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

[Sekhar Nori <nsekhar@ti.com>]

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@ti.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@ti.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@ti.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.
+		 */
 		if (of_device_is_compatible(pdev->dev.of_node,
 					    "ti,keystone-nand")) {
 			pdata->options |= NAND_NO_SUBPAGE_WRITE;
-- 
2.9.0