3.2.4.13. MMC/SD

3.2.4.13.1. Introduction

The multimedia card high-speed/SDIO (MMC/SDIO) host controller provides an interface between a local host (LH) such as a microprocessor unit (MPU) or digital signal processor (DSP) and either MMC, SD® memory cards, or SDIO cards and handles MMC/SDIO transactions with minimal LH intervention.

Main features of the MMC/SDIO host controllers:

  • Full compliance with MMC/SD command/response sets as defined in the Specification.

  • Support:

    • 4-bit transfer mode specifications for SD and SDIO cards

    • 8-bit transfer mode specifications for eMMC

    • Built-in 1024-byte buffer for read or write

    • 32-bit-wide access bus to maximize bus throughput

    • Single interrupt line for multiple interrupt source events

    • Two slave DMA channels (1 for TX, 1 for RX)

    • Designed for low power and programmable clock generation

    • Maximum operating frequency of 48MHz

    • MMC/SD card hot insertion and removal

The following image shows the MMC/SD Driver Architecture:

../../../../../_images/Mmcsd_Driver.png

3.2.4.13.2. References

  1. JEDEC eMMC homepage

  2. SD organization homepage

3.2.4.13.3. Acronyms & Definitions

Acronym

Definition

MMC

Multimedia Card

HS-MMC

High Speed MMC

SD

Secure Digital

SDHC

SD High Capacity

SDIO

SD Input/Output

3.2.4.13.4. Features

The SD driver supports the following features:

  • The driver is built in-kernel (part of vmlinux)

  • SD cards including SD High Speed and SDHC cards

  • Uses block bounce buffer to aggregate scattered blocks

3.2.4.13.5. SD: Supported High Speed Modes

3.2.4.13.6. Driver Configuration

The default kernel configuration enables support for MMC/SD(built-in to kernel).

The selection of MMC/SD/SDIO driver can be modified using the linux kernel configuration tool. Launch it by the following command:

$ make menuconfig  ARCH=arm

Building into Kernel

Ensure that the following config options are set to ‘y’:

  • CONFIG_MMC

  • CONFIG_MMC_BLOCK

  • CONFIG_MMC_SDHCI

  • CONFIG_MMC_SDHCI_OMAP (for DRA7XX and AM57XX devices)

  • CONFIG_MMC_OMAP (for AM335X and AM437X devices)

Building as Loadable Kernel Module

Depending on your configuration, any of the above options can be set to ‘m’ to build them as a module. Use the following command to install all modules tp your filesystem.

$ sudo -E make modules_install ARCH=arm INSTALL_MOD_PATH=path/to/filesystem

Boot the kernel upto kernel prompt and use modprobe to insert the driver module and all its dependencies.

$ modprobe sdhci-omap             # for DRA7XX and AM57XX devices
$ modprobe omap_hsmmc             # for AM335X and AM437X devices

If udev is running and the SD card is already inserted, the required modules will be loaded and any valid filesystem will be automatically mounted if they exist on the card.

Enabling eMMC Card Background operations support

eMMC cards need to occasionally spend some time cleaning up garbage and perform cache/buffer related operations. These are strictly on the card side and do not involve the host. They occur at one of the three levels based on the importance/severity of the operation:

  1. Normal

  2. Important

  3. Critical

If an operation is delayed for too long, it becomes critical, taking priority over the regular read/write from host. This can cause host operations to be delayed or take more time than expected. To avoid such issues the MMC HW and core driver provide a framework which can check for pending background operations and give the card some time to service them before they become critical. This feature is already part of the framework and to start using it the User needs to enable: EXT_CSD : BKOPS_EN [163] BIT 0.

This can be done using the “mmc-utils” tool from user space or using the “mmc” command in U-boot.

Command to enable bkops from userspace using mmc-utils, assuming eMMC instance to be mmcblk0

root@<machine>:mmc bkops enable /dev/mmcblk0

You can find the instance of eMMC by reading the ios timing spec form debugfs:

root@<machine>:~# cat /sys/kernel/debug/mmc0/ios
----
timing spec:    9 (mmc HS200)
---

or by looking for boot partitions, eMMC has two boot partitions mmcblk<x>boot0 and mmcblk<x>boot1

root@<machine>:/# ls /dev/mmcblk*boot*
/dev/mmcblk0boot0  /dev/mmcblk0boot1


3.2.4.13.7. Create software partitions in eMMC UDA

In eMMC, the User Data Area (UDA) HW partition is the primary storage space generally used to flash the rootfs. To prepare the UDA, use the fdisk command. For ex: fdisk /dev/mmcblkN in which N is 0 or 1. To find which integer is eMMC use the command lsblk, like so:

root@<machine>:~# lsblk
NAME         MAJ:MIN RM  SIZE RO TYPE MOUNTPOINTS
mmcblk0      179:0    0 14.8G  0 disk
mmcblk0boot0 179:32   0 31.5M  1 disk
mmcblk0boot1 179:64   0 31.5M  1 disk
mmcblk1      179:96   0 14.8G  0 disk
|-mmcblk1p1  179:97   0  128M  0 part /run/media/boot-mmcblk1p1
`-mmcblk1p2  179:98   0  1.9G  0 part /

Where the eMMC will have hardware partitions mmcblkNboot0 and mmcblkNboot1. The mmcblkN is the eMMC device.

Now we use fdisk /dev/mmcblk0 to create one software partition in UDA. For documentation on using fdisk, please go to: fdisk how-to.

3.2.4.13.8. Formatting eMMC partitions from Linux

After creating a partition/s, the partition can be formated with the mkfs command. For ex: mkfs -t ext4 /dev/mmcblkN where mmcblkN is the MMC device with the software partition to format. The general syntax for formatting disk partitions in Linux is:

mkfs [options] [-t type fs-options] device [size]

For example, to format a partition in eMMC UDA to ext4 file system:

root@<machine>:~# lsblk
NAME         MAJ:MIN RM  SIZE RO TYPE MOUNTPOINTS
mmcblk0      179:0    0 14.8G  0 disk
`-mmcblk0p1  179:1    0 14.8G  0 part /run/media/mmcblk0p1
mmcblk0boot0 179:32   0 31.5M  1 disk
mmcblk0boot1 179:64   0 31.5M  1 disk
mmcblk1      179:96   0 14.8G  0 disk
|-mmcblk1p1  179:97   0  128M  0 part /run/media/boot-mmcblk1p1
`-mmcblk1p2  179:98   0  8.8G  0 part /
root@<machine>:~# umount /run/media/mmcblk0p1
[   43.648532] EXT4-fs (mmcblk0p1): unmounting filesystem f8ecc7b8-ab1a-4240-ab4b-470d242c0539.
root@<machine>:~# mkfs -t ext4 /dev/mmcblk0p1
mke2fs 1.47.0 (5-Feb-2023)
Discarding device blocks: done
Creating filesystem with 3884800 4k blocks and 972944 inodes
Filesystem UUID: 842929dd-4e57-47b6-afa1-c03abc3100b1
Superblock backups stored on blocks:
   32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208

Allocating group tables: done
Writing inode tables: done
Creating journal (16384 blocks): done
Writing superblocks and filesystem accounting information: done