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Date:   Wed, 21 Jun 2023 18:52:20 +0200
From:   Andrew Lunn <andrew@lunn.ch>
To:     "Limonciello, Mario" <mario.limonciello@amd.com>
Cc:     Johannes Berg <johannes@sipsolutions.net>,
        Evan Quan <evan.quan@amd.com>, rafael@kernel.org,
        lenb@kernel.org, alexander.deucher@amd.com,
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        daniel@ffwll.ch, davem@davemloft.net, edumazet@google.com,
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        maarten.lankhorst@linux.intel.com, tzimmermann@suse.de,
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Subject: Re: [PATCH V4 1/8] drivers/acpi: Add support for Wifi band RF
 mitigations
Message-ID: <36902dda-9e51-41b3-b5fc-c641edf6f1fb@lunn.ch>
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On Wed, Jun 21, 2023 at 11:15:00AM -0500, Limonciello, Mario wrote:
> 
> On 6/21/2023 10:39 AM, Johannes Berg wrote:
> > On Wed, 2023-06-21 at 17:36 +0200, Andrew Lunn wrote:
> > > On Wed, Jun 21, 2023 at 01:45:56PM +0800, Evan Quan wrote:
> > > > From: Mario Limonciello <mario.limonciello@amd.com>
> > > > 
> > > > Due to electrical and mechanical constraints in certain platform designs
> > > > there may be likely interference of relatively high-powered harmonics of
> > > > the (G-)DDR memory clocks with local radio module frequency bands used
> > > > by Wifi 6/6e/7.
> > > > 
> > > > To mitigate this, AMD has introduced an ACPI based mechanism that
> > > > devices can use to notify active use of particular frequencies so
> > > > that devices can make relative internal adjustments as necessary
> > > > to avoid this resonance.
> > > Do only ACPI based systems have:
> > > 
> > >     interference of relatively high-powered harmonics of the (G-)DDR
> > >     memory clocks with local radio module frequency bands used by
> > >     Wifi 6/6e/7."
> > > 
> > > Could Device Tree based systems not experience this problem?
> > They could, of course, but they'd need some other driver to change
> > _something_ in the system? I don't even know what this is doing
> > precisely under the hood in the ACPI BIOS, perhaps it adjusts the DDR
> > memory clock frequency in response to WiFi using a frequency that will
> > cause interference with harmonics.
> 
> The way that WBRF has been architected, it's intended to be able
> to scale to any type of device pair that has harmonic issues.

So you set out to make something generic...

> In the first use (Wifi 6e + specific AMD dGPUs) that matches this
> series BIOS has the following purposes:
> 
> 1) The existence of _DSM indicates that the system may not have
> adequate shielding and should be using these mitigations.
> 
> 2) Notification mechanism of frequency use.
> 
> For the first problematic devices we *could* have done notifications
> entirely in native Linux kernel code with notifier chains.
> However that still means you need a hint from the platform that the
> functionality is needed like a _DSD bit.
> 
> It's also done this way so that AML could do some of the notifications
> directly to applicable devices in the future without needing "consumer"
> driver participation.

And then tie is very closely to ACPI.

Now, you are AMD, i get that ACPI is what you have. But i think as
kernel Maintainers, we need to consider that ACPI is not the only
thing used. Do we want the APIs to be agnostic? I think APIs used by
drivers should be agnostic.

      Andrew