linux-kernel - Re: [PATCH v2] mtd: rawnand: denali: set SPARE_AREA_SKIP

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Date:   Fri, 5 Oct 2018 16:39:14 +0200
From:   Miquel Raynal <miquel.raynal@...tlin.com>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     linux-mtd@...ts.infradead.org,
        Boris Brezillon <boris.brezillon@...tlin.com>,
        Dinh Nguyen <dinguyen@...nel.org>,
        linux-kernel@...r.kernel.org, Marek Vasut <marek.vasut@...il.com>,
        Brian Norris <computersforpeace@...il.com>,
        Richard Weinberger <richard@....at>,
        David Woodhouse <dwmw2@...radead.org>
Subject: Re: [PATCH v2] mtd: rawnand: denali: set SPARE_AREA_SKIP_BYTES
 register to 8 if unset

Hi Masahiro,

Masahiro Yamada <yamada.masahiro@...ionext.com> wrote on Fri, 28 Sep
2018 13:16:01 +0900:

> NAND devices need additional data area (OOB) for error correction,
> but it is also used for Bad Block Marker (BBM).  In many cases, the
> first byte in OOB is used for BBM, but the location actually depends
> on chip vendors.  The NAND controller should preserve the precious
> BBM to keep track of bad blocks.
> 
> In Denali IP, the SPARE_AREA_SKIP_BYTES register is used to specify
> the number of bytes to skip from the start of OOB.  The ECC engine
> will automatically skip the specified number of bytes when it gets
> access to OOB area.
> 
> The same value for SPARE_AREA_SKIP_BYTES should be used between
> firmware and the operating system if you intend to use the NAND
> device across the control hand-off.
> 
> In fact, the current denali.c code expects firmware to have already
> set the SPARE_AREA_SKIP_BYTES register, then reads the value out.
> 
> If no firmware (or bootloader) has initialized the controller, the
> register value is zero, which is the default after power-on-reset.
> In other words, the Linux driver cannot initialize the controller
> by itself.
> 
> Some possible solutions are:
> 
>  [1] Add a DT property to specify the skipped bytes in OOB
>  [2] Associate the preferred value with compatible
>  [3] Hard-code the default value in the driver
> 
> My first attempt was [1], but in the review process, [3] was suggested
> as a counter-implementation.
> (https://lore.kernel.org/patchwork/patch/983055/)
> 
> The default value 8 was chosen to match to the boot ROM of the UniPhier
> platform.  The preferred value may vary by platform.  If so, please
> trade up to a different solution.
> 
> Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
> ---
> 

Applied to nand/next.


Thanks,
Miquèl