Re: [PATCH] mtd: mtk: avoid warning in mtk_ecc_encode

From: Arnd Bergmann
Date: Fri Sep 30 2016 - 13:26:25 EST

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

- 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.