linux-kernel - Re: [PATCH v2 1/1] arm64: dts: mediatek: mt8186: Increase CCI frequency

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Message-ID: <f0403753f64eaa6e62249a1d787dc9153dd8e733.camel@mediatek.com>
Date: Mon, 26 Feb 2024 09:12:06 +0000
From: Chun-Jen Tseng (曾俊仁)
	<Chun-Jen.Tseng@...iatek.com>
To: "matthias.bgg@...il.com" <matthias.bgg@...il.com>, "wenst@...omium.org"
	<wenst@...omium.org>, "angelogioacchino.delregno@...labora.com"
	<angelogioacchino.delregno@...labora.com>, "robh+dt@...nel.org"
	<robh+dt@...nel.org>, "krzysztof.kozlowski+dt@...aro.org"
	<krzysztof.kozlowski+dt@...aro.org>, "conor+dt@...nel.org"
	<conor+dt@...nel.org>
CC: "linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linux-mediatek@...ts.infradead.org"
	<linux-mediatek@...ts.infradead.org>, "devicetree@...r.kernel.org"
	<devicetree@...r.kernel.org>, Project_Global_Chrome_Upstream_Group
	<Project_Global_Chrome_Upstream_Group@...iatek.com>
Subject: Re: [PATCH v2 1/1] arm64: dts: mediatek: mt8186: Increase CCI
 frequency

On Wed, 2024-01-10 at 12:10 +0100, AngeloGioacchino Del Regno wrote:
> Il 10/01/24 06:44, Chun-Jen Tseng (曾俊仁) ha scritto:
> > On Wed, 2023-11-29 at 14:22 +0100, AngeloGioacchino Del Regno
> > wrote:
> > > Il 14/09/23 14:10, Mark Tseng ha scritto:
> > > > The original CCI OPP table's lowest frequency 500 MHz is too
> > > > low
> > > > and causes
> > > > system stalls. Increase the frequency range to 1.05 GHz ~ 1.4
> > > > GHz
> > > > and adjust
> > > > the OPPs accordingly.
> > > > 
> > > > Fixes: 32dfbc03fc26 ("arm64: dts: mediatek: mt8186: Add CCI
> > > > node
> > > > and CCI OPP table")
> > > > 
> > > > Signed-off-by: Mark Tseng <chun-jen.tseng@...iatek.com>
> > > 
> > > You ignored my comment [1] on the v1 of this patch.
> > > 
> > > Besides, I think that you should at least keep the 500MHz
> > > frequency
> > > for a
> > > sleep-only/idle OPP to save power.
> > > 
> > > It would also be helpful to understand why you chose this new
> > > frequency range,
> > > so if you can, please put some numbers in the commit description,
> > > showing the
> > > stall in terms of requested BW vs actual BW (as I'd imagine that
> > > a 2x
> > > increase
> > > in CCI frequency means that we need *twice* the bandwidth
> > > compared to
> > > what we
> > > have for the workloads that are stalling the system).
> > > 
> > 
> > Hi AngeloGioacchino Del Regno,
> > 
> > Thanks your reminder this issue. After ajdustment CCI OPP, we also
> > do
> > power test benchmark and the result is PASS.
> > 
> 
> Sorry but `PASS` is not a number; I actually wanted a before and
> after power
> consumption measurement in microwatts.
> 
> > The original CCI table has stall issue.  When the Big CPU frequency
> > set
> > on 2.05G and CCI frequency keep on 500MHz then run CTS MediaTest
> > will
> > system stall then trigger watchdog reset SoC.
> > 
> > The CPU and CCI frequency setting are not in the same driver. So it
> > will have timing issue cause CPU stall side effect.
> > 
> 
> Are you trying to fix a frequency setting delay/desync with raising
> the
> frequency of the CCI?
> That's not the right way of doing it.
> 
> Asserting that we have a timing issue because the two frequency
> settings
> are not done by the same driver is borderline wrong - but anyway - if
> there
> is a frequency setting timing issue because of the interaction
> between the
> two drivers (cpufreq/ccifreq), the right way of eliminating the stall
> is to
> actually solve the root cause of that.
> 
> I'm insisting on this because if there's a "timing issue" this means
> that
> even though the "base" CCI frequency is higher, during a scaling up
> operation
> depending on how much the CCI gets flooded, you might *either*:
>   - Have this same stall issue again, and/or
>   - Have performance issues/drops while waiting for the CCI to scale
> up.
> 
> Even though you may not (or may...) get a stall issue again with this
> change,
> you will surely get (very short) temporary performance drops during
> scaling up.
> 
> ....and this is why your CCI frequency increase solution does *not*
> resolve
> this issue, but only partially mitigates it.
> 
> That should get solved, not partially mitigated.
> 
> Besides that, can you please tell me how to replicate the stall
> issue, making
> me able to better understand what's going on here?
> 
> Regards,
> Angelo
> 

Hi Angelo,

Thanks your review and suggestion.

This issue happen on CTS mediaTest and WebGL test.

I found that if CCI under 1.05 GHz, it will happend randomly system
stall so I do workaround modification on devfreq driver to limit CCI
level (1.05 Ghz). It look solve this issue.

BRs,

Mark Tseng



> > BRs,
> > 
> > Mark Tseng
> > 
> > > [1]:
> > > 
https://lore.kernel.org/all/799325f5-29b5-f0c0-16ea-d47c06830ed3@collabora.com/
> > > 
> > > Regards,
> > > Angelo
> 
>