MMC overview

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SUMMARY
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[edit]

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[edit]

MMC overview.png

2.1 Component description[edit]

  • 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[1].
  • VFS (Kernel space)

Virtual File System. Please refer to the VFS documentation [2].

  • MMC core/SD/MMC/SDIO (Kernel space)

The MMC core ensures compliance with MultiMediaCard (MMC)[3] / secure digital (SD)[4] / secure digital input/output (SDIO)[5].

  • 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[edit]

The MMC core handles the file system read/write calls.

3 Configuration[edit]

3.1 Kernel configuration[edit]

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[edit]

DT configuration can be done thanks to STM32CubeMX.

Please refer to the SDMMC device tree configuration.

4 How to use the framework[edit]

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[edit]

5.1 How to monitor[edit]

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[edit]

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[edit]

The MMC framework is available here .

7 References[edit]

Please refer to the following links for a full description of the MMC framework:

  1. https://en.wikipedia.org/wiki/Device_file#Block_devices
  2. VFS
  3. MultiMediaCard, embedded MultiMediaCard specification
  4. Secure Digital, secure digital specification
  5. Secure Digital Input Output, Secure Digital Input Output specification



MultimediaCard

Secure digital

Virtual File System - NEW

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)

Device Tree

System File System (See https://en.wikipedia.org/wiki/Sysfs for more details)

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