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Date:   Fri, 20 Apr 2018 21:54:40 +0200
From:   Boris Brezillon <boris.brezillon@bootlin.com>
To:     NeilBrown <neil@brown.name>
Cc:     Cyrille Pitchen <cyrille.pitchen@wedev4u.fr>,
        Marek Vasut <marek.vasut@gmail.com>,
        David Woodhouse <dwmw2@infradead.org>,
        Brian Norris <computersforpeace@gmail.com>,
        Boris Brezillon <boris.brezillon@free-electrons.com>,
        Richard Weinberger <richard@nod.at>,
        linux-mtd@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] mtd: spi-nor: clear Winbond Extended Address Reg on
 switch to 3-byte addressing.
Message-ID: <20180420215440.7b078f6c@bbrezillon>
In-Reply-To: <87sh7wrq8p.fsf@notabene.neil.brown.name>
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        <87sh7wrq8p.fsf@notabene.neil.brown.name>
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Hi Neil,

On Mon, 16 Apr 2018 09:42:30 +1000
NeilBrown <neil@brown.name> wrote:

> Winbond spi-nor flash 32MB and larger have an 'Extended Address
> Register' as one option for addressing beyond 16MB (Macronix
> has the same concept, Spansion has EXTADD bits in the Bank Address
> Register).
> 
> According to section
>    8.2.7 Write Extended Address Register (C5h)
> 
> of the Winbond W25Q256FV data sheet (256M-BIT SPI flash)
> 
>    The Extended Address Register is only effective when the device is
>    in the 3-Byte Address Mode.  When the device operates in the 4-Byte
>    Address Mode (ADS=1), any command with address input of A31-A24
>    will replace the Extended Address Register values. It is
>    recommended to check and update the Extended Address Register if
>    necessary when the device is switched from 4-Byte to 3-Byte Address
>    Mode.
> 
> So the documentation suggests clearing the EAR after switching to
> 3-byte mode.  Experimentation shows that the EAR is *always* one after
> the switch to 3-byte mode, so clearing the EAR is mandatory at
> shutdown for a subsequent 3-byte-addressed reboot to work.
> 
> Note that some SOCs (e.g. MT7621) do not assert a reset line at normal
> reboot, so we cannot rely on hardware reset.  The MT7621 does assert a
> reset line at watchdog-reset.
> 
> Signed-off-by: NeilBrown <neil@brown.name>

We should probably backport the fix. Can you add a Fixes and Cc-stable
tag?

> ---
> 
> following a helpful discussion with Marek, I've revised the description
> a little, and make the code change specific to winbond.
> I've change the OP names to RDEAR and WREAR instead of RDXA and WRXA to
> match names used in the Macronix documentation.  Winbond documentation
> doesn't provide abbreviated OP names.
> 
> Thanks,
> NeilBrown
> 
> 
>  drivers/mtd/spi-nor/spi-nor.c | 13 +++++++++++++
>  include/linux/mtd/spi-nor.h   |  2 ++
>  2 files changed, 15 insertions(+)
> 
> diff --git a/drivers/mtd/spi-nor/spi-nor.c b/drivers/mtd/spi-nor/spi-nor.c
> index d445a4d3b770..0d0af0acf8b9 100644
> --- a/drivers/mtd/spi-nor/spi-nor.c
> +++ b/drivers/mtd/spi-nor/spi-nor.c
> @@ -284,6 +284,19 @@ static inline int set_4byte(struct spi_nor *nor, const struct flash_info *info,
>  		if (need_wren)
>  			write_disable(nor);
>  
> +		if (!status && !enable &&
> +		    JEDEC_MFR(info) == SNOR_MFR_WINBOND) {
> +			/* On Winbond W25Q256FV, leaving 4byte mode causes

We use regular kernel-comment style in MTD:

			/*
			 * blablabla
			 */

Thanks,

Boris

> +			 * the Extended Address Register to be set to 1, so all
> +			 * 3-byte-address reads come from the second 16M.
> +			 * We must clear the register to enable normal behavior.
> +			 */
> +			write_enable(nor);
> +			nor->cmd_buf[0] = 0;
> +			nor->write_reg(nor, SPINOR_OP_WREAR, nor->cmd_buf, 1);
> +			write_disable(nor);
> +		}
> +
>  		return status;
>  	default:
>  		/* Spansion style */
> diff --git a/include/linux/mtd/spi-nor.h b/include/linux/mtd/spi-nor.h
> index de36969eb359..e60da0d34cc1 100644
> --- a/include/linux/mtd/spi-nor.h
> +++ b/include/linux/mtd/spi-nor.h
> @@ -62,6 +62,8 @@
>  #define SPINOR_OP_RDCR		0x35	/* Read configuration register */
>  #define SPINOR_OP_RDFSR		0x70	/* Read flag status register */
>  #define SPINOR_OP_CLFSR		0x50	/* Clear flag status register */
> +#define SPINOR_OP_RDEAR		0xc8	/* Read Extended Address Register */
> +#define SPINOR_OP_WREAR		0xc5	/* Write Extended Address Register */
>  
>  /* 4-byte address opcodes - used on Spansion and some Macronix flashes. */
>  #define SPINOR_OP_READ_4B	0x13	/* Read data bytes (low frequency) */