Re: [LINUX PATCH v12 3/3] mtd: rawnand: arasan: Add support for Arasan NAND Flash Controller

[Boris Brezillon <boris.brezillon@bootlin.com>]

On Thu, 15 Nov 2018 09:34:16 +0000
Naga Sureshkumar Relli <nagasure@xilinx.com> wrote:

> Hi Boris & Miquel,
> 
> I am updating the driver by addressing your comments, and I have one concern,  especially in anfc_read_page_hwecc(), 
> there I am checking for erased pages bit flips.
> Since Arasan NAND controller doesn't have multibit error detection beyond 24-bit( it can correct up to 24 bit),
> i.e. there is no indication from controller to detect uncorrectable error beyond 24bit.

Do you mean that you can't detect uncorrectable errors, or just that
it's not 100% sure to detect errors above max_strength?

> So I took some error count as default value(MULTI_BIT_ERR_CNT  16, I put this based on the error count that 
> I got while reading erased page on Micron device).
> And during a page read, will just read the error count register and compare this value with the default error count(16) and if it is more 
> Than default then I am checking for erased page bit flips.

Hm, that's wrong, especially if you set ecc_strength to something > 16.

> I am doubting that this will not work in all cases.

It definitely doesn't.

> In my case it is just working because the error count that it got on an erased page is 16.
> Could you please suggest a way to do detect erased_page bit flips when reading a page with HW-ECC?.

I'm a bit lost. Is the problem only about bitflips in erase pages, or
is it also impacting reads of written pages that lead to uncorrectable
errors.

Don't you have a bit (or several bits) reporting when the ECC engine
was not able to correct data? I you do, you should base the "detect
bitflips in erase pages" logic on this information.