[PATCH] nand_read_subpage vs. S3C244x NAND: non-word reads

[Ben Dooks]

On Sat, Nov 01, 2008 at 01:15:05AM -0200, Werner Almesberger wrote:
> In recent kernels, the optimized OOB reads in nand_read_subpage often
> make requests which aren't an exact multiple of a word, which caused
> nand/s3c2410.c to do a partial read, yielding chaos and mayhem.
> 
> I'm not entirely sure whether nand_read_subpage is to blame for making
> non-word requests, or whether nand/s3c2410.c is to blame for not
> supporting them, but here's a patch for the latter.

As noted on the openmoko list, I think we can do 256byte subpage reads
as long as they are aligned to 256bytes. We could make the ECC code
deal with non-256 byte power-of-two aligned blocks without huge
changes but my belief is that we cannot support anything that isn't
a power of two.

I think the best thing to do is to either force the caller to read
a power of two (pref. >4 bytes), so either we need some form of flag
to say this, or change the behaviour of the callers to never try this.
 
> Note that s3c2410_nand_write_buf has a similar issue. RNDIN is used in
> nand_write_oob_syndrome, so this may also be a real problems.
> 
> - Werner
> 
> ---------------------------------- cut here -----------------------------------
> 
> fix-s3c-nand-read-bytes.patch
> 
> With the introduction of optimized OOB reads in nand_read_subpage,
> the length of the data requested may not be a multiple of four bytes.
> 
> This caused a partial read on the 2440, leading to false ECC errors
> and, worse, attempts to "correct" them.
> 
> Note that there is a similar issue in s3c2440_nand_write_buf, which
> doesn't seem to cause trouble yet.
> 
> Signed-off-by: Werner Almesberger <werner at openmoko.org>
> 
> ---
> 
> Index: ktrack/drivers/mtd/nand/s3c2410.c
> ===================================================================
> --- ktrack.orig/drivers/mtd/nand/s3c2410.c	2008-11-01 00:29:45.000000000 -0200
> +++ ktrack/drivers/mtd/nand/s3c2410.c	2008-11-01 00:38:10.000000000 -0200
> @@ -530,7 +530,14 @@
>  static void s3c2440_nand_read_buf(struct mtd_info *mtd, u_char *buf, int len)
>  {
>  	struct s3c2410_nand_info *info = s3c2410_nand_mtd_toinfo(mtd);
> +
>  	readsl(info->regs + S3C2440_NFDATA, buf, len / 4);
> +	if (unlikely(len & 3)) {
> +		u32 data;
> +
> +		data = readl(info->regs + S3C2440_NFDATA);
> +		memcpy(buf + (len & ~3), &data, len & 3);
> +	}

note you'll still fail for len in the 0..3 region, as you'll move
len+4 bytes, and then copy them over the data you just read.

>  }
>  
>  static void s3c2410_nand_write_buf(struct mtd_info *mtd, const u_char *buf, int len)
> 
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-- 
Ben (ben at fluff.org, http://www.fluff.org/)

  'a smiley only costs 4 bytes'