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: <20180515223516.16e907b3@bbrezillon>
Date:   Tue, 15 May 2018 22:35:16 +0200
From:   Boris Brezillon <boris.brezillon@...tlin.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     jagdish.gediya@....com,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Jane Wan <Jane.Wan@...ia.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Richard Weinberger <richard@....at>,
        Marek Vasut <marek.vasut@...il.com>, ties.bos@...ia.com,
        prabhakar.kushwaha@....com,
        "open list:MEMORY TECHNOLOGY..." <linux-mtd@...ts.infradead.org>,
        Shreeya Patel <shreeya.patel23498@...il.com>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Brian Norris <computersforpeace@...il.com>,
        Shawn Guo <shawnguo@...nel.org>,
        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, 15 May 2018 23:23:02 +0300
Andy Shevchenko <andy.shevchenko@...il.com> wrote:

> 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))

I made a mistake in my code sample, it's

				if (srcbuf[i] & BIT(j))

If you look at v6, you'll see it's been fixed by Jane.

>                                       m++;
>                     }
> 
> ...and now it looks obvious:
> 
> m += hweight...(srcbuf[i])
> 
> _If_ nbufs is power of two we may use primitive helper.
>