Not able to boot into fastboot mode for db410c

Hi, All

with pressing Vol- + powering up, I could not boot my db410c board to fastboot mode.
On the serial it shows:

Format: Log Type - Time(microsec) - Message - Optional Info
Log Type: B - Since Boot(Power On Reset), D - Delta, S - Statistic
S - QC_IMAGE_VERSION_STRING=BOOT.BF.3.0-00261
S - IMAGE_VARIANT_STRING=HAAAANAAA
S - OEM_IMAGE_VERSION_STRING=C-BPATTH
S - Boot Config, 0x000002e1
S - Core 0 Frequency, 0 MHz
B - 1545 - PBL, Start
B - 3490 - bootable_media_detect_entry, Start
B - 139786 - bootable_media_detect_success, Start
B - 139790 - elf_loader_entry, Start
B - 143580 - auth_hash_seg_entry, Start
B - 143791 - auth_hash_seg_exit, Start
B - 158602 - elf_segs_hash_verify_entry, Start
B - 217830 - PBL, End
B - 180163 - SBL1, Start
B - 230702 - pm_device_init, Start
D - 14243 - pm_device_init, Delta
B - 245433 - boot_flash_init, Start
D - 0 - boot_flash_init, Delta
B - 249459 - boot_config_data_table_init, Start
D - 33611 - boot_config_data_table_init, Delta - (0 Bytes)
B - 287645 - CDT version:3,Platform ID:24,Major ID:1,Minor ID:0,Subtype:0
B - 293837 - sbl1_ddr_set_params, Start
B - 297558 - cpr_init, Start
D - 0 - cpr_init, Delta
B - 303170 - Pre_DDR_clock_init, Start
D - 183 - Pre_DDR_clock_init, Delta
D - 0 - sbl1_ddr_set_params, Delta
B - 315675 - pm_driver_init, Start
D - 6832 - pm_driver_init, Delta
B - 331169 - clock_init, Start
D - 61 - clock_init, Delta
B - 341295 - Image Load, Start
D - 24552 - QSEE Image Loaded, Delta - (460664 Bytes)
B - 365878 - Image Load, Start
D - 427 - SEC Image Loaded, Delta - (2048 Bytes)
B - 373045 - sbl1_efs_handle_cookies, Start
D - 2958 - sbl1_efs_handle_cookies, Delta
B - 380823 - Image Load, Start
D - 11498 - QHEE Image Loaded, Delta - (51952 Bytes)
B - 392352 - Image Load, Start
D - 12962 - RPM Image Loaded, Delta - (149492 Bytes)
B - 405345 - Image Load, Start
D - 13847 - APPSBL Image Loaded, Delta - (503244 Bytes)
B - 419222 - QSEE Execution, Start
D - 61 - QSEE Execution, Delta
B - 424926 - SBL1, End
D - 247111 - SBL1, Delta
S - Flash Throughput, 236000 KB/s (1167400 Bytes, 4941 us)
S - DDR Frequency, 400 MHz
Android Bootloader - UART_DM Initialized!!!
[0] [0] welcome to lk

[10] [10] platform_init()
[10] [10] target_init()
[40] [40] SDHC Running in HS200 mode
[70] [70] Done initialization of the card
[80] [80] pm8x41_get_is_cold_boot: cold boot
[90] [90] Not able to search the panel:
[90] [90] Display not enabled for 24 HW type
[100] [100] Target panel init not found!
[100] [100] pm8x41_get_is_cold_boot: cold boot
[110] [110] fastboot_init()
[110] [110] Loading keystore failed status 5 [210] [210] USB init ept @ 0x8f697000
[230] [230] udc_start()

It worked before I flashed the system.img generated according to the instructions from here:
https://github.com/96boards/documentation/blob/master/ConsumerEdition/DragonBoard-410c/BuildSource/AOSP.md

which cause the problem like this when I flash the system.img generated:
22:39:04 liuyq: generic$ fastboot flash system system.img
target reported max download size of 268435456 bytes
erasing ‘system’…
OKAY [ 0.148s]
sending sparse ‘system’ 1/3 (256993 KB)…
FAILED (data transfer failure (Protocol error))
finished. total time: 0.457s
22:39:19 liuyq: generic$

Anyone knows what’s the problem here?

hmm, not sure if this is solution, but it seems help me to enter fastboot mode again.
so just list here for reference:

  1. unplug the micro usb first,
  2. press Vol- + powering up
  3. plug the micro usb again.
    run fastboot devices on the host side, you will see it listed there.