linux-kernel - Re: [PATCH] thermal/drivers/intel/int340x: Add DLVR support

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Date:   Tue, 04 Apr 2023 10:55:28 -0700
From:   srinivas pandruvada <srinivas.pandruvada@...ux.intel.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     rui.zhang@...el.com, daniel.lezcano@...aro.org,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] thermal/drivers/intel/int340x: Add DLVR support

On Tue, 2023-04-04 at 18:46 +0200, Rafael J. Wysocki wrote:
> On Tue, Apr 4, 2023 at 6:39 PM srinivas pandruvada
> <srinivas.pandruvada@...ux.intel.com> wrote:
> > 
> > On Mon, 2023-04-03 at 20:37 +0200, Rafael J. Wysocki wrote:
> > > On Fri, Mar 31, 2023 at 6:53 PM Srinivas Pandruvada
> > > <srinivas.pandruvada@...ux.intel.com> wrote:
> > > > 
> > > > Add support for DLVR (Digital Linear Voltage Regulator)
> > > > attributes,
> > > > which can be used to control RFIM.
> > > > Here instead of "fivr" another directory "dlvr" is created with
> > > > DLVR
> > > > attributes:
> > > > 
> > > > /sys/bus/pci/devices/0000:00:04.0/dlvr
> > > > ├── dlvr_freq_mhz
> > > > ├── dlvr_freq_select
> > > > ├── dlvr_hardware_rev
> > > > ├── dlvr_pll_busy
> > > > ├── dlvr_rfim_enable
> > > > └── dlvr_spread_spectrum_pct
> > > > 
> > > > Attributes
> > > > dlvr_freq_mhz (RO):
> > > > Current DLVR PLL frequency in MHz.
> > > > 
> > > > dlvr_freq_select (RW):
> > > > Sets DLVR PLL clock frequency.
> > > > 
> > > > dlvr_hardware_rev (RO):
> > > > DLVR hardware revision.
> > > > 
> > > > dlvr_pll_busy (RO):
> > > > PLL can't accept frequency change when set.
> > > > 
> > > > dlvr_rfim_enable (RW):
> > > > 0: Disable RF frequency hopping, 1: Enable RF frequency
> > > > hopping.
> > > > 
> > > > dlvr_spread_spectrum_pct (RW)
> > > > A write to this register updates the DLVR spread spectrum
> > > > percent
> > > > value.
> > > 
> > > How is this attribute going to be used by user space in practice?
> > 
> > Spread spectrum percent helps to reduce the DLVR clock noise to
> > meet
> > regulatory compliance. This spreading % increases bandwidth of
> > signal
> > transmission and hence reduces the effects of interference, noise,
> > and
> > signal fading.
> 
> The above information should be added to the documentation I think.
> 
> Still, I would like to know when user space is going to write to it
> and how it is going to find out what value to write.
As specified in the
https://docs.kernel.org/driver-api/thermal/intel_dptf.html
This is all related to reduce interference with WiFi radio frequencies.

The algorithm should be read current dlvr_freq_mhz,
dlvr_spread_spectrum_pct, current WiFi frequency (channel has a fix
freq), find the error in WiFi frame error rates (From WiFi module), and
do small adjustment +- to dlvr_freq. While changing the dlvr
frequencies you may induce more interference so you spread the signal
to reduce S/N ratio using this percent knob.

> 
> > > Also should it be split like the frequency one (for consistency)?
> > 
> > This is a RW field and is applied immediately unlike frequency,
> > where
> > it is two step process. First you specify and enable and then see
> > the
> > effect. So they are two fields.
> 
> I was talking about dlvr_freq_mhz (RO) and dlvr_freq_select (RW). 
> I'm
> not sure how the above is related to them TBH.
The frequency is the base signal to which interference must be reduced.
A good explanation for this technique
https://www.eetimes.com/tutorial-on-spread-spectrum-technology/

Thanks,
Srinivas