[U-Boot] MMC write failed - Can't read partition table

[DaveKucharczyk]

Hi, we've been using a new SD card for 6+ months without issues, but recently
noticed issues with newer releases. I'm not sure if it's the SD cards, our
burner stations or our build process. Were using U-Boot 2014.07

The problem is that we can't read/write to the SD card during boot. This
happens intermittently on freshly burned cards. 75% of the time it works.

Our bootcmd starts with ext2load mmc ... but obviously it fails due to
"can't read partition table". 

Manually debugging in U-Boot if I run mmc rescan then it can read the
partition table, but if I reboot the system then the issue comes back upon
next power-up. 

If I run rescan AND then saveenv then everything works after that. 

My questions are:

1. Is it good practice to run mmc rescan as one of the first things in
bootcmd?

2. I'm a little confised as to why saveenv would cause the SD card to work
again *persistently*. 

3. Any recommendations on getting root cause as to what it could be (SD
card, our burner stations or our build process)?


Troubleshooting steps below...

MX51 U-Boot > mmc info
Device: FSL_SDHC
Manufacturer ID: 87
OEM: 494f
Name: 004GB
Tran Speed: 50000000
Rd Block Len: 512
SD version 3.0
High Capacity: Yes
Capacity: 3.8 GiB
Bus Width: 1-bit
MX51 U-Boot > mmc part

Partition Map for MMC device 0  --   Partition Type: DOS

Part    Start Sector    Num Sectors     UUID            Type
** Can't read partition table on 0:0 **
MX51 U-Boot > mmc rescan
MX51 U-Boot > mmc part

Partition Map for MMC device 0  --   Partition Type: DOS

Part    Start Sector    Num Sectors     UUID            Type
  1     31000           3844000         00000000-01     83
  2     3882750         4037750         00000000-02     83
MX51 U-Boot > saveenv
Saving Environment to MMC...

reboot and everything works. 





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