TS-4100 eMMC
| Note: | This section is incomplete at this time. |
The i.MX6UL SD card controller supports the MMC specification, the TS-4100 includes a soldered down eMMC IC to provide on-board flash media.
Our default software image contains 3 partitions:
| Device | Contents |
|---|---|
| /dev/mmcblk1 | eMMC block device |
| /dev/mmcblk1boot0 | eMMC boot partition |
| /dev/mmcblk1boot1 | eMMC boot partition |
| /dev/mmcblk1p1 | Full Debian linux partition |
This platform includes an eMMC device, a soldered down MMC flash device. Our off the shelf builds are 4 GB, but up to 64 GB are available for customized builds. The eMMC flash appears to Linux as an SD card at /dev/mmcblk1. Our default programming of the eMMC is the same as the SD card image for standard partitions, but includes additional boot partitions that are used by U-Boot and are not affected by the eMMC partition table.
The eMMC module has a similar concern by default to SD cards in that they should not be powered down during a write/erase cycle. However, this eMMC module includes support for setting a fuse for a "Write Reliability" mode, and a "psuedo SLC (pSLC)" mode. With both of these enabled all writes will be atomic to 512 B and each NAND cell will be treated as a single layer rather than a multi-layer cell. If a sector is being written during a power loss, a block is guaranteed to have either the old or new data. Even in cases where the wrong data is present on the next boot, fsck is often able to deal with the older data being present in a 512 B block. The downsides to setting these modes are that it will reduce the overall write speed and halve the available space on the eMMC to roughly 1.759 GB. Please note that even with these settings, Technologic Systems strongly recommends designing the end application to eliminate any situations where a power-loss event can occur while any disk is mounted as read/write. The TS-SILO option for the TS-4100 can help to eliminate the dangerous situation.
The "mmc-utils" package is used to enable these modes. The command is pre-installed on the latest image. Additionally we have created a script to safely enable the write reliability and pSLC modes. Since the U-Boot binary and environment reside on the eMMC, care must be taken to save the current state of the boot partitions, enable the modes, restore the boot partitions, and re-enable proper booting options. This script can be used in combination with the production mechanism scripting to complete these steps as part of an end application production process.
| WARNING: | Enabling these modes causes all data on the disk to become invalid and must be rewritten. Do not attempt to run the 'mmc' commands from the script individually, all steps in the script must occur as they are or the unit may be unable to boot. If there are any failures of the script, care must be taken to resolve any issues while the unit is still booted or it may fail to boot in the future. |
| WARNING: | The script is only compatible with Rev. B or newer PCBs. Running the script on any previous PCB revision WILL result in the unit being unable to boot! There is no safe way to enable these modes on previous PCB revisions. |
| Note: | Enabling these modes is a one-way operation, it is not possible to undo them once they are made. Because of this, setting these eMMC modes will invalidate Technologic Systems' return/replacement warranty on the unit. See the warranty section for more information on this. |
The 'emmc_reliability' script can be found in the TS-4100 utilities github repository.
The script must be run when boot from any media other than eMMC, such as SD, NFS, or USB. No partition of the eMMC disk can be mounted when these commands are run. Doing so may result in corruption or inability for the unit to boot. Once the pSLC mode is enabled, all data on the disk will become invalid. This means the partition table will need to be re-created, the filesystems formatted, and all filesystem contents re-written to disk. This is why we recommend using this script in conjunction with the production mechanism scripting. The 'emmc_reliability' script can be run first, then the rest of the production script can create and format the partitions as well as write data to disk.
The script requires a single argument, the device node of the eMMC disk, and will output verbosely to stderr. Any specific errors will also be printed out on stderr.
Example usage:
./emmc_reliability /dev/mmcblk1
Upon successful run, the script will return 0. Any errors will return a positive code. See the script for detailed error code information.