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Date:   Thu, 7 Jul 2022 09:37:57 -0500
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Subject: Re: [PATCH 5/8] arm64: Create cache sysfs directory without ACPI PPTT
 for hardware prefetch control
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To:     "tarumizu.kohei@fujitsu.com" <tarumizu.kohei@fujitsu.com>,
        'Sudeep Holla' <sudeep.holla@arm.com>
Cc:     "catalin.marinas@arm.com" <catalin.marinas@arm.com>,
        "will@kernel.org" <will@kernel.org>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "bp@alien8.de" <bp@alien8.de>,
        "dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>,
        "x86@kernel.org" <x86@kernel.org>, "hpa@zytor.com" <hpa@zytor.com>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
References: <20220125071414.811344-1-tarumizu.kohei@fujitsu.com>
 <20220125071414.811344-6-tarumizu.kohei@fujitsu.com>
 <20220126103614.pcrcuc2kzklkq3xa@bogus>
 <OSBPR01MB2037E92722D372D48F54438E80269@OSBPR01MB2037.jpnprd01.prod.outlook.com>
From:   Jeremy Linton <jeremy.linton@arm.com>
In-Reply-To: <OSBPR01MB2037E92722D372D48F54438E80269@OSBPR01MB2037.jpnprd01.prod.outlook.com>
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Hi,

On 2/1/22 05:56, tarumizu.kohei@fujitsu.com wrote:
>> I am assuming this is ACPI enabled system.
> 
> Yes, it is ACPI enabled system.
> 
>> This looks bit hacky in my opinion. Before I explore better way of adding it, I would
>> like to check if you have explored ways to add PPTT reading these registers from
>> UEFI/EDK2 as PPTT has other topology information which you will need anyways.
>> That would simplify handling of these cacheinfo sysfs in the kernel. Let me know
>> what are your thoughts ?
> 
> The latest firmware of ARM64 machine, FX700 with the A64FX processor
> does not support PPTT.
> I think adding PPTT is the best way to generate cacheinfo sysfs.
> However, it is difficult to modify the firmware to add PPTT, so
> it is not clear when it will be possible.
> Therefore, I would like to implement the function in the kernel on
> the condition that firmware does not support PPTT.

As a bit of a late comment here, I assume you tried injecting the PPTT 
via the initrd (directions in 
admin-guide/acpi/initrd_table_override.txt) then? That is one of the 
usual kernel workarounds for broken/missing ACPI tables.

As mentioned above, besides not providing appropriate topology 
information to userspace, not having the PPTT is also possibly causing 
suboptimal scheduling decisions in the kernel itself.