Ludovic Rousseau's blog: OS X El Capitan missing feature: add support of TAG_IFD_POLLING_THREAD_WITH

Friday, December 25, 2015

OS X El Capitan missing feature: add support of TAG_IFD_POLLING_THREAD_WITH_TIMEOUT

This is part of the series: "OS X El Capitan and smart cards: known bugs".

This is not a bug since this feature was never present in any PC/SC from Apple.

com.apple.ifdreader.slotd does not support TAG_IFD_POLLING_THREAD_WITH_TIMEOUT and friends

The equivalent of pcscd on Mac OS X since Yosemite (10.10) is com.apple.ifdreader.slotd. See "OS X Yosemite and smart cards status". This process is in charge of loading the smart card reader driver. The API between pcscd or com.apple.ifdreader.slotd and the driver is defined in IFDHandler API.

The IFDHandler API evolved during the development of pcsc-lite and the CCID driver to support new features. Apple forked pcsc-lite in 2002 (see "Evolution of Apple pcsc-lite (from Jaguar to Mavericks)") and never merged back the changes I made in pcsc-lite and the IFDHandler API since that time.

Card status polling

In the early versions of pcsc-lite (when the smart card readers where using a RS 232 serial port because USB was not invented yet) the card state (card present or card absent) was checked using a regular polling (every 200 ms) of the reader. Every 200 ms a command is sent to the reader to know if a card is present or not.

The problem is that pcscd is then continuously running (and pcscd was reported by the powertop tool as one of the processes that prevent the CPU to sleep and consume less power)

Another problem is that you have to wait up to 200 ms to detect that a card has been inserted or removed in a reader.

Polling replaced

The CCID specification provides a way to indicate that a card event occurred. This mechanism uses an USB interrupt end point. See "New CCID 1.4.0 and card movement notification mechanism" for more details.

The new mechanism, introduced in the pcsc-lite and the CCID driver 2010, allows to let libusb-1.0 notify the driver that a card event occurred. The change was introduced in the CCID driver version 1.4.0 (with the update from libusb-0.1 to libusb-1.0) and in pcsc-lite 1.6.2 released at the same time.

The mechanism uses the IFDHandler function IFDHGetCapabilities to request the driver about three features:

This allows pcscd to delegate to the driver the cart event management. pcscd will call a function in the driver that will block until a card event occurs or the timeout expires.

Benefits to implement it

If such a mechanism is implemented by Apple in com.apple.ifdreader.slotd we could have, at least, three benefits.

better reactivity when a smart card is inserted (or removed)
lower CPU power consumption
far less log messages when you debug a smart card driver (mostly useful to smart card reader driver developers ☺)