linux-kernel - Re: [PATCH] mtd: mtk: avoid warning in mtk_ecc

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Message-Id: <201609301925.17577.arnd@arndb.de>
Date:   Fri, 30 Sep 2016 19:25:17 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Boris Brezillon <boris.brezillon@...e-electrons.com>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        Brian Norris <computersforpeace@...il.com>,
        Richard Weinberger <richard@....at>,
        Matthias Brugger <matthias.bgg@...il.com>,
        linux-mtd@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mtd: mtk: avoid warning in mtk_ecc_encode

On Friday 30 September 2016, Boris Brezillon wrote:
> > +     /* copy into possibly unaligned OOB region with actual length */
> > +     memcpy(data + bytes, eccdata, len);
> 
> Is it better than
> 
>         for (i = 0; i < len; i += 4) {
>                 u32 val = __raw_readl(ecc->regs + ECC_ENCPAR(i / 4));
> 
>                 memcpy(data + bytes + i, &val, min(len, 4));
>         }
> 
> I'm probably missing something, but what's the point of creating a
> temporary buffer of 112 bytes on the stack since you'll have to copy
> this data to the oob buffer at some point?


I tried something like that first, but wasn't too happy with it for
a number of small reasons:

- __raw_readl in a driver is not usually the right API, __memcpy32_from_io
  uses it internally, but it's better for a driver not to rely on that,
  in case we need some barriers (which we may in factt need for other drivers).

- the min(len,4) expression is incorrect, fixing that makes it more complicated
  again

- I didn't like to call memcpy() multiple times, as that might get turned
  into an external function call (the compiler is free to optimize small
  memcpy calls or not).

I agree that he 112 byte buffer isn't ideal either, it just seemed to
be the lesser annoyance.

	Arnd