linux-kernel - Re: [PATCH 4/6] lib: rework bitmap

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Message-ID: <20190428165745.GX9224@smile.fi.intel.com>
Date:   Sun, 28 Apr 2019 19:57:45 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Yury Norov <yury.norov@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Amritha Nambiar <amritha.nambiar@...el.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Tobin C . Harding" <tobin@...nel.org>,
        Will Deacon <will.deacon@....com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Vineet Gupta <vineet.gupta1@...opsys.com>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Yury Norov <ynorov@...vell.com>, linux-kernel@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>,
        Steffen Klassert <steffen.klassert@...unet.com>
Subject: Re: [PATCH 4/6] lib: rework bitmap_parse()

On Sat, Apr 27, 2019 at 08:29:34PM -0700, Yury Norov wrote:
> bitmap_parse() is ineffective and full of opaque variables and opencoded
> parts. It leads to hard understanding of it. This rework includes:
>  - remove bitmap_shift_left() call from the cycle. Now it makes the
>    complexity of the algorithm as O(nbits^2). In the suggested approach
>    the input string is parsed in reverse direction, so no shifts needed;
>  - relax requirement on a single comma and no white spaces between chunks.
>    It is considered useful in scripting, and it aligns with
>    bitmap_parselist();
>  - split bitmap_parse() to small readable helpers;
>  - make an explicit calculation of the end of input line at the
>    beginning, so users of the bitmap_parse() won't bother doing this.

> +static inline bool in_str(const char *start, const char *ptr)
> +{
> +	return start <= ptr;
> +}
> +

I don't see how it's better than explicit use. Moreover, explicit use shows the
exact condition in-line. Even by used characters it's longer.

> +static const char *bitmap_get_hex32_rev(const char *start,
> +					const char *end, u32 *num)

In kernel few functions to work with hex u32 named foo_x32(). I would rather
use that. Besides, we spell "reverse" in full.

> +{
> +	u32 ret = 0;
> +	int c, i;
> +
> +	if (hex_to_bin(*end) < 0)
> +		return ERR_PTR(-EINVAL);
> +
> +	for (i = 0; i < 32; i += 4) {
> +		c = hex_to_bin(*end--);
> +		if (c < 0)

Perhaps we may need similar patch for hex_to_bin() as in the commit
9888a588ea96 ("lib/hexdump.c: return -EINVAL in case of error in hex2bin()")

> +			return ERR_PTR(-EINVAL);

> +
> +		ret |= c << i;
> +
> +		if (!in_str(start, end) || __end_of_region(*end))
> +			goto out;
> +	}
> +
> +	if (hex_to_bin(*end) >= 0)
> +		return ERR_PTR(-EOVERFLOW);
> +out:
> +	*num = ret;
> +	return end;
> +}

-- 
With Best Regards,
Andy Shevchenko