[RFC] NuttX use littlefs for F-RAM parameter storage

[dagar]

• currently for parameter storage on F-RAM (aka ramtron) we export parameters as bson saved to a character device ramtron partition
• the file is overwritten in place and there's no checksum
• littlefs is a fail-safe filesystem designed to handle random power failures
  • all file operations have strong copy-on-write guarantees and if power is lost the filesystem will fall back to the last known good state

nsh> df -h
  Filesystem    Size      Used  Available Mounted on
  binfs           0B        0B         0B /bin
  cromfs        280K      280K         0B /etc
  vfat         7388M      800K      7387M /fs/microsd
  littlefs       32K        2K        30K /fs/mtd_params
  procfs          0B        0B         0B /proc


nsh> param dump
[param] reading from /fs/mtd_params/parameters.bson

BSON document size 814
BSON_INT32:  CAL_ACC0_ID = 2424858
BSON_INT32:  CAL_ACC0_PRIO = 50
BSON_DOUBLE: CAL_ACC0_TEMP = 45.680267
BSON_DOUBLE: CAL_ACC0_XOFF = -0.025817
BSON_DOUBLE: CAL_ACC0_YOFF = 0.005446
BSON_DOUBLE: CAL_ACC0_ZOFF = -0.433016
BSON_INT32:  CAL_ACC1_ID = 2490386
BSON_INT32:  CAL_ACC1_PRIO = 50
BSON_DOUBLE: CAL_ACC1_TEMP = 41.005436
BSON_DOUBLE: CAL_ACC1_XOFF = -0.203097
BSON_DOUBLE: CAL_ACC1_YOFF = 0.076597
BSON_DOUBLE: CAL_ACC1_ZOFF = -0.179166
BSON_INT32:  CAL_ACC2_ID = 3670026
BSON_INT32:  CAL_ACC2_PRIO = 50
BSON_DOUBLE: CAL_ACC2_TEMP = 45.096695
BSON_DOUBLE: CAL_ACC2_XOFF = -0.003840
BSON_DOUBLE: CAL_ACC2_YOFF = 0.106350
BSON_DOUBLE: CAL_ACC2_ZOFF = -0.199815
BSON_INT32:  CAL_GYRO0_ID = 2424858
BSON_INT32:  CAL_GYRO0_PRIO = 50
BSON_DOUBLE: CAL_GYRO0_TEMP = 45.850945
BSON_DOUBLE: CAL_GYRO0_XOFF = -0.010202
BSON_DOUBLE: CAL_GYRO0_YOFF = 0.014070
BSON_DOUBLE: CAL_GYRO0_ZOFF = -0.011514
BSON_INT32:  CAL_GYRO1_ID = 2490386
BSON_INT32:  CAL_GYRO1_PRIO = 50
BSON_DOUBLE: CAL_GYRO1_TEMP = 40.992916
BSON_DOUBLE: CAL_GYRO1_XOFF = -0.002029
BSON_DOUBLE: CAL_GYRO1_YOFF = 0.013790
BSON_DOUBLE: CAL_GYRO1_ZOFF = -0.000258
BSON_INT32:  CAL_GYRO2_ID = 3670026
BSON_INT32:  CAL_GYRO2_PRIO = 50
BSON_DOUBLE: CAL_GYRO2_TEMP = 45.104542
BSON_DOUBLE: CAL_GYRO2_XOFF = -0.010561
BSON_DOUBLE: CAL_GYRO2_YOFF = -0.018770
BSON_DOUBLE: CAL_GYRO2_ZOFF = -0.002687
BSON_DOUBLE: EKF2_MAG_DECL = -6.149145
BSON_EOO
BSON decoded 814 bytes (double:25, string:0, bin:0, bool:0, int32:12, int64:0)


nsh> ls -ls /fs/mtd_params
/fs/mtd_params:
 drwxrwxrwx       0 .
 drwxrwxrwx       0 ..
 -rwxrwxrwx     814 parameters.bson

nsh> cp /fs/mtd_params/parameters.bson /fs/mtd_params/parameters_backup.bson

nsh> ls -ls /fs/mtd_params
/fs/mtd_params:
 drwxrwxrwx134378631 .
 drwxrwxrwx       0 ..
 -rwxrwxrwx     814 parameters.bson
 -rwxrwxrwx     814 parameters_backup.bson

nsh> df -h
  Filesystem    Size      Used  Available Mounted on
  binfs           0B        0B         0B /bin
  cromfs        281K      281K         0B /etc
  vfat         7388M      800K      7387M /fs/microsd
  littlefs       32K        3K        29K /fs/mtd_params
  procfs          0B        0B         0B /proc

TODO:

• how would we handle migration?
  • first try to import any valid data from ramtron as character device, then stop bch and instead mount as littlefs
  • explicitly format littlefs only after successful import
  • automatically try to mount littlefs (with no autoformat) first going forward
• update parameter system to export to new file each time
  • auto delete previous exports as needed to free up space

[dagar]

Once this is in place we can also start using this safe storage area for things like the UAVCAN database rather than have an SD card dependency.

[dagar]

Continuing in #21134.