K5 kindle paperwhite1 bricked， HELP！

I use an usb-ttl device connected with my kindle. It shows below text:


Board: Unknown
Boot Reason: [POR]
Boot Device: MMC
Board id:
S/N:
Invalid board id! Can't determine system type for RAM init.. bailing!
DRAM: 0KB
Using default environment.


I searched in google and this forum.
I try an uboot which can boot in fastboot mode directly. It not wok.
I download the uboot source code, add some code to run the idme cmd: write the serial and pcbsn。I got an u-boot.bin after compiled.

I try imx_usb_loader, mfg_tool and sb_loader to load u-boot.bin, all not working.
I checked the imx_usb_loader source code. It first run the plugin code, then load all the u-boot.bin. The second step always fail. Then how about skip the first step.? I change the u-boot.lds (comment the flash_header.o, so it don't load) and run:
sb_loader -trans 0xF8007000 -f u-boot.bin
sb_loader -exec 0xF8007000

The sb_loader tell me "-trans is ok" and "-exec is ok", but nothing happens on serial port.

I want to know how to make the imx_usb_loader or sb_loader work properly?

Some confuse with the usb download mode. Is it transfer the u-boot.bin in RAM?
If it is, why address is 0xF8007000? The RAM size is 256MB. If not, where exactly it transferred?

Really sorry for my poor English.