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

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Date:   Mon, 11 Sep 2023 08:15:51 +0300
From:   Matti Vaittinen <mazziesaccount@...il.com>
To:     Jonathan Cameron <jic23@...nel.org>
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

On 9/8/23 21:45, Jonathan Cameron wrote:
> 
>> +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..

Great. Then I think it means we really can treat the combined value of 
the registers as s16 BE. Should make this much simpler :)

> 
>> +		 * 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;
>> +}

-- 
Matti Vaittinen
Linux kernel developer at ROHM Semiconductors
Oulu Finland

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