linux-kernel - Re: [PATCH v1 03/18] arm64: dts: mediatek: mt6795: xperia-m5: Enable Frequency Hopping

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Date:   Mon, 27 Mar 2023 10:09:35 +0200
From:   AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>
To:     Pavel Machek <pavel@....cz>
Cc:     matthias.bgg@...il.com, qii.wang@...iatek.com, robh+dt@...nel.org,
        krzysztof.kozlowski+dt@...aro.org, jassisinghbrar@...il.com,
        houlong.wei@...iatek.com, linux-i2c@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, kernel@...labora.com,
        phone-devel@...r.kernel.org, ~postmarketos/upstreaming@...ts.sr.ht
Subject: Re: [PATCH v1 03/18] arm64: dts: mediatek: mt6795: xperia-m5: Enable
 Frequency Hopping

Il 25/03/23 22:11, Pavel Machek ha scritto:
> On Fri 2023-03-24 18:54:41, AngeloGioacchino Del Regno wrote:
>> Enable FHCTL with Spread Spectrum for MAINPLL, MPLL and MSDCPLL
>> as found on the downstream kernel for this smartphone.
>> Which one to enable, and at what SSC percentage, was found by
>> dumping the debugging data from a running downstream kernel and
>> checking the downstream code.
> 
> What advantages does this have? Lower EMI emissions?
> 

Hello Pavel,

this is done for multiple reasons, main one being lower EMI, but the effects
on functionality, depending on the actual board, may be quite dramatic and
ranging from lower WiFi and/or modem signal "sensitivity" (as in, for example,
having to automatically lower signal gain because of signal disturbance from
the conducted EMI ripple), to even a complete platform crash due to clocking
issues.

This is not about accounting for hardware quirks because having this situation
is totally normal and expected, especially on "particular" usecases such as
smartphones, where the actual board is typically much constrained in terms of
space, and where on the same board you have multiple components generating noise,
either wanted (a nfc/wifi/bt/2g/3g/4g/5g/something-else signal), or unwanted.

Cheers!
Angelo