[U-Boot] RFC: porting u-boot to sequoia based nand booting board

[Scott Wood]

On Tue, 17 May 2011 13:49:44 -0400
Alex Waterman <awaterman at dawning.com> wrote:

> 
> Scott,
> 
> On 05/17/2011 01:05 PM, Scott Wood wrote:
> > On Tue, 17 May 2011 10:11:14 -0400
> > Alex Waterman <awaterman at dawning.com> wrote:
> > 
> >> I have seen issues with the nand_read_byte16() function in nand_base.c; it seems like the cpu_to_le16() should be the other way around: le16_to_cpu(). Other than that no bugs as far as I am aware.
> > 
> > What is the specific problem you're seeing?  The use of these endian macros
> > is a bit abusive and ugly (what's really wanted is native-endian I/O
> > accessors -- readw() has an implicit le16_to_cpu()), and should have been
> > done internally to the read_word() implementation rather than made part of
> > the API, but functionally it should be correct.
> 
> When I was getting our NAND to work, it seemed like that function was always returning 0. I fixed it by writing a read_byte() function like this:
> 
> /*
>  * Read a byte from the NDFC. 
>  */
> static uint8_t tiger_read_byte(struct mtd_info *mtd){
> 
> 	uint16_t word;
> 	struct nand_chip *chip = mtd->priv;
>   
> 	word = readw(chip->IO_ADDR_R);
> 
> 	return (uint8_t) word;
> 
> }
> 
> It looked to me like the readw() function was returning the data in the correct CPU endianness (at least for PPC) and that the cpu_to_le16() was swapping the bytes such that the cast down to a uint8_t was getting the unset high order byte from the 16 bit read.

If readw() is returning the bytes in the correct endianness, then that
means the register is little-endian.

It's not clear to me what the default assumption in nand_base.c is,
though.  read_byte16() suggests the default is native endian, but
read_buf() and read_word() suggest it's little endian. :-(

Is there any currently working host-big-endian platform with 16-bit NAND
that doesn't override these functions?

BTW, as for read_word(), looking at its only user, I think nand_block_bad()
should be checking a 16-bit bad block marker on 16-bit NAND, rather than the
low byte.  Why is there a separate mechanism for checking bad block markers
than the one in nand_bbt.c (not the bbt itself, but the code used to read
the markers to create the bbt)?  As long as a BBT is used, I don't think
read_word() will ever matter.

-Scott