[PATCH v3 01/13] spi: dt-bindings: allow spi-max-frequency to specify a frequency pair

[Miquel Raynal]

Hi Conor,

>> >> > Right, and this I guess is what scuppers letting the controller driver
>> >> > sort the configuration out itself and leaving the property as-is.
>> >> > It could be that the speed in spi-max-frequency is lower than the "base
>> >> > speed" of the controller but because of board routing or device
>> >> > capability that the tuned mode is still required, right?
>> >> 
>> >> I do not actually expect any tuned mode/frequency to be mandatory.
>> >
>> > I think you misunderstood my use of "required", I meant that the new
>> > property/information was needed in the scenario I described, not that it
>> > should be a required property in a binding.
>> 
>> Yes I misunderstood the term indeed. However I still fail to catch what
>> you meant here, I'm sorry. Would you mind rephrasing?
>
> I was talking about a scenario where you want to use the tuned mode to
> achieve the maximum rate because of the device and/or board configuration,
> but the rate is below the point where the controller would need tuning.
> Say the controller needs tuning above 8 Hz but the conditions require
> tuning to achieve more than 5 Hz. In this example, if the device can do
> 6 Hz, spi-max-frequency (in the current form) would be set to 6 Hz, and
> the controller would not enable the tuned mode, leading to problems
> because the inflection point determined from the controller compatible
> of 8 Hz would not have been reached.

I don't think this is a real situation. If the "conditions", as you say
(ie. PCB routing, mostly) require tuning above 5, then spi-max-frequency
should be 5. It is the frequency that is reachable without any
tuning. Tuning is just a plus. If tuning fails, we fallback to the
regular "base" speed, which just works.

Thanks,
Miquèl