nand "BCH decoding failed" when using bch8_hw_romcode ecc mode

[Sascha Hauer]

On Fri, Mar 18, 2022 at 11:32:36AM +0100, Tibault Damman wrote:
> Gah, I just saw how that mail was sent, let me try that again:
> 
> 
> Because the data looks right(?), despite the error prints, I tried
> ubiformat again from barebox, then booted linux from SD, and attached
> the ubi nand partition in linux... which worked fine.
> All volumes and data were there.
> 
> Very confused about what's going wrong here.

You can correctly write to the NAND and can even correctly read the
data, that's good news.

Reading is done in omap_gpmc_read_page_bch_rom_mode(). Here we have this
to read the OOB data:

> 	p = chip->oob_poi;
> 
> 	p += omap_gpmc_read_buf_manual(chip, p, 2, 5);
> 
> 	for (i = 0; i < 4; i++) {
> 		p += omap_gpmc_read_buf_manual(chip, p, 13, i + 1);
> 		p += omap_gpmc_read_buf_manual(chip, p, 1, 5);
> 	}

omap_gpmc_read_buf_manual() is called with uneven numbers of bytes. This
goes down to omap_read_buf_pref():

> 	/* take care of subpage reads */
> 	if (len % 4) {
> 		if (info->nand.options & NAND_BUSWIDTH_16)
> 			readsw(info->cs_base, buf, (len % 4) / 2);
> 		else
> 			readsb(info->cs_base, buf, len % 4);
> 		p = (u32 *) (buf + len % 4);
> 		len -= len % 4;
> 	}

This can't work properly for 16bit NANDs.

I don't know how to do this correctly though, you'll have to experiment
a bit here.

Have a look at "md -s /dev/nand0.raw 0+2112", then you can see how the
OOB data should look like and compare it with the data read in
omap_gpmc_read_page_bch_rom_mode(). The ".raw" is important, I saw you
printed 2112 bytes from a non raw device. I meant to print a single
page, on a non raw device that would be 2048. You might have to enable
CONFIG_MTD_RAW_DEVICE to get the raw device.

Sascha

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