The MMC (MultiMediaCard) / SD (secure digital) / SDIO (secure digital input/output) subsystem implements a standard Linux® host driver to interface with MMC / SD memory cards or SDIO cards.
1 Framework purpose
The purpose of this article is to introduce the MMC Linux® subsystem (MMC / SD) by:
- providing general information
- describing the main components/stakeholders
The SDIO is addressed in the WLAN overview.
2 System overview
2.1 Component description
- User space applications handle file I/O management to view the card memory as a disk, whereas programs that perform raw I/O accesses see the memory as a block device.
- VFS (Kernel space)
Virtual File System. Please refer to the VFS documentation .
- MMC core/SD/MMC/SDIO (Kernel space)
- SDMMC driver (Kernel space) / SDMMC (hardware)
The SDMMC driver handles:
- the registers, the clock, the interrupt and the IDMA control.
- the communications over the bus based on command/response and data transfers.
Please refer to the SDMMC internal peripheral.
2.2 API description
The MMC core handles the file system read/write calls.
3.1 Kernel configuration
The MMC framework is activated by default in ST deliveries. If a specific configuration is needed, this section indicates how the MMC framework can be activated/inactivated in the kernel.
The MMC framework can be activated in the kernel configuration via Linux® Menuconfig tool: Menuconfig or how to configure kernel
[*] Device Drivers [*] MMC/SD/SDIO card support <*> HW reset support for eMMC <*> Simple HW reset support for MMC <*> MMC block device driver (16) Number of minors per block device ... <*> ARM AMBA Multimedia Card Interface support [*] STMicroelectronics STM32 SDMMC Controller
3.2 Device tree configuration
DT configuration can be done thanks to STM32CubeMX.
Please refer to the SDMMC device tree configuration.
4 How to use the framework
A file system, which handles read/write/erase operations, can be used with the MMC framework. Please refer to the EXT4 support through MMC.
5 How to trace and debug the framework
5.1 How to monitor
The sysfs interface provides detailed information on each mmc device:
root:~# cat /sys/kernel/debug/mmc0/ios clock: 50000000 Hz vdd: 21 (3.3 ~ 3.4 V) bus mode: 2 (push-pull) chip select: 0 (don't care) power mode: 2 (on) bus width: 2 (4 bits) timing spec: 2 (sd high-speed) signal voltage: 0 (3.30 V) driver type: 0 (driver type B)
5.2 How to trace
For details on dynamic trace usage, refer to How to use the kernel dynamic debug.
root:~# echo "file drivers/mmc/* +p" > /sys/kernel/debug/dynamic_debug/control
6 Source code location
The MMC framework is available here .
Please refer to the following links for a full description of the MMC framework:
- MultiMediaCard, embedded MultiMediaCard specification
- Secure Digital, secure digital specification
- Secure Digital Input Output, Secure Digital Input Output specification
Secure digital input/output
Application programming interface
SDIO is an SD-size card with extended input/output functions
former spelling for e•MMC ('e' in italic)