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The MMC Layer will become available when the Kernel 2.6.24 merge window opens. In his posting on the Linux Kernel mailing list (lkml) developer Pierre Ossman writes that the subsystem has been reworked and that two major milestones, and many minor changes, have been achieved. The new subsystem is the first to provide native support for the SDIO and SPI standards – added incentive for deployment on mobile and embedded devices.

SDIO (Secure Digital Input Output) is based on the SD memory card standard and gives users the ability to connect external devices such as mass memory, NICs, or GPS receivers via a SDIO slot. The new SDIO stack provided by the MMC Layer currently supports three drivers. Standard GPS devices can be accessed using the "sdio_uart" driver. In theory more UART devices (Universal Asynchronous Receiver Transmitter) should be accessible in future. The "libertas_sdio" driver supports WLAN cards with Marvell's 8686 Libertas WiFi chip. However, it will not be introduced until shortly before the merge window closes due to unresolved dependencies between the SDIO maintainer's tree and the original "libertas" tree. The third driver, "hci_sdio", is for Bluetooth devices and will integrate with the kernel's Bluetooth subsystem.

Maintainer Ossman warns that the new stack design may make porting of more drivers a complicated process: on a brighter note, this may lead to more stable drivers that remove the need for hacks and workarounds.

The second major feature is SPI (Serial Peripheral Interface) support. This will allow users to address and use SPI controllers for MMC, SD and SDIO, as long as they keep to the standard. Pierre Ossman admits that many controllers do not do so. Developer David Brownell is working on the SPI branch, but not on a permanent basis. This is why Ossman is looking for a new maintainer with sufficient experience of the Linux SPI interface, and time to spare.

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