Re: ACPI support for System Control and Management Interface (SCMI)

From: Hanjun Guo <guohanjun@huawei.com>
To: Souvik Chakravarty <Souvik.Chakravarty@arm.com>,
	Sudeep Holla <Sudeep.Holla@arm.com>
Cc: Thanu Rangarajan <Thanu.Rangarajan@arm.com>,
	John Garry <john.garry@huawei.com>,
	Linuxarm <linuxarm@huawei.com>,
	Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>,
	Matteo Carlini <Matteo.Carlini@arm.com>,
	Jonathan Cameron <jonathan.cameron@huawei.com>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>,
	Charles Garcia-Tobin <Charles.Garcia-Tobin@arm.com>
Subject: Re: ACPI support for System Control and Management Interface (SCMI)
Date: Sat, 18 Apr 2020 10:16:14 +0800	[thread overview]
Message-ID: <e5e56ffc-6c7b-4e4c-7a85-5cc5c557e960@huawei.com> (raw)
In-Reply-To: <DBBPR08MB4790D0779B7933EEF5C1689F82DA0@DBBPR08MB4790.eurprd08.prod.outlook.com>

Hi Sudeep, Souvik,

On 2020/4/14 20:08, Souvik Chakravarty wrote:
>> From: Sudeep Holla <sudeep.holla@arm.com>
>> Subject: Re: ACPI support for System Control and Management Interface
>> (SCMI)
>>
>> On Fri, Apr 10, 2020 at 10:26:41AM +0800, Hanjun Guo wrote:
>>> Hi Sudeep,
>>>
>>> We are working on the enablement of SCMI for Kunepeng920, which is a
>>> server chip using ACPI, but I didn't see the ACPI support in the
>>> mainline kernel for SCMI.
>>>
>>> Kunpeng920 and the upcomming Kunpeng930 have thermal sensors and
>> other
>>> system management features which needs the SCMI for abstraction layer,
>>> do we have plan to support ACPI for SCMI?
>>>
>>
>> Yes and No.
>>
>> Yes, because the same SCMI compliant firmware needs to work with ACPI as
>> SCMI is designed so.
>>
>> No, because we don't plan to add another SCMI layer below ACPI in the
>> kernel. ACPI specification was extended to support special PCC types to
>> support the same. IIUC things like sensors are supported using power meter
>> objects in ACPI and you must be able to use PCC opregion for it or any
>> custom mechanism to achieve that. However one need to add PCC opregion
>> handler in Linux. The main idea is you must use existing ACPI methods and
>> we don't plan to add anything special for SCMI.
> 
> Additionally, if you already have an SCMI compliant firmware that uses 
> Shared Memory based transport, it is compatible with PCC Type 3 Channels > in ACPI.

We had a disscusion interally (with firmware folks) and this works
for our sensor usecase too, thanks!

> 
>>
>> SCMI Perf protocol is compatible with CPPC. We can discuss details once you
>> explore PCC opregion or any alternative you come up with. I am no ACPI
>> firmware expert and I reckon you may have other interesting ideas.

Sensor works via PCC channels, and SCMI based clock/reset/cpufreq also
have other abstraction level to make it work on ACPI based system,
so for now no more requirements are needed, I will update if we got
new ones :)

Thanks
Hanjun

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