Re: [PATCH v2 3/5] dt-bindings: iio: adc: Add adi,ad4052

[David Lechner <dlechner@baylibre.com>]

On 6/2/25 4:17 AM, Jorge Marques wrote:
> On Tue, Apr 29, 2025 at 10:45:20AM -0500, David Lechner wrote:
>> On 4/29/25 8:48 AM, Jorge Marques wrote:
>>> Hi David, 
>>>
>>> I didn't went through your's and Jonathan's ad4052.c review yet,
>>> but for the trigger-source-cells I need to dig deeper and make
>>> considerable changes to the driver, as well as hardware tests.
>>> My idea was to have a less customizable driver, but I get that it is
>>> more interesting to make it user-definable.
>>
>> We don't need to make the driver support all possibilities, but the devicetree
>> needs to be as complete as possible since it can't be as easily changed in the
>> future.
>>
> 
> Ack.
> 
> I see that the node goes in the spi controller (the parent). To use the
> same information in the driver I need to look-up the parent node, then
> the node. I don't plan to do that in the version of the driver, just an
> observation.
> 
> There is something else I want to discuss on the dt-bindings actually.
> According to the schema, the spi-max-frequency is:
> 
>   > Maximum SPI clocking speed of the device in Hz.
> 
> The ad4052 has 2 maximum speeds: Configuration mode (lower) and ADC Mode
> (higher, depends on VIO). The solution I came up, to not require a
> custom regmap spi bus, is to have spi-max-frequency bound the
> Configuration mode speed,

The purpose of spi-max-frequency in the devicetree is that sometimes
the wiring of a complete system makes the effective max frequency
lower than what is allowed by the datasheet. So this really needs
to be the absolute highest frequency allowed.

> and have ADC Mode set by VIO regulator
> voltage, through spi_transfer.speed_hz. At the end of the day, both are
> bounded by the spi controller maximum speed.

If spi_transfer.speed_hz > spi-max-frequency, then the core SPI code
uses spi-max-frequency. So I don't think this would actually work.

> 
> My concern is that having ADC mode speed higher than spi-max-frequency
> may be counter-intuitive, still, it allows to achieve the max data sheet
> speed considering VIO voltage with the lowest code boilerplate.
> 
> Let me know if I can proceed this way before submitting V3.