On Tue, 17 May 2011 13:49:44 -0400
Alex Waterman <awater...@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 <awater...@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
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