Thanks to plug-and-play devices like PC cards, you can backup gigabytes of data onto a stamp-sized device or enable your laptop with diverse networking technologies.
Last month’s column looked at the Personal Computer Memory Card International Association (PCMCIA) core support present in the Linux kernel and discussed how devices having PCMCIA or Compact Flash (CF) interfaces could use services provided by the kernel to instantly enable themselves with diverse technologies.
PCMCIA is a 16-bit data transfer interface specification that has evolved to include support for higher speeds in the form of 32-bit CardBus cards. Since CardBus is closer to the Peripheral Component Interconnect (PCI) bus, recent versions of Linux have moved hot-plug support for CardBus devices from the PCMCIA layer to the PCI layer. The term PC Card is used to refer to either PCMCIA or CardBus cards.
This month, let’s continue and look at some popular PCMCIA/CF cards and see how they interact with the PCMCIA core and the rest of the kernel. Let’s also implement a simple driver for a multifunction CardBus card.
PCMCIA Networking over Serial Transports
Many networking technologies use a serial transport mechanism to communicate with host devices. serial_cs, the generic serial card services driver allows the rest of the operating system to see the PCMCIA/CF card as a serial device. The first unused serial device, /dev/ttySX, gets allotted to the card. serial_cs thus emulates a serial port over cards that feature technologies like General Packet Radio Service (GPRS), Global Systems for Mobile communications (GSM), or Global Positioning System (GPS). It also allows Bluetooth PCMCIA/CF cards that use a serial transport to transfer Host Control Interface (HCI)…
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