linux-kernel - Re: [PATCH v2 2/7] iio: adc: ti-adc128s052: Simplify using be16_to

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Message-ID: <1189b539-adb4-46f9-9783-c6577b57a304@gmail.com>
Date: Thu, 3 Apr 2025 08:16:43 +0300
From: Matti Vaittinen <mazziesaccount@...il.com>
To: David Lechner <dlechner@...libre.com>,
 Matti Vaittinen <matti.vaittinen@...rohmeurope.com>
Cc: Jonathan Cameron <jic23@...nel.org>, Lars-Peter Clausen
 <lars@...afoo.de>, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Nuno Sa <nuno.sa@...log.com>,
 Javier Carrasco <javier.carrasco.cruz@...il.com>, linux-iio@...r.kernel.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/7] iio: adc: ti-adc128s052: Simplify using
 be16_to_cpu()

On 03/04/2025 00:04, David Lechner wrote:
> On 4/2/25 1:09 AM, Matti Vaittinen wrote:
>> The register data is 12-bit big-endian data. Use be16_to_cpu() to do
>> the conversion, and simple bitwise AND for masking to make it more
>> obvious.
>>
>> Signed-off-by: Matti Vaittinen <mazziesaccount@...il.com>
>> ---
>> Revision history:
>> v1 => v2:
>>   - Fix commit msg to reflect the fact there was no bug
>>   - Drop Fixes tag
>>   - Use union for rx / tx buffer to avoid casting
>>   - Keep the shared message protected by the mutex
>> ---
>>   drivers/iio/adc/ti-adc128s052.c | 18 ++++++++++--------
>>   1 file changed, 10 insertions(+), 8 deletions(-)
>>
>> diff --git a/drivers/iio/adc/ti-adc128s052.c b/drivers/iio/adc/ti-adc128s052.c
>> index a456ea78462f..3e69a5fce010 100644
>> --- a/drivers/iio/adc/ti-adc128s052.c
>> +++ b/drivers/iio/adc/ti-adc128s052.c
>> @@ -28,32 +28,34 @@ struct adc128 {
>>   	struct regulator *reg;
>>   	struct mutex lock;
>>   
>> -	u8 buffer[2] __aligned(IIO_DMA_MINALIGN);
>> +	union {
>> +		__be16 rx_buffer;
>> +		u8 tx_buffer[2];
>> +	} __aligned(IIO_DMA_MINALIGN);
>>   };
>>   
>>   static int adc128_adc_conversion(struct adc128 *adc, u8 channel)
>>   {
>>   	int ret;
>> +	char *msg = &adc->tx_buffer[0];
>>   
>>   	mutex_lock(&adc->lock);
>>   
>> -	adc->buffer[0] = channel << 3;
>> -	adc->buffer[1] = 0;
>> +	msg[0] = channel << 3;
>> +	msg[1] = 0;
>>   
>> -	ret = spi_write(adc->spi, &adc->buffer, 2);
>> +	ret = spi_write(adc->spi, msg, sizeof(adc->tx_buffer));
>>   	if (ret < 0) {
>>   		mutex_unlock(&adc->lock);
>>   		return ret;
>>   	}
>>   
>> -	ret = spi_read(adc->spi, &adc->buffer, 2);
>> -
>> +	ret = spi_read(adc->spi, &adc->rx_buffer, 2);
>>   	mutex_unlock(&adc->lock);
>> -
>>   	if (ret < 0)
>>   		return ret;
>>   
>> -	return ((adc->buffer[0] << 8 | adc->buffer[1]) & 0xFFF);
>> +	return be16_to_cpu(adc->rx_buffer) & 0xFFF;
> 
> 
> The cast isn't exactly beautiful, but this would save a lot of
> lines of diff and a few lines of code by avoiding the need for
> the union and the local msg variable.
> 
> 	return be16_to_cpup((__be16 *)adc->buffer) & 0xFFF;

Thanks again for the review David :)

I am unsure which way to go. I kind of like having the __be16 in the 
struct, as it immediately yells "data from device is big-endian". OTOH, 
I've never loved unions (and, it silences the above "yelling" quite a 
bit). I still think this might be the first time I really see a valid 
use-case for an union :) And, you're right this adds more lines, 
besides, the cast doesn't look that ugly to me. Yet, I originally had a 
cast probably as simple as this (and I also had the __be16 in the 
struct), and Jonathan suggested using union to avoid it...

At the end of the day, I suppose I am Okay with any of these 3 
approaches. Original cast, union or this cast you suggest. Jonathan, any 
preferences on your side?

> 
>>   }
>>   
>>   static int adc128_read_raw(struct iio_dev *indio_dev,
> 

Yours,
	-- Matti