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

[Naga Sureshkumar Relli]

H Boris,

> -----Original Message-----
> From: Boris Brezillon [mailto:boris.brezillon@xxxxxxxxxxx]
> Sent: Monday, November 19, 2018 1:13 AM
> To: Naga Sureshkumar Relli <nagasure@xxxxxxxxxx>
> Cc: miquel.raynal@xxxxxxxxxxx; richard@xxxxxx; dwmw2@xxxxxxxxxxxxx;
> computersforpeace@xxxxxxxxx; marek.vasut@xxxxxxxxx; linux-mtd@xxxxxxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; nagasuresh12@xxxxxxxxx; robh@xxxxxxxxxx; Michal Simek
> <michals@xxxxxxxxxx>
> Subject: Re: [LINUX PATCH v12 3/3] mtd: rawnand: arasan: Add support for Arasan NAND
> Flash Controller
> 
> On Thu, 15 Nov 2018 09:34:16 +0000
> Naga Sureshkumar Relli <nagasure@xxxxxxxxxx> 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?
Yes, in Arasan NAND controller there is no way to detect uncorrectable errors beyond 24-bit.
> 
> > 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.
Ok
> 
> > I am doubting that this will not work in all cases.
> 
> It definitely doesn't.
Ok
> 
> > 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.
Yes, it is for both. But in case of read errors that we can't detect beyond 24-bit, then the answer from HW design team
Is that the flash part is bad.
Unfortunately till now we haven't ran into that situation(read errors of written pages beyond 24-bit).
But we are hitting this because of erased page reading(needed in case of ubifs).

> 
> 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.
Bit reporting for several bit errors is there only for Hamming(1bit correction and 2bit detection) but not in BCH.

Thanks,
Naga Sureshkumar Relli