linux-kernel - Re: [PATCH v2 11/53] mtd: nand: denali: fix bitflips calculation in handle

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Date:   Thu, 23 Mar 2017 09:12:43 +0100
From:   Boris Brezillon <boris.brezillon@...e-electrons.com>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     linux-mtd@...ts.infradead.org,
        Laurent Monat <laurent.monat@...uantique.com>,
        thorsten.christiansson@...uantique.com,
        Enrico Jorns <ejo@...gutronix.de>,
        Jason Roberts <jason.e.roberts@...el.com>,
        Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>,
        Dinh Nguyen <dinguyen@...nel.org>,
        Marek Vasut <marek.vasut@...il.com>,
        Brian Norris <computersforpeace@...il.com>,
        Graham Moore <grmoore@...nsource.altera.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Chuanxiao Dong <chuanxiao.dong@...el.com>,
        Jassi Brar <jaswinder.singh@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Richard Weinberger <richard@....at>,
        Cyrille Pitchen <cyrille.pitchen@...el.com>
Subject: Re: [PATCH v2 11/53] mtd: nand: denali: fix bitflips calculation in
 handle_ecc()

On Thu, 23 Mar 2017 16:02:02 +0900
Masahiro Yamada <yamada.masahiro@...ionext.com> wrote:

> Hi Boris,
> 
> 2017-03-23 5:57 GMT+09:00 Boris Brezillon <boris.brezillon@...e-electrons.com>:
> > On Wed, 22 Mar 2017 23:07:18 +0900
> > Masahiro Yamada <yamada.masahiro@...ionext.com> wrote:
> >  
> >> +     do {
> >> +             err_addr = ioread32(denali->flash_reg + ECC_ERROR_ADDRESS);
> >> +             err_sector = ECC_SECTOR(err_addr);
> >> +             err_byte = ECC_BYTE(err_addr);
> >> +
> >> +             err_cor_info = ioread32(denali->flash_reg + ERR_CORRECTION_INFO);
> >> +             err_cor_value = ECC_CORRECTION_VALUE(err_cor_info);
> >> +             err_device = ECC_ERR_DEVICE(err_cor_info);
> >> +
> >> +             /* reset the bitflip counter when crossing ECC sector */
> >> +             if (err_sector != prev_sector)
> >> +                     bitflips = 0;
> >> +
> >> +             if (ECC_ERROR_UNCORRECTABLE(err_cor_info)) {
> >> +                     /*
> >> +                      * if the error is not correctable, need to look at the
> >> +                      * page to see if it is an erased page. if so, then
> >> +                      * it's not a real ECC error
> >> +                      */
> >> +                     ret = -EBADMSG;  
> >
> > You should never return -EBADMSG directly. Just increment
> > ecc_stats.failed and let the core return -EBADMSG to the upper layer.
> >  
> 
> Here, -EBADMSG is used like that returned from  ->ecc.correct()
> 
> 
> Please notice denali_read_page() never returns -EBADMSG.
> 
>  -EBADMSG is used as a mark "we need erased page check".
> 
> 
> I think nand_read_page_syndrome() does similar;
> -EBADMSG is used internally.

That's not exactly what happens. nand_read_page_syndrome() calls
ecc->correct() for each chunk, and if this method returns -EBADMSG (and
nand_check_erased_ecc_chunk() returns -EBADMSG too) it increments the
ecc_stats.failed counter.

Here you check all chunks in the same function and only increment
ecc_stats.failed once in denali_read_page() even if several chunks are
uncorrectable.
You handle_ecc() should act like nand_read_page_syndrome() WRT ECC
checking: check each block one by one, call
nand_check_erased_ecc_chunk() if needed, increment ecc_stats.failed
when an uncorrectable error is detected, and return max_bitflips at the
end.