Message-ID: <CAGb2v65h8zaxoEKeqdT8BZD9t=4gf0QM7zBnhuDoiEhHQLKduw@mail.gmail.com>
Date: Thu, 5 Sep 2024 20:26:15 +0800
From: Chen-Yu Tsai <wens@...e.org>
To: Dragan Simic <dsimic@...jaro.org>
Cc: Andre Przywara <andre.przywara@....com>, linux-sunxi@...ts.linux.dev, 
	jernej.skrabec@...il.com, samuel@...lland.org, 
	linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org, 
	robh@...nel.org, krzk+dt@...nel.org, conor+dt@...nel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: dts: allwinner: a64: Move CPU OPPs to the SoC dtsi file

Hi,

On Thu, Sep 5, 2024 at 8:17 PM Dragan Simic <dsimic@...jaro.org> wrote:
>
> Hello,
>
> Just checking, any further thoughts about this patch?

Sorry, but I feel like it's not really worth the churn. There's not
really a problem to be solved here. What you are arguing for is more
about aesthetics, and we could argue that having them separate makes
it easier to read and turn on/off.

And even though the GPU OPPs are in the dtsi, it's just one OPP acting
as a default clock rate.


ChenYu

> On 2024-08-17 06:25, Dragan Simic wrote:
> > Hello Andre,
> >
> > On 2024-08-15 19:15, Andre Przywara wrote:
> >> On Thu, 15 Aug 2024 18:34:58 +0200
> >> Dragan Simic <dsimic@...jaro.org> wrote:
> >>> On 2024-08-14 18:11, Chen-Yu Tsai wrote:
> >>> > On Wed, Aug 14, 2024 at 1:52 PM Dragan Simic <dsimic@...jaro.org>
> >>> > wrote:
> >>> >>
> >>> >> Move the Allwinner A64 CPU OPPs to the A64 SoC dtsi file and,
> >>> >> consequently,
> >>> >> adjust the contents of the affected board dts(i) files appropriately,
> >>> >> to
> >>> >> "encapsulate" the CPU OPPs into the SoC dtsi file.
> >>> >>
> >>> >> Moving the CPU OPPs to the SoC dtsi file, instead of requiring the
> >>> >> board
> >>> >> dts(i) files to include both the SoC dtsi file and the CPU OPP dtsi
> >>> >> file,
> >>> >> reduces the possibility for incomplete SoC data inclusion and improves
> >>> >> the
> >>> >> overall hierarchical representation of data.  Moreover, the CPU OPPs
> >>> >> are
> >>> >> not used anywhere but together with the SoC dtsi file, which
> >>> >> additionally
> >>> >> justifies the folding of the CPU OPPs into the SoC dtsi file.
> >>> >>
> >>> >> No functional changes are introduced, which was validated by
> >>> >> decompiling and
> >>> >> comparing all affected board dtb files before and after these changes.
> >>> >>  When
> >>> >> compared with the decompiled original dtb files, the updated dtb files
> >>> >> have
> >>> >> some of their blocks shuffled around a bit and some of their phandles
> >>> >> have
> >>> >> different values, as a result of the changes to the order in which the
> >>> >> building blocks from the parent dtsi files are included into them, but
> >>> >> they
> >>> >> still effectively remain the same as the originals.
> >>> >
> >>> > IIRC, this was a conscious decision requiring board dts files to set
> >>> > their
> >>> > CPU supply before OPPs are given. The bootloader does not boot the SoC
> >>> > at the highest possible OPP / regulator voltage, so if the OPPs are
> >>> > given
> >>> > but the supply is not, the kernel will attempt to raise the frequency
> >>> > beyond what the current voltage can supply, causing it to hang.
> >>
> >> Yes, this is what I remember as well: this forces boards to opt in to
> >> DVFS, otherwise they get a fixed 816 MHz. Since there is only one OPP
> >> table for all boards with that SoC, I think it's reasonable to ask for
> >> this, since the cooling could not be adequate for higher frequencies
> >> in
> >> the first place, or the power supply is not up to par.
> >
> > If the cooling isn't capable enough to dissipate the additional heat
> > generated at higher frequencies, the thermal governor is there to
> > handle
> > that by lowering the operating frequency.  If the PSU isn't capable to
> > provide an additional watt or two, I think a better PSU is needed. :)
> > No reasonably sized PSU should work at ~100% of its power output.
> >
> > On top of that, all currently supported A64-based boards have the CPU
> > OPPs defined and CPU DVFS enabled, so no such issues are possible
> > there.
> > Though, there could be some issues with new A64-based boards, which is
> > discussed further below.
> >
> >>> > Now that all existing boards have it properly enabled, there should be
> >>> > no
> >>> > need for this. However I would appreciate a second opinion.
> >>
> >> Well, since there is no way to opt *out* now, I am somewhat reluctant
> >> to
> >> just have this. What is the actual problem we are solving here? After
> >> all
> >> there is just one OPP table for all A64 boards, so there is less
> >> confusion
> >> about what to include in each board file. Which IIUC is a more
> >> complicated
> >> situation on the Rockchip side.
> >
> > Well, this patch doesn't solve some real problem, but it makes the
> > things
> > neater and a bit more clean.  The things are more complicated with
> > Rockchip
> > SoCs, but following the concept of "encapsulating" the CPU OPPs into
> > the
> > A64 SoC dtsi makes things neater.  Moreover, the A64 GPU OPPs are
> > already
> > in the A64 SoC dtsi, so we could also say that folding the A64 CPU OPPs
> > into the SoC dtsi follows the A64 GPU OPPs.
> >
> >> I still have to try "operating-points-v2", but at least on the H616
> >> side
> >> putting a 'status = "disabled";' into the OPP node didn't prevent it
> >> from
> >> probing. Otherwise this would have been a nice compromise, I think.
> >>
> >>> Good point, thanks for the clarification.  This is quite similar to
> >>> how
> >>> board dts(i) files for Rockchip SoCs need to enable the SoC's
> >>> built-in
> >>> TSADC for temperature sensing, before the CPU thermal throttling can
> >>> actually work and prevent the SoC from overheating, etc.
> >>>
> >>> The consensus for Rockchip boards is that it's up to the authors and
> >>> reviewers of the board dts(i) files to make sure that the built-in
> >>> TSADC
> >>> is enabled, etc.  With that approach in mind, and knowing that all
> >>> Allwinner
> >>> A64 board dts(i) files are in good shape when it comes to the
> >>> associated
> >>> voltage regulators, I think it's fine to follow the same approach of
> >>> "encapsulating" the CPU OPPs into the A64 SoC dtsi file.
> >>
> >> As mentioned above, I am not so sure about this. With this patch here,
> >> *every* board gets DVFS. And while this seems to be fine when looking
> >> at
> >> the current DTs in the tree (which have it anyway), it creates a
> >> potentially dangerous situation for new boards.
> >>
> >> So pragmatically speaking, this patch would be fine, but it leaves me
> >> a
> >> bit uneasy about future or downstream boards.
> >
> > Frankly, I wouldn't be worried about that.  When a new A64-based board
> > is added, it should be verified that CPU DVFS works as expected, etc.,
> > before the new board dts file is accepted upstream.
> >
> > Maybe we could take into account some possible issues when someone
> > starts
> > putting together a new A64-based board dts file, but there are already
> > many dangerous things that someone can do in the process, such as
> > messing
> > up various regulators and voltages unrelated to the CPU DVFS, so
> > everyone
> > putting a new board dts file together simply have to know what are they
> > doing.  I see no way for escaping from that need.
> >

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