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

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Date:   Mon, 28 May 2018 11:12:27 +0200
From:   Andrzej Pietrasiewicz <andrzej.p@...sung.com>
To:     Marcus Folkesson <marcus.folkesson@...il.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

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?

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

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?

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

> 
> 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?

Andrzej