linux-kernel - Re: [PATCH v3 2/3] Documentation: usb: add documentation for USB CCID Gadget Device

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Date:   Tue, 29 May 2018 13:27:23 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Marcus Folkesson <marcus.folkesson@...il.com>,
        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>,
        Ruslan Bilovol <ruslan.bilovol@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kate Stewart <kstewart@...uxfoundation.org>
Cc:     linux-usb@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/3] Documentation: usb: add documentation for USB CCID
 Gadget Device

On 05/29/2018 11:50 AM, Marcus Folkesson wrote:
> Add documentation to give a brief description on how to use the
> CCID Gadget Device.
> This includes a description for all attributes followed by an example on
> how to setup the device with ConfigFS.
> 
> Signed-off-by: Marcus Folkesson <marcus.folkesson@...il.com>
> ---
> 
> Notes:
>     v3:
>     	- correct the grammer (thanks Randy)
>     v2:
>     	- add the missing changelog text
> 
>  Documentation/usb/gadget_ccid.rst | 267 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 267 insertions(+)
>  create mode 100644 Documentation/usb/gadget_ccid.rst
> 
> diff --git a/Documentation/usb/gadget_ccid.rst b/Documentation/usb/gadget_ccid.rst
> new file mode 100644
> index 000000000000..524fe9e6ac19
> --- /dev/null
> +++ b/Documentation/usb/gadget_ccid.rst
> @@ -0,0 +1,267 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +============
> +CCID Gadget
> +============
> +
> +:Author: Marcus Folkesson <marcus.folkesson@...il.com>
> +
> +Introduction
> +============
> +
> +The CCID Gadget will present itself as a CCID device to the host system.
> +The device supports two endpoints for now; BULK IN and BULK OUT.
> +These endpoints are exposed to userspace via /dev/ccidg*.
> +
> +All CCID commands are sent on the BULK-OUT endpoint. Each command sent to the CCID
> +has an associated ending response. Some commands can also have intermediate
> +responses. The response is sent on the BULK-IN endpoint.
> +See Figure 3-3 in the CCID Specification [1]_ for more details.
> +
> +The CCID commands must be handled in userspace since the driver is only working
> +as a transport layer for the TPDUs.

I think that it would be helpful to tell us what the naming of the /dev/ccidg*
endpoints looks like.  Also, how to distinguish the BULK-IN from the BULK-OUT
endpoint.

> +
> +
> +CCID Commands
> +--------------
> +
> +All CCID commands begins with a 10-byte header followed by an optional
> +data field depending on message type.
> +
> ++--------+--------------+-------+----------------------------------+
> +| Offset | Field        | Size  | Description                      |
> ++========+==============+=======+==================================+
> +| 0      | bMessageType | 1     | Type of message                  |
> ++--------+--------------+-------+----------------------------------+
> +| 1      | dwLength     | 4     | Message specific data length     |
> +|        |              |       |                                  |
> ++--------+--------------+-------+----------------------------------+
> +| 5      | bSlot        | 1     | Identifies the slot number       |
> +|        |              |       | for this command                 |
> ++--------+--------------+-------+----------------------------------+
> +| 6      | bSeq         | 1     | Sequence number for command      |
> ++--------+--------------+-------+----------------------------------+
> +| 7      | ...          | 3     | Fields depends on message type   |
> ++--------+--------------+-------+----------------------------------+
> +| 10     | abData       | array | Message specific data (OPTIONAL) |
> ++--------+--------------+-------+----------------------------------+
> +
> +
> +Multiple CCID gadgets
> +----------------------
> +
> +It is possible to create multiple instances of the CCID gadget, however,
> +a much more flexible way is to create one gadget and set the `nslots` attribute
> +to the number of desired CCID devices.
> +
> +All CCID commands specify which slot is the receiver in the `bSlot` field
> +of the CCID header.
> +
> +Usage
> +=====
> +
> +Access from userspace
> +----------------------
> +All communication is by read(2) and write(2) to the corresponding /dev/ccidg* device.
> +Only one file descriptor is allowed to be open to the device at a time.


Reviewed-by: Randy Dunlap <rdunlap@...radead.org>


thanks,
-- 
~Randy