linux-kernel - Re: [PATCH RFC 1/2] ACPI/PPTT: Add acpi_pptt_get_package

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Date:   Tue, 28 Jan 2020 14:04:19 +0000
From:   John Garry <john.garry@...wei.com>
To:     Sudeep Holla <sudeep.holla@....com>
CC:     <rjw@...ysocki.net>, <lenb@...nel.org>, <jeremy.linton@....com>,
        <arnd@...db.de>, <olof@...om.net>, <linux-kernel@...r.kernel.org>,
        <linux-acpi@...r.kernel.org>, <guohanjun@...wei.com>,
        <gregkh@...uxfoundation.org>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        wanghuiqiang <wanghuiqiang@...wei.com>
Subject: Re: [PATCH RFC 1/2] ACPI/PPTT: Add acpi_pptt_get_package_info() API

On 28/01/2020 12:34, Sudeep Holla wrote:

Hi Sudeep,

> On Tue, Jan 28, 2020 at 07:14:18PM +0800, John Garry wrote:
>> The ACPI PPTT ID structure (see 6.2 spec, section 5.2.29.3) allows the
>> vendor to provide an identifier (or vendor specific part number) for a
>> particular processor hierarchy node structure. That may be a processor
>> identifier for a processor node, or some chip identifier for a processor
>> package node.
>>
> 
> Unfortunately, there were plans to deprecate this in favour of the new
> SOC_ID SMCCC API[1]. I am not sure if you or anyone in your company have
> access to UEFI ASWG mantis where you can look for the ECR for the PPTT
> Type 2 deprecation.

I wasn't aware and I can't get access...

Personally I would rather PPTT ID structure have a fixed field 
definition in future spec versions, rather than deprecate.

 From checking here, nobody has even used it (properly) for processor 
package nodes:
https://github.com/tianocore/edk2-platforms/tree/master/Platform

  I understand it's not ideal, but we need to converge,
> please take a look at both before further discussion.

I can only check the SMCCC extension which you pointed me at.

> 
> I personally would not prefer to add the support when I know it is getting
> deprecated. I am not sure on kernel community policy on the same.

So I need a generic solution for this, as my userspace tool requires a 
generic solution.

> 
> 
> [...]
> 
>>
>> The ID structure table has a number of fields, which are left open to
>> interpretation per implementation. However the spec does provide reference
>> examples of how the fields could be used. As such, just provide the
>> table fields directly in the API, which the caller may interpret (probably
>> as per spec example).
>>
> 
> The "open for interpretation" part is why it's not being favoured anymore
> by silicon vendors as OEM/ODMs can override the same.
> 
>> https://lore.kernel.org/linux-arm-kernel/1579876505-113251-6-git-send-email-john.garry@huawei.com/
>>
> Ah, there's already quite a lot of dependency built for this feature :(

Not really. It's only an RFC ATM, and my requirement is a sysfs file to 
read the SoC id(s) (under ACPI FW). So I would still expect to be able 
to support this from the SMCCC extension method.

> 
> --
> Regards,
> Sudeep
> 
> [1] https://developer.arm.com/docs/den0028/c
> .
> 

Cheers,
John