linux-kernel - Re: [PATCH v5 2/2] mtd: rawnand: use bit-wise majority to recover the contents of ONFI parameter

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Message-ID: <CAHp75VeEhB=+x0toCtSvrq0zBLXVCASzN2BUDDgNParSYZc5BQ@mail.gmail.com>
Date:   Tue, 15 May 2018 23:23:02 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Boris Brezillon <boris.brezillon@...tlin.com>
Cc:     Shreeya Patel <shreeya.patel23498@...il.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Jane Wan <Jane.Wan@...ia.com>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Marek Vasut <marek.vasut@...il.com>, ties.bos@...ia.com,
        prabhakar.kushwaha@....com,
        "open list:MEMORY TECHNOLOGY..." <linux-mtd@...ts.infradead.org>,
        jagdish.gediya@....com, Richard Weinberger <richard@....at>,
        Shawn Guo <shawnguo@...nel.org>,
        Brian Norris <computersforpeace@...il.com>,
        David Woodhouse <dwmw2@...radead.org>
Subject: Re: [PATCH v5 2/2] mtd: rawnand: use bit-wise majority to recover the
 contents of ONFI parameter

On Tue, May 15, 2018 at 11:03 AM, Boris Brezillon
<boris.brezillon@...tlin.com> wrote:
> On Tue, 15 May 2018 10:46:00 +0300
> Andy Shevchenko <andy.shevchenko@...il.com> wrote:
>
>> On Tue, May 15, 2018 at 10:35 AM, Boris Brezillon
>> <boris.brezillon@...tlin.com> wrote:
>> > On Mon, 14 May 2018 20:54:36 +0300
>> > Andy Shevchenko <andy.shevchenko@...il.com> wrote:

>> >> >                         for (k = 0; k < nbufs; k++) {
>> >> >                                 const u8 *srcbuf = srcbufs[j];
>> >> >
>> >> >                                 if (srcbuf[i] & BIT(k))
>> >> >                                         m++;
>> >> >                         }
>> >>
>> >> ...which is effectively hweightXX().
>> >
>> > No it's not.
>>
>> I don't see how "not". In the loop everithing except m and k are
>> invariants. What did I miss?
>
> We're not counting the number of bits set in an uXX var, but the number
> of set bits at the same position in different buffers.

...on big picture. The excerpt above is hweight() against srcbuf[i].

Let's rewrite it like this:

                    const u8 *srcbuf = srcbufs[j];

                    for (k = 0; k < nbufs; k++) {
                               if (srcbuf[i] & BIT(k))
                                      m++;
                    }

...and now it looks obvious:

m += hweight...(srcbuf[i])

_If_ nbufs is power of two we may use primitive helper.

-- 
With Best Regards,
Andy Shevchenko