> >>
> >> Andrzej
> > 
> >>> +	  since the kernel itself does not implement CCID/TPDU/APDU
> > Oops, the driver does handle CCID.
> 
> Which parts of code do this handling?

My bad, I was thinking about the USB descriptors and endpoints setup.
That is of cause not part of the CCID protocol.

> 
> Is there any kind of state machine usual for protocols?
> If the protocol is stateless then isn't it just a data format then?

The protocol is stateless.

> 
> Which part of this handling must be done in kernel and why?
> 
> Does the said handling do anything other than forwarding the
> traffic between USB and a character device?

No, it forward the CCID messages to the character device to be handled
by the application.

> 
> What is the character device used for? I know: read, write and poll.
> But why? To do what?

It is used for the application to fetch, interpret and then perform actions depending on
commands.

> 
> > 
> > Well, yes, It needs an application that perform the "smartcard operations", such as
> > generate keys or sign data, as this depends on how it should be used.
> > 
> > The actual smartcard operations could for example be in software,
> > use a crypto engine in SoC or external HSM (Hardware Security Module).
> > 
> > Without the application, the gadget shows up as a smart card reader
> > with an unconnected smartcard.
> > 
> 
> Does showing up as anything require anything other than merely
> providing USB descriptors?

I guess.

> 
> Andrzej

Thank you,
Marcus