linux-kernel - Re: [PATCH 4/4] mtd: nand: mxc

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Message-ID: <ZiDCKGlG4MZ23Tqo@pengutronix.de>
Date: Thu, 18 Apr 2024 08:48:08 +0200
From: Sascha Hauer <s.hauer@...gutronix.de>
To: Miquel Raynal <miquel.raynal@...tlin.com>,
	Richard Weinberger <richard@....at>,
	Vignesh Raghavendra <vigneshr@...com>
Cc: linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] mtd: nand: mxc_nand: disable subpage reads

On Wed, Apr 17, 2024 at 09:13:31AM +0200, Sascha Hauer wrote:
> The NAND core enabled subpage reads when a largepage NAND is used with
> SOFT_ECC. The i.MX NAND controller doesn't support subpage reads, so
> clear the flag again.
> 
> Signed-off-by: Sascha Hauer <s.hauer@...gutronix.de>
> ---
>  drivers/mtd/nand/raw/mxc_nand.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/mtd/nand/raw/mxc_nand.c b/drivers/mtd/nand/raw/mxc_nand.c
> index f44c130dca18d..19b46210bd194 100644
> --- a/drivers/mtd/nand/raw/mxc_nand.c
> +++ b/drivers/mtd/nand/raw/mxc_nand.c
> @@ -1667,6 +1667,8 @@ static int mxcnd_probe(struct platform_device *pdev)
>  	if (err)
>  		goto escan;
>  
> +	this->options &= ~NAND_SUBPAGE_READ;
> +

Nah, it doesn't work like this. It turns out the BBT is read using
subpage reads before we can disable them here.

This is the code in nand_scan_tail() we stumble upon:

	/* Large page NAND with SOFT_ECC should support subpage reads */
	switch (ecc->engine_type) {
	case NAND_ECC_ENGINE_TYPE_SOFT:
		if (chip->page_shift > 9)
			chip->options |= NAND_SUBPAGE_READ;
		break;

	default:
		break;
	}

So the code assumes subpage reads are ok when SOFT_ECC is in use, which
in my case is not true. I guess some drivers depend on the
NAND_SUBPAGE_READ bit magically be set, so simply removing this code is
likely not an option.  Any ideas what to do?

Sascha


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