Command Register Settings Description Notes

Transcription

1 Technical Note e.mmc Automotive 5.0 Cache Features TN-FC-50: e.mmc Automotive 5.0 Cache Features Introduction Introduction This technical note introduces an optional cache feature defined in the e.mmc JEDEC specification (JEDEC Standard No. 84-B50) and describes this feature availability in the Micron automotive e.mmc 5.0 device. Cache Definition and Access Table : Cache Enable Commands Cache is temporary volatile storage space, typically used to reduce write access time contrasted with the time required to access nonvolatile storage space. Commands for enabling and disabling cache are shown in the table below. Command Register Settings Description Notes ON OFF FLUSH CACHE_CTRL = On (byte [33] of EXT_CSD register) CACHE_CTRL = Off (byte [33] of EXT_CSD register) FLUSH_CACHE = On (byte [3] of EXT_CSD register) Turns on cache. Turns off cache, triggering flush of data to nonvolatile storage area. Enables host to flush cache to nonvolatile storage. Note:. At power-up, e.mmc cache is disabled by default; high-end systems that perform fast WRITE operations typically include an ON command to enable cache. Products and specifications discussed herein are subject to change by Micron without notice.

2 Cache Commands Automotive e.mmc 5.0 The e.mmc device contains a controller-embedded SRAM device, part of which can be used to enable cache to store WRITE command data, as shown in the table below. Table : Cache Functionality vs. Host Commands Command Name Code Status Description Notes SINGLE BLOCK WRITE CMD4 Data0 = Busy MULTIPLE BLOCK WRITE RELIABLE WRITE When cache is enabled, the data path from the e.mmc interface, through the SRAM device, to the nonvolatile memory depends on chunk size, write mode (open-ended or close-ended), and access type (random or sequential), all factors which affect performance. Closeended Openended FORCE-PROGRAMMING WRITE REPLAY PROTECTED MEMORY BLOCK (RPMB) WRITE CMD3 + CMD5 CMD5 + CMD CMD3 bit[3] set + CMD4/5 CMD3 bit[4] set + CMD4/5 Data0 = Busy Data0 = Busy Applies data caching for a single block. Applies data caching for multiple, predefined blocks. Applies data caching for multiple blocks that are not predefined. Programs data directly to nonvolatile storage (bypassing cache) while cache is turned on. Alternative to RELIABLE WRITE command. Programs data directly to nonvolatile storage (bypassing cache) while cache is turned on. BOOT PARTITION WRITE ERASE, TRIM, DISCARD CMD38 SANITIZE CMD6 Cleans up the unmapped data in the cache. SLEEP, POWER-OFF NOTIFICATION CMD5, CMD6 TN-FC-50: e.mmc Automotive 5.0 Cache Features Cache Commands Automotive e.mmc 5.0 Puts to sleep mode or notifies the power off. HW RESET Moves the device in pre-idle state. 4 SW RESET CMD0 Moves the device in idle state. 4, 3. Status for these commands might apply to nonvolatile memory programming or to volatile cache programming.. These commands imply a cache flush before their execution. 3. Systems typically use the RELIABLE WRITE command to protect sensitive data bypassing cache. 4. HW and SW RESET invalidates data in the cache area.

3 Performance Performance Enabling the cache can improve e.mmc bus level performance. For example, Micron measured that enabling the cache for a 64GB e.mmc 5.0 device brought an average sequential write performance improvement of 40% for the various chunks. Random write performance improved by approximately 300% with a 4KB chunk. Power Loss Management Linux Enable and Flush Cache When performing a WRITE operation while cache is enabled, the host must compensate for data that could be lost during a power failure. To facilitate power loss management, the CACHE_SIZE (byte [5:49] of EXT_CSD) provides information about the maximum amount of data potentially at risk. CACHE_SIZE represents data both in the SRAM and nonvolatile NAND storage devices that is potentially unsafe if a power loss occurs during an internal flush. To protect against power loss, the host can flush data to the nonvolatile storage by writing to the FLUSH_CACHE byte and turning the cache off. Also, the RELIABLE WRITE operation and WRITE operations with a forced programming bit set protect against power loss, making both old and new data inside the e.mmc device available for the host at the next power-on. Table 3: Linux Support for e.mmc Cache Action Description Notes Cache is enabled During initialization, when the e.mmc driver sets the cache to ON., Cache is flushed When either the REQ_FLUSH register or the REQ_FUA flag is set per file system request, which typically happens when file system metadata are written to the media. Every time emmc_suspend is called, which occurs during e.mmc system shutdown or during e.mmc system suspend (SUSPEND or RUNTIME SUSPEND).. The related Linux function is mmc_init_card () in Linux/drivers/mmc/core/mmc.c.. In the latest Linux kernel 4.4, e.mmc cache is enabled automatically; in older kernel versions, such as kernel 3.4, e.mmc cache might be eventually disabled by setting MMC_CAP_CACHE_CTRL = 0. Mitigate Power-Loss Effects To ensure a robust system and mitigate against power loss, systems with Linux OS and EXT4 file system can adopt e.mmc with cache ON and enable the JBD journal with the proper mount options. Specific implementation depends on the user and possible options are shown in the table below. 3

4 Linux Table 4: Implementation Options to Mitigate Power Loss Option Option Settings Description Notes Data = Ordered Barrier = Data = Journaled Barrier = Implies that the journal tracks changes uncommited to the main part of the file system by recording metadata, such as file creation, file append. Conversely to this, data are forced directly to the main file system prior to its metadata being committed to the journal. Implies that the journal commit actually updates the file system. This commit is safely performed by forcing a cache flush and by using the RELIABLE WRITE operation on the e.mmc, which protect against power loss. Linux driver implement these operations when the file system raises the REQ_FUA flag. Data are committed into the journal before being written to the main file system. This protect both file system and data but performances decrease. Implies that the journal commit actually updates the file system. This commit is safely performed by forcing a cache flush and by using the RELIABLE WRITE operation on the e.mmc, which protect against power loss. Linux driver implement these operations when the file system raises the REQ_FUA flag.,. This combination protects the file system itself against power loss.. By default, recent Linux distributions bounded to Android OS come with the option Barrier = 0 (as known as no barrier). This speeds the journal commit by using a standard WRITE operation. It is applicable in systems using complementary battery or robust power-loss detect circuitry as a power-loss prevention/backup. 4

5 Revision History Revision History Rev. B 5/7 Rev. A 5/6 Updated Cache Functionality vs. Host Commands table: Added HW and SW RESET commands Initial release 8000 S. Federal Way, P.O. Box 6, Boise, ID , Tel: Sales inquiries: Micron and the Micron logo are trademarks of Micron Technology, Inc. All other trademarks are the property of their respective owners. This data sheet contains minimum and maximum limits specified over the power supply and temperature range set forth herein. Although considered final, these specifications are subject to change, as further product development and data characterization sometimes occur. 5