Re: [PATCH 03/11] MIPS: support RAM beyond 32-bit

[Jiaxun Yang]

在2023年10月9日十月 下午4:59，Gregory CLEMENT写道：
> Hello Jiaxun,
>
>> 在2023年10月4日十月 下午5:10，Gregory CLEMENT写道：
>>> From: Vladimir Kondratiev <vladimir.kondratiev@xxxxxxxxx>
>>>
>>> Support platforms where RAM is mapped beyond 32-bit.
>>>
>>> The kernel parameter ddr32_alias allows to setup the alias to point
>>> outside the first 4 GB of memory.
>>
>> Are you trying to fix the problem that if kernel text is loaded in
>> XKPHYS there is no way to to set EBASE to that region?
>
> Yes that exactly we try to fix.
>
>>
>> The common practice for other 64bit MIPS system is to load kernel
>> in KSEG0 and add low 4G mirror with rest of the high memory to buddy
>> system. By doing this Kernel still have access to all memory beyond
>> 32 bit, the only draw back is Kernel's text and data can't be relocted
>> beyond 32-bit.
>>
>> Loading kernel into KSEG0 (i.e. with KBUILD_SYM32) have significant benefit
>> on performance, so I think you shouldn't try to load kernel into XKPHYS
>> without a good reason, but it might be helpful to add a BUG_ON at
>> CPS driver to handle such situation.
>
> I guess that being in KSEG0 allows to use shorter pointer.  But in our
> case the RAM is physically connected beyond 32bits, so it is not
> accessible in KSEG0.

For most system there should be a mirror of part of DDR which is accessible
at KSEG0 and kernel runs from here. As per my interpretion of your code EyeQ5
is also doing this? If not could you please briefly describe the memory map?

For Kernel in KSEG0 the pointer is still 64bit but we can use fewer inst
to load ABS pointer into register, see [1].

>>
>> Btw: Is your target hardware publicly available? Folks at CIP United
>> are looking for EyeQ5 boards for a while, they are supporting MIPS R6
>> support at various projects.
>
> We use evaluation boards and I don't know if they are publicly
> available.
>
> Gregory
>
[1]: https://elinux.org/images/1/1f/New-tricks-mips-linux.pdf

Thanks
- Jiaxun