Re: [PATCH v2 09/10] iio: adc: ad7476: Support ROHM BD79105

[Matti Vaittinen <mazziesaccount@gmail.com>]

On 08/08/2025 00:28, Andy Shevchenko wrote:
> On Thu, Aug 7, 2025 at 11:35 AM Matti Vaittinen
> <mazziesaccount@gmail.com> wrote:
>>
>> The ROHM BD79105 is a simple 16-bit ADC accessible via SPI*.
>>
>> The BD79105 has a CONVSTART pin, which must be set high to start the ADC
>> conversion. Unlike with the ad7091 and ad7091r which also have a
>> CONVSTART pin, the BD79105 requires that the pin must remain high also
>> for the duration of the SPI access.
>>
>> (*) Couple of words about the SPI. The BD79105 has pins named as
>> CONVSTART, SCLK, DIN and DOUT. For the curious reader, DIN is not SPI
>> ISO.
>>
>> DIN is a signal which can be used as a chip-select. When DIN is pulled
>> low, the ADC will output the completed measurement via DOUT as SCLK is
>> clocked. According to the data-sheet, the DIN can also be used for
>> daisy-chaining multiple ADCs. Furthermore, DOUT can be used also for a
>> 'data-ready' -IRQ. These modes aren't supported by this driver.
>>
>> Support reading ADC scale and data from the BD79105 using SPI, when DIN
>> is used as a chip-select.
> 
> ...
> 
>> +static void bd79105_convst_enable(struct ad7476_state *st)
>> +{
>> +       if (!st->convst_gpio)
>> +               return;
> 
> Still consider this unneeded churn. 3us delay is tolerable in almost
> any setup with this driver.

Very simple and 100% clear if (), which avoids uninterruptible 
busy-wait. I _really_ 100% disagree with you.

Luckily Nuno suggested a solution which I think will mitigate the need 
to fight over this. :)


>> +       gpiod_set_value(st->convst_gpio, 1);
>> +       /* Worst case, 2790 nS required for conversion */
> 
> nS --> ns (SI unit for seconds is 's')
> 
>> +       ndelay(2790);
>> +}
> 
> ...
> 
>> +       /*
>> +        * The BD79105 starts ADC data conversion when the CONVSTART line is
>> +        * set HIGH. The CONVSTART must be kept HIGH until the data has been
>> +        * read from the ADC.
> 
> Is this terminology in absolute levels of the pin or logical ones
> (that implied active-low)? If it's the latter, use active/inactive
> instead as the GPIO subsystem does.

Ah. This is electrical high. I have:
adi,conversion-start-gpios = <&gpio2 2 GPIO_ACTIVE_HIGH>;
in the DT. Open to suggestions how to make this more obvious. Thanks!

Yours,
	-- Matti