linux-kernel - Re: [PATCH] acpi: Allow ignoring _OSC CPPC v2 bit via kernel parameter

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Date: Tue, 18 Jun 2024 14:25:41 -0500
From: Aaron Rainbolt <arainbolt@...cus.org>
To: Mario Limonciello <mario.limonciello@....com>
Cc: linux-acpi@...r.kernel.org, rafael@...nel.org, lenb@...nel.org,
	linux-kernel@...r.kernel.org, mmikowski@...cus.org,
	Perry.Yuan@....com
Subject: Re: [PATCH] acpi: Allow ignoring _OSC CPPC v2 bit via kernel
 parameter

On Tue, Jun 18, 2024 at 01:58:07PM -0500, Mario Limonciello wrote:
> On 6/18/2024 13:52, Aaron Rainbolt wrote:
> > On Tue, Jun 18, 2024 at 01:35:57PM -0500, Mario Limonciello wrote:
> > > On 6/18/2024 13:30, Aaron Rainbolt wrote:
> > > > On Tue, Jun 18, 2024 at 12:09:19PM -0500, Mario Limonciello wrote:
> > > > > On 6/17/2024 21:54, Aaron Rainbolt wrote:
> > > > > > acpi: Allow ignoring _OSC CPPC v2 bit via kernel parameter
> > > > > > 
> > > > > > The _OSC is supposed to contain a bit indicating whether the hardware
> > > > > > supports CPPC v2 or not. This bit is not always set, causing CPPC v2 to
> > > > > > be considered absent. This results in severe single-core performance
> > > > > > issues with the EEVDF scheduler.
> > > > > > 
> > > > > > To work around this, provide a new kernel parameter,
> > > > > > "processor.ignore_osc_cppc_bit", which may be used to ignore the _OSC
> > > > > > CPPC v2 bit and act as if the bit was enabled. This allows CPPC to be
> > > > > > properly detected even if not "enabled" by _OSC, allowing users with
> > > > > > problematic hardware to obtain decent single-core performance.
> > > > > > 
> > > > > > Tested-by: Michael Mikowski <mmikowski@...cus.org>
> > > > > > Signed-off-by: Aaron Rainbolt <arainbolt@...cus.org>
> > > > > 
> > > > > This sounds like a platform bug and if we do accept a patch like this I
> > > > > think we need a lot more documentation about the situation.
> > > > 
> > > > It is a platform bug, yes. See my previous email,
> > > > https://lore.kernel.org/linux-acpi/d01b0a1f-bd33-47fe-ab41-43843d8a374f@kfocus.org/T/#u
> > > > (I meant to send this email as a reply to that one, but failed to do so.)
> > > > 
> > > > > Can you please share more information about your hardware:
> > > > > 1) Manufacturer?
> > > > 
> > > > Carbon Systems, models Iridium 14 and Iridium 16.
> > > > 
> > > > > 2) CPU?
> > > > 
> > > > Intel Core i5-13500H.
> > > > 
> > > > > 3) Manufacturer firmware version?
> > > > 
> > > > The systems use an AMI BIOS with version N.1.10CAR01 according to
> > > > dmidecode. This is the latest BIOS available from the manufacturer.
> > > > 
> > > > > 4) If it's AMD what's the AGESA version?
> > > > 
> > > > Both affected systems are Intel-based and use heterogenous cores, not AMD.
> > > > 
> > > > > And most importantly do you have the latest system firmware version from
> > > > > your manufacturer?  If not; please upgrade that first.
> > > > 
> > > > We are using the latest firmware. (We're trying to work with the ODM to
> > > > potentially get a firmware update, but since this affects more than just
> > > > us and a firmware update may not be possible for everyone, this would
> > > > likely be worth providing a kernel-level workaround for.)
> > > > 
> > > > I can easily provide more detailed information - would the full output of
> > > > 'dmidecode' and 'acpidump' be useful?
> > > 
> > > Does your BIOS offer any options for these?
> > > 
> > > Intel(R) SpeedStep(TM)
> > > Intel Speed Shift Technology(TM)
> > > 
> > > I believe you need those enabled for this to work properly.
> > 
> > Neither option is available in the BIOS settings UI, however our ODM
> > confirmed that both Intel Speed Shift Technology and Intel Turbo Boost Max
> > Technology 3.0 are enabled by default. They did not mention SpeedStep,
> > but I assume SpeedStep is working since frequency scaling in general
> > works and the kernel patch fixes the issue.
> 
> Got it.  If those are enabled I think it would be good to get comments from
> Rafael and Srinivas about your specific situation then.
> 
> But regarding the patch, if they are agreeable to this "kind" of knob for
> debugging I personally think it's better to have cpc_supported_by_cpu() look
> at the kernel command line than plumb arguments from the module down through
> every function.

Just to be clear since I'm not all too familiar with how kernel params work,
should core_param be used here? Or is there a variable that allows
accessing the entire command line to look through it? I don't think I can
use module_param in 'arch/x86/kernel/acpi/cppc.c', core_param has a
comment over it describing it as "historical" so I don't think I should
use it, and early_param looks like something one is only supposed to use
in code that runs very early at kernel startup. I can probably figure it
out on my own, but a quick pointer would be helpful.