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Date: Fri, 14 Jun 2024 03:44:36 +0100
From: "Jiaxun Yang" <jiaxun.yang@flygoat.com>
To: "Huacai Chen" <chenhuacai@kernel.org>
Cc: "Jianmin Lv" <lvjianmin@loongson.cn>, "Xuerui Wang" <kernel@xen0n.name>,
 loongarch@lists.linux.dev, linux-kernel@vger.kernel.org,
 "stable@vger.kernel.org" <stable@vger.kernel.org>
Subject: Re: [PATCH 2/2] LoongArch: Fix ACPI standard register based S3 support
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=E5=9C=A82024=E5=B9=B46=E6=9C=8814=E6=97=A5=E5=85=AD=E6=9C=88 =E4=B8=8A=E5=
=8D=883:32=EF=BC=8CHuacai Chen=E5=86=99=E9=81=93=EF=BC=9A
> On Fri, Jun 14, 2024 at 10:25=E2=80=AFAM Jiaxun Yang <jiaxun.yang@flyg=
oat.com> wrote:
>>
>>
>>
>> =E5=9C=A82024=E5=B9=B46=E6=9C=8814=E6=97=A5=E5=85=AD=E6=9C=88 =E4=B8=8A=
=E5=8D=883:11=EF=BC=8CHuacai Chen=E5=86=99=E9=81=93=EF=BC=9A
>> > Hi, Jiaxun,
>> >
>> > On Fri, Jun 14, 2024 at 12:41=E2=80=AFAM Jiaxun Yang <jiaxun.yang@f=
lygoat.com> wrote:
>> >>
>> >> Most LoongArch 64 machines are using custom "SADR" ACPI extension
>> >> to perform ACPI S3 sleep. However the standard ACPI way to perform
>> >> sleep is to write a value to ACPI PM1/SLEEP_CTL register, and this
>> >> is never supported properly in kernel.
>> > Maybe our hardware is insane so we need "SADR", if so, this patch m=
ay
>> > break real hardware. What's your opinion, Jianmin?
>>
>> I understand why your hardware need SADR. Most systems DDR self-refre=
sh
>> mode needs to be setup by firmware.
> _S3 is also a firmware method, why we can't use it to setup self-refre=
sh?

That's the problem from ACPI spec. As per ACPI spec _S3 method only tells
you what should you write into PM1 or SLEEP_CTL register, it will NOT pe=
rform
actual task to enter sleeping. (See 16.1.3.1 [1])

On existing LoongArch hardware _S3 method is only used to mark presence =
of S3
state. This is violating ACPI spec, but I guess we must live with it.

Furthermore, on Loongson hardware you have to disable access to DDR memo=
ry
to access DDR controller's configuration registers. Which means self-ref=
resh
code must run from BIOS ROM.

[1]: https://uefi.org/htmlspecs/ACPI_Spec_6_4_html/16_Waking_and_Sleepin=
g/sleeping-states.html

Thanks
- Jiaxun

>
> Huacai
>
>>
>> There is no chance that it may break real hardware. When firmware sup=
plied
>> SADR it will always use SADR. The fallback only happens when _S3 meth=
od exist
>> but no SADR supplied, which won't happen on real hardware.
>>
>> For QEMU we don't have stub firmware but standard compliant SEEP_CTL =
is
>> sufficient for entering sleep mode, thus we need this fallback path.
>>
>> Thanks
>>
>> >
>> > Huacai
>> >
>> >>
>>
>> --
>> - Jiaxun
>>

--=20
- Jiaxun