我使用的是 BeagleBone 板，默认 Debian 映像已闪存到内部 emmc。
我试图理解下面的 U-boot 消息，这些消息是我在设备启动时通过串行收到的，特别是它如何mmc0 is current device首先报告，然后又指出mmc1(part 0) is current device。
这里发生了什么？ mmc0是内部emmc内存还是SD卡，为什么引导加载程序从mmc0切换到mmc1？
U-Boot SPL 2018.09-00002-g0b54a51eee (Sep 10 2018 - 19:41:39 -0500)
Trying to boot from MMC2
Loading Environment from EXT4...
** Unable to use mmc 0:1 for loading the env **
U-Boot 2018.09-00002-g0b54a51eee (Sep 10 2018 - 19:41:39 -0500), Build:
jenkins-github_Bootloader-Builder-65
CPU : AM335X-GP rev 2.1
I2C: ready
DRAM: 512 MiB
No match for driver 'omap_hsmmc'
No match for driver 'omap_hsmmc'
Some drivers were not found
Reset Source: Power-on reset has occurred.
RTC 32KCLK Source: External.
MMC: OMAP SD/MMC: 0, OMAP SD/MMC: 1
Loading Environment from EXT4...
** Unable to use mmc 0:1 for loading the env **
Board: BeagleBone Black
<ethaddr> not set. Validating first E-fuse MAC
BeagleBone Black:
Model: SeeedStudio BeagleBone Green Wireless:
BeagleBone: cape eeprom: i2c_probe: 0x54:
BeagleBone: cape eeprom: i2c_probe: 0x55:
BeagleBone: cape eeprom: i2c_probe: 0x56:
BeagleBone: cape eeprom: i2c_probe: 0x57:
Net: eth0: MII MODE
Could not get PHY for cpsw: addr 0
cpsw, usb_ether
Press SPACE to abort autoboot in 2 seconds
board_name=[A335BNLT] ...
board_rev=[GW1A] ...
switch to partitions #0, OK
mmc0 is current device
SD/MMC found on device 0
switch to partitions #0, OK
mmc0 is current device
Scanning mmc 0:1...
gpio: pin 56 (gpio 56) value is 0
gpio: pin 55 (gpio 55) value is 0
gpio: pin 54 (gpio 54) value is 0
gpio: pin 53 (gpio 53) value is 1
switch to partitions #0, OK
mmc0 is current device
gpio: pin 54 (gpio 54) value is 1
Checking for: /uEnv.txt ...
Checking for: /boot.scr ...
Checking for: /boot/boot.scr ...
Checking for: /boot/uEnv.txt ...
** Invalid partition 3 **
** Invalid partition 4 **
** Invalid partition 5 **
** Invalid partition 6 **
** Invalid partition 7 **
switch to partitions #0, OK
mmc1(part 0) is current device
Scanning mmc 1:1...
gpio: pin 56 (gpio 56) value is 0
gpio: pin 55 (gpio 55) value is 0
gpio: pin 54 (gpio 54) value is 0
gpio: pin 53 (gpio 53) value is 1
switch to partitions #0, OK
mmc1(part 0) is current device
gpio: pin 54 (gpio 54) value is 1
Checking for: /uEnv.txt ...
Checking for: /boot.scr ...
Checking for: /boot/boot.scr ...
Checking for: /boot/uEnv.txt ...
gpio: pin 55 (gpio 55) value is 1
2099 bytes read in 7 ms (292 KiB/s)
Loaded environment from /boot/uEnv.txt
Checking if uname_r is set in /boot/uEnv.txt...
gpio: pin 56 (gpio 56) value is 1
Running uname_boot ...
loading /boot/vmlinuz-4.14.71-ti-r80 ...
10416640 bytes read in 663 ms (15 MiB/s)
答案1
在本例中，mmc0 是 SD 卡，mmc1 是 eMMC。事情变得不太清楚，因为“尝试启动”消息与 SoC 文档中编号的设备相匹配，编号从 1 开始，而后面的消息遵循正常约定，编号从 0 开始。
该板上的默认启动命令将搜索多个位置以查找要启动的有效“事物”，以便用户可以轻松尝试其他内容，它将查找 SD 卡（然后在其上的位置中查找）启动然后回退到 eMMC。