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

[Santhosh Kumar K <s-k6@ti.com>]

Hello Miquel and Pratyush,

On 18/11/25 19:12, Pratyush Yadav wrote:
> On Wed, Nov 05 2025, Miquel Raynal wrote:
> 
>> Hello Santhosh,
>>
>>>>>      - On tuning failure, retry by re-running spi_mem_needs_tuning() with
>>>>> the second best set of ops (max throughput - 1)
>>>> I would like to challenge this need. Can the same calibration fail if
>>>> attempted multiple times (eg. because of the heat?) If yes, then we need
>>>> a fallback indeed. Otherwise, I'd be in favor of just failing the
>>>> probe. Calibration is an opt-in -> users must allow a higher frequency
>>>> than they use to in order to enable the feature?
>>>
>>> It's possible the same calibration will fail intermittently for
>>> different reasons (temperature changes, as you mentioned). If tuning
>>> fails, the driver should fallback to the non-PHY frequency so the flash
>>> continues operating with slower reads/writes rather than failing the
>>> probe (availability should be prioritized, right?).
>>
>> Agreed, if the tuning may fail we must fallback in this case. However
>> there is another situation that must be handled in this case: once
>> tuning is done and we want to use PHY-optimized paths, we must fallback
>> to more basic/slower reads if for some external reason, they start
>> failing, right?
> 
> How would you even detect that your tuning is out-of-date because of
> temperature changes? You would need some sort of on-flash ECC to detect
> that. I think many of the flashes that support DDR reads at high
> frequencies also have ECC, but AFAIK the SPI NOR core does not support
> it.
> 
> Anyway, I think we should limit the scope of the problem. Let's first
> start with the expectation that the tuning supports the whole operation
> range of the device. This was true at least for the spi-cadence-quadspi
> tuning that I worked on when I was at TI. The tuning parameters had
> enough margin to ensure it worked for the device's whole temperature
> range.
> 
> If there is a tuning algorithm that can't do that, then we can extend
> the core to either do ECC or perhaps let temperature sensors signal the
> need for re-calibration.
> 
> But for now I think it is easiest to just ignore the problem and focus
> on the other ones like how to get the calibration pattern and how to do
> the tuning.
> 
>>
>> The obvious choice in this case would be to let this error handling to
>> the controller driver. Re-using the same operation at a lower speed
>> would be suboptimal, because the fastest operation at a high speed might
>> not be the most efficient at slower speeds due to the number of dummy
>> cycles needed,. But I believe this is negligible based on the fact that
>> we already are in degraded mode at that stage.
>>
>> However, this may conflict with:
>> - read retries
>> - continuous reads (?)
>>
>> So in practice the fallback might be needed on the SPI NAND/NOR side
>> (this can be further discussed).

Just to summarize, fallback logic during probe:
    - If the controller reports a tuning failure, the spi-mem client may
either retry tuning with the next-best (max-1) operation or fallback to
the non-PHY, slower operation and adjust the dummy cycles accordingly to
use the optimal non-PHY variant.

And yes, for now the priority is to have a robust probe-time tuning flow
before addressing any runtime tuning concerns.

>>
>> But once we solve this, comes a similar problem on the write side. How
>> do we know if a write will or did fail because of a temperature change?
>> What may be the heuristics to fallback in this case?
> 
> Santhosh, do you have any numbers on write performance improvements? I
> am curious if it is even worth the effort.

There's no real performance gain for SPI NOR, but SPI NAND shows notable
improvement wrt. page size.

Write performance numbers from AM62A SK with W35N01JW OSPI NAND:

    - without PHY: 6 MB/s
    - with PHY: 9.2 MB/s

Thanks,
Santhosh.

> 
>>
>> Thanks,
>> Miquèl
>