[PATCH] spi-nor: sfdp: Allow configuring unknown flashes using SFDP

[Vignesh Raghavendra]

On 5/27/21 9:15 PM, Petr Malat wrote:
> On Thu, May 27, 2021 at 07:22:19PM +0530, Vignesh Raghavendra wrote:
>>>>> This change allows adding a support for flashes with correct SFDP
>>>>> without recompilation of the kernel by setting sfdp-compatible property
>>>>> in their node. Alternatively, sfdp_compatible module option can be used
>>>>> to list JEDEC IDs of flashes, whose SFDP can be trusted. Star "*" can
>>>>> be used to match all JEDEC IDs.
>>>>
>>>> I have skimmed through the patch. Before I look at it more closely, I 
>>>> want to understand the use case for this patch. Why would you not want 
>>>> to recompile the kernel when adding support for new hardware? Do you 
>>>> want the ability to support flashes on devices that have already been 
>>>> deployed in the field? Is it something that comes up frequently?
>>> In my case the kernel is loaded from a USB mass storage device, which
>>> can be preproduced and on stock (with the kernel already on it). With
>>> my patch I can change the flash vendor without the need of updating
>>> the image on already existing USB mass storage devices.
>>>
>>> The patch is also useful for people who use distribution kernel as they
>>> will not have to wait until (and if) the distribution updates it.
>>
>> Don't need separate DT property or cmdline argument. There is no way to
>> know if the SFDP tables are 100% valid when kernel is working with a
>> "generic flash".
>> Flashes are often replaced with newer revisions of the part that may/may
>> not have valid SFDP tables.
>>
>> So just rely on SFDP, when no valid entry is found in the manufacturer's
>> flash_info[] while printing out a warning to user that this is just best
>> effort support.
> 
> OK, I will rework it to fallback to SFDP by default. I put the safeguard
> there to avoid a case when one has a flash requiring workarounds and then
> does a kernel downgrade to a release where the flash is not known and uses
> it. Should I make an opposite flag sfdp-incompatible or do we consider this
> a theoretical scenario not worth of addressing in the code?
>  

We can guarantee new kernels don't regress wrt older version on
upgrading. But anyone downgrading kernel should expect some HW support
to be missing (not just SPI flash but many other things in general).
I don't see a need to work about such scenario

Regards
Vignesh