linux-kernel - Re: [PATCH 2/3] iio: pressure: Support ROHM BU1390

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Message-ID: <20230908194509.6a4f4adc@jic23-huawei>
Date:   Fri, 8 Sep 2023 19:45:09 +0100
From:   Jonathan Cameron <jic23@...nel.org>
To:     Matti Vaittinen <mazziesaccount@...il.com>
Cc:     Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Angel Iglesias <ang.iglesiasg@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Andreas Klinger <ak@...klinger.de>, linux-iio@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] iio: pressure: Support ROHM BU1390


> +static int bm1390_read_temp(struct bm1390_data *data, int *temp)
> +{
> +	u8 temp_reg[2] __aligned(2);
> +	__be16 *temp_raw;
> +	int ret;
> +	s16 val;
> +	bool negative;
> +
> +	ret = regmap_bulk_read(data->regmap, BM1390_REG_TEMP_HI, &temp_reg,
> +			       sizeof(temp_reg));
> +	if (ret)
> +		return ret;
> +
> +	if (temp_reg[0] & 0x80)
> +		negative = true;
> +	else
> +		negative = false;
> +
> +	temp_raw = (__be16 *)&temp_reg[0];
> +	val = be16_to_cpu(*temp_raw);
> +
> +	if (negative) {
> +		/*
> +		 * Two's complement. I am not sure if all supported
> +		 * architectures actually use 2's complement represantation of

AFAIK they do. Many IIO drivers would be broken if not..

> +		 * signed ints. If yes, then we could just do the endianes
> +		 * conversion and say this is the s16 value. However, as I
> +		 * don't know, and as the conversion is pretty simple. let's
> +		 * just convert the signed 2's complement to absolute value and
> +		 * multiply by -1.
> +		 */
> +		val = ~val + 1;
> +		val *= -1;
> +	}
> +
> +	*temp = val;
> +
> +	return 0;
> +}