Re: [PATCH] iio: adc: ad4000: Avoid potential double data word read

[David Lechner <dlechner@baylibre.com>]

On 4/15/25 1:02 PM, Andy Shevchenko wrote:
> On Tue, Apr 15, 2025 at 3:21 PM Marcelo Schmitt
> <marcelo.schmitt@analog.com> wrote:
>>
>> Currently, SPI-Engine offload module always sends 32-bit data elements to
>> DMA engine. Appropriately, when set for SPI offloading, the IIO driver uses
>> 32 storagebits for IIO ADC channel buffer elements. However, setting SPI
>> transfer length according to storagebits (32-bits in case of offload) can
>> lead to unnecessarily long transfers for ADCs that are 16-bit or less
>> precision. Adjust AD4000 single-shot read to run transfers of 2 bytes when
>> that is enough to get all ADC data bits.
> 
> ...
> 
>>         xfers[1].rx_buf = &st->scan.data;
>> -       xfers[1].len = BITS_TO_BYTES(chan->scan_type.storagebits);
>> +       xfers[1].len = chan->scan_type.realbits > 16 ? 4 : 2;
> 
> But wouldn't be logical to have
> 
>        xfers[1].len = BITS_TO_BYTES(chan->scan_type.realbits);
> 
> ?
> 

No, SPI expects 1, 2 or 4 bytes, never 3. If realbits is 18, we
need len = 4.

It would have to be:

	xfers[1].len = roundup_pow_of_two(BITS_TO_BYTES(chan->scan_type.realbits));

But that gets too long for 1 line, so I prefer what Marcelo wrote.

Maybe an idea for another day:

#define SPI_LEN_FOR_BITS(bits) roundup_pow_of_two(BITS_TO_BYTES(bits))

There are a couple of places in spi/ that could use this and several
iio drivers.