Re: [RFC PATCH 01/10] spi: spi-mem: Introduce support for tuning controller

["Michael Walle" <mwalle@kernel.org>]

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On Wed Dec 3, 2025 at 10:50 AM CET, Miquel Raynal wrote:
>
>>>> I think we should start with the requirement to have the pattern flashed
>>>> already and figure out how SPI NOR or SPI NAND can discover that
>>>> (perhaps via NVMEM?).
>>
>> But we should also keep in mind that certain flashes might return
>> tuning data during the dummy cycles. I.e. the PHY might probably be
>> tuned on each read and there is no need for any pre-programmed
>> pattern.
>>
>> I'm not saying it should be implemented, but the current
>> implementation should be that flexible that it will be easy to add
>> that later.
>
> Conceptually, yes, but in practice, I know no controller capable of
> using just a few cycles every transfer to calibrate themselves
> automatically and reaching such an optimized speed state as the cadence
> controller is capable of ATM.

Then have a look at the flexspi controller. I.e. look at the LS1028A
reference manual "18.5.15.1 Data Learning with Flash providing
preamble bit".  The sequence is a follows:

<CMD> <ADDR> <MODE> <DUMMY> <LEARN> <READ>

There's an example with the learning pattern as short as 8 bit, or
- I guess - 8 clock cycles.

> Despite the end result being close, I would still consider this other
> way to optimize the I/Os somewhat orthogonal. If someone has some
> knowledge to share about the training patterns sent during the dummy
> cycles, I am all ears though.

There's also a short chapter about the training. Basically, it will
just compare the read bits with a predefined value (which is max
32 bit long) of 16 different clock phases. Which one is chosen is
not answered though (ideally it should be the one at the center of
all matching clock phases).

Now how good that tuning actually is, I don't know. But the
procedure sounds sane. I'm also not sure whether this (any?) tuning
will account for different I/O trace lengths of if it is assumed
that they have to be trace length matched for multi IO flashes.

-michael

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