Re: [RFC] Correct memory layout reporting for "jedec,lpddr2" and related bindings

[Krzysztof Kozlowski <krzk@kernel.org>]

On 28/07/2022 02:22, Julius Werner wrote:
>>> By "use case" I mean our particular platform and firmware requirements
>>> -- or rather, the realities of building devices with widely
>>> multi-sourced LPDDR parts. One cannot efficiently build firmware that
>>> can pass an exact vendor-and-part-specific compatible string to Linux
>>> for this binding for every single LPDDR part used on such a platform.
>>
>> Why cannot? You want to pass them as numerical values which directly map
>> to vendor ID and some part, don't they?
> 
> Yes, but the current compatible string format also requires the exact
> part number, of which there are many thousands and it's impossible to
> build a list in advance. Even for vendors, hardcoding 255 strings in a
> tight firmware space would be an unnecessary burden. 

There are 25 for LPDDR2/3 and and 12 for LPDD4 (although many reserved
so it might grow to ~32). You will not have 255 of them, although I
actually don't insist on that - we can code manufacturer ID as well.

> There's also an
> update problem -- firmware may be built and signed and burned into ROM
> long before the assembly of the final mainboard. Board manufacturers
> want to be able to just drop-in replace a newly-sourced LPDDR part in
> their existing production line without having to worry if the existing
> (and possibly no longer changeable) firmware contains a string table
> entry for this part.
> 
> If you just want the compatible string to be unique, encoding the
> numbers like Doug suggested (e.g. jedec,lpddr3-ff-0100) would work for
> us.
> 
>> If we talk about standard, then DT purpose is not for autodetectable
>> pieces. These values are autodetectable, so such properties should not
>> be encoded in DT.
> 
> But the DT is the only interface that we have to pass information from
> firmware to kernel and userspace. Where else should these properties
> be encoded? They are auto-detectable, but not for the kernel itself
> (only for memory-training firmware running in SRAM). Maybe the usual
> rules of thumb don't apply here, because unlike all other peripheral
> controllers the memory controller is special in that the kernel cannot
> simply reinitialize it and get the same information from the original
> source again.

True, I thought these registers are aliased or also exposed as memory
controllers, but at least for one MC I don't see it so kernel cannot
read them.

Best regards,
Krzysztof