[RFC 6/9] regmap: Speed up _regmap_raw_write_impl() for large buffers

[Mark Brown]

On Sun, Feb 16, 2020 at 06:21:14PM +0100, Noralf Trønnes wrote:

> When writing a 3MB buffer the unwritable check in _regmap_raw_write_impl()
> adds a ~20ms overhead on a Raspberry Pi 4.
> Amend this by avoiding the check if it's not necessary.

This is a generic optimization, why is it mixed in with the rest of this
series?  There is no dependency either way :(

>  	/* Check for unwritable registers before we start */
> -	for (i = 0; i < val_len / map->format.val_bytes; i++)
> -		if (!regmap_writeable(map,
> -				     reg + regmap_get_offset(map, i)))
> -			return -EINVAL;
> +	if (map->max_register || map->writeable_reg || map->wr_table) {
> +		for (i = 0; i < val_len / map->format.val_bytes; i++)
> +			if (!regmap_writeable(map,
> +					      reg + regmap_get_offset(map, i)))
> +				return -EINVAL;
> +	}

This is going to break if there is any change to the implementation of
regmap_writeable().  The code should at least be next to that if not
actually shared so that this doesn't happen.  I'd suggest implementing a
function regmap_writeable_range() and then making regmap_writeable()
call that.
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