Re: [PATCH 4/7] mtd: rawnand: sunxi: Fix ECC strength maximization

From: Miquel Raynal <miquel.raynal@bootlin.com>
To: Samuel Holland <samuel@sholland.org>
Cc: Richard Weinberger <richard@nod.at>,
	Vignesh Raghavendra <vigneshr@ti.com>,
	Chen-Yu Tsai <wens@csie.org>,
	Jernej Skrabec <jernej.skrabec@gmail.com>,
	Boris Brezillon <bbrezillon@kernel.org>,
	Brian Norris <computersforpeace@gmail.com>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-mtd@lists.infradead.org,
	linux-sunxi@lists.linux.dev
Subject: Re: [PATCH 4/7] mtd: rawnand: sunxi: Fix ECC strength maximization
Date: Tue, 3 Jan 2023 15:41:54 +0100	[thread overview]
Message-ID: <20230103154155.7064e59e@xps-13> (raw)
In-Reply-To: <6940e60e-834d-9cc6-c3b8-657b100ea5db@sholland.org>

Hi Samuel,

samuel@sholland.org wrote on Mon, 2 Jan 2023 11:06:20 -0600:

> Hi Miquèl,
> 
> On 1/2/23 10:45, Miquel Raynal wrote:
> >>>> This is already accounted for in the subtraction for OOB, since the BBM
> >>>> overlaps the first OOB dword. With this change, the driver picks the
> >>>> same ECC strength as the vendor driver.
> >>>>
> >>>> Fixes: 4796d8655915 ("mtd: nand: sunxi: Support ECC maximization")
> >>>> Signed-off-by: Samuel Holland <samuel@sholland.org>
> >>>> ---
> >>>>
> >>>>  drivers/mtd/nand/raw/sunxi_nand.c | 3 +--
> >>>>  1 file changed, 1 insertion(+), 2 deletions(-)
> >>>>
> >>>> diff --git a/drivers/mtd/nand/raw/sunxi_nand.c b/drivers/mtd/nand/raw/sunxi_nand.c
> >>>> index 1bddeb1be66f..1ecf2cee343b 100644
> >>>> --- a/drivers/mtd/nand/raw/sunxi_nand.c
> >>>> +++ b/drivers/mtd/nand/raw/sunxi_nand.c
> >>>> @@ -1643,8 +1643,7 @@ static int sunxi_nand_hw_ecc_ctrl_init(struct nand_chip *nand,
> >>>>  		ecc->size = 1024;
> >>>>  		nsectors = mtd->writesize / ecc->size;
> >>>>  
> >>>> -		/* Reserve 2 bytes for the BBM */
> >>>> -		bytes = (mtd->oobsize - 2) / nsectors;
> >>>> +		bytes = mtd->oobsize / nsectors;    
> >>>
> >>> I'm sorry but I don't think we can make this work. This change would
> >>> break all existing users...    
> >>
> >> OK, it is not too much of an issue because I can manually specify the
> >> ECC parameters in the devicetree. Do you think it makes sense to fix
> >> this when adding new hardware variants/compatible strings?  
> > 
> > Actually, looking at the code again, I don't get how the above diff
> > could be valid. The "maximize strength" logic (in which this diff is)
> > looks for the biggest region to store ECC bytes. These bytes cannot
> > be stored on the BBM, which "mtd->oobsize - 2" tries to avoid, so we
> > cannot get rid of this.  
> 
> Right, we cannot overlap the BBM, but the BBM is accounted for in the
> line below:
> 
>   /* 4 non-ECC bytes are added before each ECC bytes section */
>   bytes -= 4;
> 
> Normally those 4 bytes are all free OOB, but for the first ECC step,
> those are split into 2 free bytes and 2 BBM bytes:
> 
>   /*
>    * The first 2 bytes are used for BB markers, hence we
>    * only have 2 bytes available in the first user data
>    * section.
>    */
>   if (!section && ecc->engine_type == NAND_ECC_ENGINE_TYPE_ON_HOST) {
>           oobregion->offset = 2;
>           oobregion->length = 2;
> 
>           return 0;
>   }
> 
> So if we subtract 4 bytes for the each free OOB area, including the
> first one, and also subtract 2 bytes for the BBM, we are double-counting
> the BBM. I should have made my commit message clearer. But I am going to
> drop this patch anyway.

Ah, yes, you are absolutely right, then.

Thanks,
Miquèl

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