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Date:   Tue, 10 Oct 2023 09:55:40 +0100
From:   "Jiaxun Yang" <jiaxun.yang@flygoat.com>
To:     "Gregory CLEMENT" <gregory.clement@bootlin.com>,
        "paulburton@kernel.org" <paulburton@kernel.org>,
        "Thomas Bogendoerfer" <tsbogend@alpha.franken.de>,
        "linux-mips@vger.kernel.org" <linux-mips@vger.kernel.org>,
        "Rob Herring" <robh+dt@kernel.org>,
        "Krzysztof Kozlowski" <krzysztof.kozlowski+dt@linaro.org>,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org
Cc:     "Vladimir Kondratiev" <vladimir.kondratiev@intel.com>,
        "Tawfik Bayouk" <tawfik.bayouk@mobileye.com>,
        "Alexandre Belloni" <alexandre.belloni@bootlin.com>,
        =?UTF-8?Q?Th=C3=A9o_Lebrun?= <theo.lebrun@bootlin.com>,
        "Thomas Petazzoni" <thomas.petazzoni@bootlin.com>
Subject: Re: [PATCH 03/11] MIPS: support RAM beyond 32-bit
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=E5=9C=A82023=E5=B9=B410=E6=9C=889=E6=97=A5=E5=8D=81=E6=9C=88 =E4=B8=8B=E5=
=8D=884:59=EF=BC=8CGregory CLEMENT=E5=86=99=E9=81=93=EF=BC=9A
> Hello Jiaxun,
>
>> =E5=9C=A82023=E5=B9=B410=E6=9C=884=E6=97=A5=E5=8D=81=E6=9C=88 =E4=B8=8B=
=E5=8D=885:10=EF=BC=8CGregory CLEMENT=E5=86=99=E9=81=93=EF=BC=9A
>>> From: Vladimir Kondratiev <vladimir.kondratiev@intel.com>
>>>
>>> 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 b=
enefit
>> on performance, so I think you shouldn't try to load kernel into XKPH=
YS
>> 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 accessi=
ble
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