linux-kernel - Re: [PATCH] Revert "mtd: davinci-nand: disable subpage write for keystone-nand"

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Date:   Thu, 30 Mar 2017 15:38:54 +0530
From:   Sekhar Nori <nsekhar@...com>
To:     Boris Brezillon <boris.brezillon@...e-electrons.com>,
        Richard Weinberger <richard@....at>
CC:     David Woodhouse <dwmw2@...radead.org>,
        Brian Norris <computersforpeace@...il.com>,
        Marek Vasut <marek.vasut@...il.com>,
        Cyrille Pitchen <cyrille.pitchen@...el.com>,
        <linux-mtd@...ts.infradead.org>, <linux-kernel@...r.kernel.org>,
        Muralidharan Karicheri <m-karicheri2@...com>,
        Franklin S Cooper Jr <fcooper@...com>
Subject: Re: [PATCH] Revert "mtd: davinci-nand: disable subpage write for
 keystone-nand"

On Thursday 30 March 2017 03:30 PM, Boris Brezillon wrote:
> On Thu, 30 Mar 2017 13:13:58 +0530
> Sekhar Nori <nsekhar@...com> wrote:
> 
>> Hi Boris,
>>
>> On Wednesday 29 March 2017 08:33 PM, Boris Brezillon wrote:
>>> On Mon, 27 Mar 2017 15:56:55 +0530
>>> Sekhar Nori <nsekhar@...com> wrote:
>>>   
>>>> This partly reverts commit 28c015a9daabe4ed3aeb0ccf669a3f1c2b8b81d5.
>>>>
>>>> Since f6d7c1b5598b ("mtd: nand: davinci: Reinitialize the HW ECC
>>>> engine in 4bit hwctl") using subpage and 4bit HWECC works.
>>>>
>>>> Revert the original workaround made for this. Tested with
>>>> mtd_subpagetest and UBIFS on K2E, K2HK, K2L EVMs.
>>>>
>>>> The addition of keystone specific compatible to davinci_nand_of_match[]
>>>> contained in original commit is retained.  
>>>
>>> Hm, it sounds like a bad idea to re-enable sub-page writes afterwards.
>>> You might break existing systems using UBI, because UBI detects the
>>> sub-page write feature and decides to place the VID header at a
>>> different position depending on that.
>>> To sum-up, you're likely to break users that have already flashed a UBI
>>> image with the old driver.  
>>
>> Okay, I didn't realize that.
>>
>>> If you want to re-enable sub-page writes, just switch back to the
>>> "ti,davinci-nand" compatible.  
>>
>> How about a comment on top of the disable subpage write code explaining 
>> that its not really needed anymore ? Something like this patch:
>>
>> ---8<---
>> From: Sekhar Nori <nsekhar@...com>
>> Date: Thu, 30 Mar 2017 13:08:30 +0530
>> Subject: [PATCH] mtd: nand: davinci: add comment on NAND subpage write status
>>  on keystone
>>
>> Add a comment clarifying that NAND subpage write on keystone works,
>> but is not being enabled in the interest of backward compatibility.
>>
>> Signed-off-by: Sekhar Nori <nsekhar@...com>
>> ---
>>  drivers/mtd/nand/davinci_nand.c | 11 +++++++++++
>>  1 file changed, 11 insertions(+)
>>
>> diff --git a/drivers/mtd/nand/davinci_nand.c b/drivers/mtd/nand/davinci_nand.c
>> index 27fa8b87cd5f..5bf8be1cfc8c 100644
>> --- a/drivers/mtd/nand/davinci_nand.c
>> +++ b/drivers/mtd/nand/davinci_nand.c
>> @@ -581,6 +581,17 @@ static struct davinci_nand_pdata
>>  			"ti,davinci-nand-use-bbt"))
>>  			pdata->bbt_options = NAND_BBT_USE_FLASH;
>>  
>> +		/*
>> +		 * Since kernel v4.8, this driver has been fixed to enable
>> +		 * use of 4-bit hardware ECC with subpages and verified on
>> +		 * TI's keystone EVMs (K2L, K2HK and K2E).
>> +		 * However, in the interest of not breaking systems using
>> +		 * existing UBI partitions, sub-page writes are not being
>> +		 * (re)enabled. If you want to use subpage writes on Keystone
>> +		 * platforms (i.e. do not have any existing UBI partitions),
>> +		 * then remove the if() block below or pass "ti,davinci-nand"
>> +		 * as the compatible in your device-tree file.
> 
> Hm, I'm not sure we should suggest to remove these lines from the code.
> Just suggest to use "ti,davinci-nand" compatible.

Thats fine with me. Today, this if() block is the only place where
ti,keystone-nand is used. If in future it gets used for something else
(yet unknown), then the comment might start to get misleading. Thats why
just removing the code seemed like a safer option to me.

But the main intention was to document that subpage is not really broken
on K2 platform. So, what you suggest will do as well.

Thanks,
Sekhar