[U-Boot] eMMC boot partition booting and SMDK5250 support

[Tom Rini <trini@ti.com>]

Hey guys,

I've posted a patch to support the eMMC boot partitions found on the TI
DRA7xx EVM the other day, and I've been looking at what exactly happens
with the 'mmc open ...' and 'mmc close ...' commands the
CONFIG_SUPPORT_EMMC_BOOT enables.  I've got a few questions about how
exactly the support works on the SMDK5250 and what your platform
requirements are, so we can re-work the code to be a bit more generic.

Why do we have it as "open" and "close" ?  Is this because it's a
version 4.2[1] style device?  I'm working with 4.3-compatible (it's
actually 4.41, but same method just larger partitions) one and that can
be done with setting EXT_CSD[179] (BOOT_CONFIG / Boot configuration) _and_
EXT_CSD[177] to a specific and different value than we've hard-coded
now.

I would like to change things to be:
"mmc open <device>" - Note required for eMMC 4.2 boot method.
"mmc close <device>" - Same
"mmc bootpart <device> <partition>" - Used for eMMC 4.3 and later boot
sequence option 1/2
"mmc bootbus <device> <value>" - Note used in all boot sequences

This would make the SMDK5250 sequence be (based on how I think it works
today):
mmc dev ...
mmc open
mmc write ...
mmc bootpart ...
mmc bootbus ...
mmc close

And the DRA7xx sequence be:
mmc dev ...
mmc write ...
mmc bootpart ...
mmc bootbus ...

Comments?  I plan to start down this path shortly, thanks!

[1]:
http://www.micron.com/~/media/Documents/Products/Technical%20Note/NAND%20Flash/tn2918.pdf
-- 
Tom
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