[U-Boot] [PATCH 1/3] powerpc/p5040: enable NAND boot support

[Xie Shaohui-B21989]

> > > This needs more explanation.  What sort of image am I supposed to
> > get
> > > when I build "P5040DS_NAND"?  Where is the PBI?
> > [S.H] there is already a readme for p3041/p5020/p4080
> > (doc/README.pblimage), With the PBL tool, we will get a ramboot image
> > "u-boot.pbl".
> > and also the PBI, it is shared by P3/P4/P5.
> 
> The question is how is the user to know that something called "pblimage"
> is relevant to booting from NAND on P5040?  Maybe a README.corenet_ds (or
> board/freescale/corenet_ds/README), that points to README.pblimage?
[S.H] Yes. A README for corenet_ds board will be more useful, 
then a ramboot section points to README.pblimage.

> 
> You don't even update strings like "P3041/P5020" in README.pblimage to
> include P5040. :-)
[S.H] Thanks for pointing it out. I will update the README.pblimage in next version.

 Why is P4080 excluded, BTW?  I realize that P4080DS
> doesn't have NAND, but the file talks about chips, not boards.
[S.H] there is no P4080DS_NAND entry in boards.cfg...

> 
> > > What is the long-term plan for fixing the problem of the
> > environment not
> > > being available until after relocation?  With SPL we could use
> > > CONFIG_NAND_ENV_DST (which has some issues, but they're fixable).
> > [S.H] this will need export some NAND read APIs (like load ENV stuff
> > from CONFIG_NAND_ENV_DST), then they can be reused before relocation
> > even not using SPL.
> > Please suggest.
> 
> The full NAND subsystem will not work before relocation.  Your options
> are to either use SPL (note that you'll have no 4K limitation in this
> case, so you can do SPD DDR init), or make the SPL code available in non-
> SPL context somehow.  I think the former would be easier.
[S.H] Thanks. I will assess the effort base on my case.

> 
> > > It would also be nice to include instructions for configuring the
> > board
> > > to boot from NAND in a README
> > [S.H] This exist in the above readme.
> 
> I only see hardware switch settings.  Is there no way to soft-boot into
> NAND (similar to using "pixis/qixis altbank" to boot into an alternate
> NOR bank)?  Our e500v2-based boards have been able to do this...
[S.H] we do have these soft-boot instructions for corenet_ds board.
And yes, they should go in README of corenet_ds board, 
but seems we don't have such a README.

Best Regards, 
Shaohui Xie