3.3.4.25. MMC/SD¶

3.3.4.25.1. 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

MMC/SD Driver Architecture

3.3.4.25.2. References¶

1. JEDEC eMMC Homepage [https://www.jedec.org/category/technology-focus-area/flash-memory-ssds-ufs-emmc]
2. SD ORG Homepage [https://www.sdcard.org]

3.3.4.25.3. Acronyms & Definitions¶

Table: HSMMC Driver: Acronyms

3.3.4.25.4. 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

Features NOT supported

• Polling I/O mode

3.3.4.25.5. Supported High Speed Modes¶

Table: MMC1/SD

^*(1)* - Does not have power cycle support. So if a card fails to enumerate in UHS mode, it doesn’t fall back to high speed mode.

Important Info: Certain UHS cards doesn’t enumerate in UHS cards. Find the list of functional UHS cards here: https://processors.wiki.ti.com/index.php/Linux_Core_MMC/SD_User%27s_Guide#Testing_Information

Known Workaround: For cards which doesn’t enumerate in UHS mode, removing the PULLUP resistor in CLK line and changing the GPIO to PULLDOWN increases the frequency in which the card enumerates in UHS modes.

Table: MMC2/EMMC

3.3.4.25.6. Driver Configuration¶

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

The selection of MMC/SD/SDIO driver can be modified as follows: start Linux Kernel Configuration tool.

$ make menuconfig  ARCH=arm

• Select Device Drivers from the main menu.

...
...
Kernel Features  --->
Boot options  --->
CPU Power Management  --->
Floating point emulation  --->
Userspace binary formats  --->
Power management options  --->
[*] Networking support  --->
Device Drivers  --->
...
...

Building into Kernel

• Select MMC/SD/SDIO card support from the menu.

...
...
[*] USB support  --->
< > Ultra Wideband devices (EXPERIMENTAL)  --->
<*> MMC/SD/SDIO card support  --->
< > Sony MemoryStick card support (EXPERIMENTAL)  --->
...
...

• Select OMAP HSMMC driver

...
[ ] MMC debugging
[ ] Assume MMC/SD cards are non-removable (DANGEROUS)
   *** MMC/SD/SDIO Card Drivers ***
<*> MMC block device driver
[*]  Use bounce buffer for simple hosts
...
<*>   TI OMAP High Speed Multimedia Card Interface support
...

Building as Loadable Kernel Module

• To build the above components as modules, press ‘M’ key after navigating to config entries preceded with ‘< >’ as shown below:

...
...
[*] USB support  --->
< > Ultra Wideband devices (EXPERIMENTAL)  --->
<M> MMC/SD/SDIO card support  --->
< > Sony MemoryStick card support (EXPERIMENTAL)  --->
...

• Select OMAP HSMMC driver to be built as module

...
[ ] MMC debugging
[ ] Assume MMC/SD cards are non-removable (DANGEROUS)
   *** MMC/SD/SDIO Card Drivers ***
<*> MMC block device driver
[*]  Use bounce buffer for simple hosts
...
<*>   TI OMAP High Speed Multimedia Card Interface support
...

• After doing module selection, exit and save the kernel configuration when prompted.
• Now build the kernel and modules form Linux build host as

$ make uImage
$ make modules

• Following modules will be built

mmc_core.ko
mmc_block.ko
omap_hsmmc.ko

• Boot the newly built kernel and transfer the above mentioned .ko files to the filesystem
• Navigate to the directory containing these modules and insert them form type the following commands in console to insert the modules in specified order:

# insmod mmc_core.ko
# insmod mmc_block.ko
# insmod omap_hsmmc.ko

• If ‘udev’ is running and the SD card is already inserted, the devices nodes will be created and filesystem will be automatically mounted if exists on the card.

Suspend to Memory support

This driver supports suspend to memory functionality. To use the same, the following configuration is enabled by default.

• Select Device Drivers from the main menu.

...
...
Kernel Features  --->
Boot options  --->
CPU Power Management  --->
Floating point emulation  --->
Userspace binary formats  --->
Power management options  --->
[*] Networking support  --->
Device Drivers  --->
...
...

• Select MMC/SD/SDIO card support from the menu.

...
...
[*] USB support  --->
< > Ultra Wideband devices (EXPERIMENTAL)  --->
<*> MMC/SD/SDIO card support  --->
< > Sony MemoryStick card support (EXPERIMENTAL)  --->
...
...

• Select Assume MMC/SD cards are non-removable option.

...
[ ] MMC debugging
[*] Assume MMC/SD cards are non-removable (DANGEROUS)
*** MMC/SD/SDIO Card Drivers ***
<*> MMC block device driver
[*]  Use bounce buffer for simple hosts
...
<*>   TI OMAP High Speed Multimedia Card Interface support
...

Enabling eMMC Card Background operations support

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@dra7xx-evm:mmc bkops enable /dev/mmcblk0

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

root@dra7xx-evm:~# cat /sys/kernel/debug/mmc0/ios
----
timing spec:    9 (mmc HS200)
---

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

root@dra7xx-evm:/# ls /dev/mmcblk*boot*
/dev/mmcblk0boot0  /dev/mmcblk0boot1