netdev - Re: [RFC 3/7] regmap: allow upshifting register addresses before performing operations

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Date:   Fri, 24 Mar 2023 13:08:47 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Maxime Chevallier <maxime.chevallier@...tlin.com>
Cc:     Mark Brown <broonie@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        rafael@...nel.org, Colin Foster <colin.foster@...advantage.com>,
        Vladimir Oltean <vladimir.oltean@....com>,
        Lee Jones <lee@...nel.org>, davem@...emloft.net,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        thomas.petazzoni@...tlin.com
Subject: Re: [RFC 3/7] regmap: allow upshifting register addresses before
 performing operations

> index da8996e7a1f1..2ef53936652b 100644
> --- a/drivers/base/regmap/internal.h
> +++ b/drivers/base/regmap/internal.h
> @@ -31,7 +31,7 @@ struct regmap_format {
>  	size_t buf_size;
>  	size_t reg_bytes;
>  	size_t pad_bytes;
> -	size_t reg_downshift;
> +	int reg_shift;

Maybe ssize_t to keep with the pattern of using size_t. However,
ssize_t is somewhat over sized for a bit shift. So maybe just s8?

>  	size_t val_bytes;
>  	void (*format_write)(struct regmap *map,
>  			     unsigned int reg, unsigned int val);
> diff --git a/drivers/base/regmap/regmap.c b/drivers/base/regmap/regmap.c
> index 726f59612fd6..c4cde4f45b05 100644
> --- a/drivers/base/regmap/regmap.c
> +++ b/drivers/base/regmap/regmap.c
> @@ -814,7 +814,7 @@ struct regmap *__regmap_init(struct device *dev,
>  
>  	map->format.reg_bytes = DIV_ROUND_UP(config->reg_bits, 8);
>  	map->format.pad_bytes = config->pad_bits / 8;
> -	map->format.reg_downshift = config->reg_downshift;
> +	map->format.reg_shift = config->reg_shift;
>  	map->format.val_bytes = DIV_ROUND_UP(config->val_bits, 8);
>  	map->format.buf_size = DIV_ROUND_UP(config->reg_bits +
>  			config->val_bits + config->pad_bits, 8);
> @@ -1679,7 +1679,13 @@ static void regmap_set_work_buf_flag_mask(struct regmap *map, int max_bytes,
>  static unsigned int regmap_reg_addr(struct regmap *map, unsigned int reg)
>  {
>  	reg += map->reg_base;
> -	return reg >> map->format.reg_downshift;
> +
> +	if (map->format.reg_shift > 0)
> +		reg >>= map->format.reg_shift;
> +	else if (map->format.reg_shift < 0)
> +		reg <<= -(map->format.reg_shift);

I was wondering about negative shifts. It is apparently undefined
behaviour. So this construct is required.

	   Andrew