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

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Message-ID: <4f099833-5aa6-47cc-917c-7a466cb644b9@gmail.com>
Date: Mon, 7 Apr 2025 08:23:07 +0300
From: Matti Vaittinen <mazziesaccount@...il.com>
To: Jonathan Cameron <jic23@...nel.org>
Cc: David Lechner <dlechner@...libre.com>,
 Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
 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 05/04/2025 20:29, Jonathan Cameron wrote:
> On Thu, 3 Apr 2025 08:16:43 +0300
> Matti Vaittinen <mazziesaccount@...il.com> wrote:
> 
>> 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];
> As below. Maybe
> 		__be16 buffer16;
> 		u8 buffer[2];

Ok.

>>>> +	} __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));
> 
> I'd get rid of msg as it's now just confusing given we are
> using the sizeof() here.

Ok.

>>>>    	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);
> 
> sizeof(adc->rx_buffer)

I was thinking of this but went with raw 2 - because we need to read 
exactly 2 bytes from the device. Sizeof buffer is matter of software 
where as the 2 bytes is dictated by the device. (Sure the size of buffer 
needs to be large enough).

I don't care it that much though, so I can go with the sizeof() if 
that's what you prefer. Just explaining that the '2' here was a 
conscious choice :)

>>>>    	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;
> 
> The cast only works because we have forced the alignment for DMA safety.
> That to me is a little fragile.
> 
> You could do get_unaligned_be16() which doesn't need the cast then carry
> on using the original buffer.
>>
>> 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?
> 
> Majority of the diff is really about renaming buffer to tx_buffer.
> Could just not bother doing that and instead have buffer and buffer16
> as the two union elements. With msg gone as suggested above, then the diff
> becomes only a few lines and you get to keep the nicety of it being either
> a pair of u8s or a __be16.

I was tempted to try using the spi_write_then_read() - but I suppose 
this may be kind of a hot path.

I'll go with (not)renaming the buffer and dropping the msg, to squeeze 
the diff.

Yours,
	-- Matti