Message-ID: <20220704215214.52d1816f@slackpad.lan>
Date:   Mon, 4 Jul 2022 21:52:14 +0100
From:   Andre Przywara <andre.przywara@....com>
To:     Samuel Holland <samuel@...lland.org>
Cc:     Jernej Škrabec <jernej.skrabec@...il.com>,
        Chen-Yu Tsai <wens@...e.org>, Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Icenowy Zheng <icenowy@...c.io>,
        linux-arm-kernel@...ts.infradead.org, linux-sunxi@...ts.linux.dev,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v11 3/6] arm64: dts: allwinner: Add Allwinner H616 .dtsi
 file

On Mon, 4 Jul 2022 13:44:18 -0500
Samuel Holland <samuel@...lland.org> wrote:

Hi Samuel,

> On 7/4/22 8:30 AM, Andre Przywara wrote:
> > On Sat, 02 Jul 2022 23:16:53 +0200
> > Jernej Škrabec <jernej.skrabec@...il.com> wrote:  
> >> Dne četrtek, 30. junij 2022 ob 02:04:10 CEST je Andre Przywara napisal(a):  
> >>> On Tue, 03 May 2022 21:05:11 +0200
> >>> Jernej Škrabec <jernej.skrabec@...il.com> wrote:  
> >>>> Dne petek, 29. april 2022 ob 01:09:30 CEST je Andre Przywara napisal(a):    
> >>>>> This (relatively) new SoC is similar to the H6, but drops the (broken)
> >>>>> PCIe support and the USB 3.0 controller. It also gets the management
> >>>>> controller removed, which in turn removes *some*, but not all of the
> >>>>> devices formerly dedicated to the ARISC (CPUS).
> >>>>> And while there is still the extra sunxi interrupt controller, the
> >>>>> package lacks the corresponding NMI pin, so no interrupts for the PMIC.
> >>>>>
> >>>>> The reserved memory node is actually handled by Trusted Firmware now,
> >>>>> but U-Boot fails to propagate this to a separately loaded DTB, so we
> >>>>> keep it in here for now, until U-Boot learns to do this properly.
> >>>>>
> >>>>> Signed-off-by: Andre Przywara <andre.przywara@....com>
> >>>>> ---
> >>>>>
> >>>>>  .../arm64/boot/dts/allwinner/sun50i-h616.dtsi | 574 ++++++++++++++++++
> >>>>>  1 file changed, 574 insertions(+)
> >>>>>  create mode 100644 arch/arm64/boot/dts/allwinner/sun50i-h616.dtsi
> >>>>>
> >>>>> diff --git a/arch/arm64/boot/dts/allwinner/sun50i-h616.dtsi
> >>>>> b/arch/arm64/    
> >>>>
> >>>> boot/dts/allwinner/sun50i-h616.dtsi
> >>>>     
> >>>>> new file mode 100644
> >>>>> index 000000000000..cc06cdd15ba5
> >>>>> --- /dev/null
> >>>>> +++ b/arch/arm64/boot/dts/allwinner/sun50i-h616.dtsi
> >>>>> @@ -0,0 +1,574 @@
> >>>>> +// SPDX-License-Identifier: (GPL-2.0+ OR MIT)
> >>>>> +// Copyright (C) 2020 Arm Ltd.
> >>>>> +// based on the H6 dtsi, which is:
> >>>>> +//   Copyright (C) 2017 Icenowy Zheng <icenowy@...c.io>
> >>>>> +
> >>>>> +#include <dt-bindings/interrupt-controller/arm-gic.h>
> >>>>> +#include <dt-bindings/clock/sun50i-h616-ccu.h>
> >>>>> +#include <dt-bindings/clock/sun50i-h6-r-ccu.h>
> >>>>> +#include <dt-bindings/reset/sun50i-h616-ccu.h>
> >>>>> +#include <dt-bindings/reset/sun50i-h6-r-ccu.h>
> >>>>> +
> >>>>> +/ {
> >>>>> +	interrupt-parent = <&gic>;
> >>>>> +	#address-cells = <2>;
> >>>>> +	#size-cells = <2>;
> >>>>> +
> >>>>> +	cpus {
> >>>>> +		#address-cells = <1>;
> >>>>> +		#size-cells = <0>;
> >>>>> +
> >>>>> +		cpu0: cpu@0 {
> >>>>> +			compatible = "arm,cortex-a53";
> >>>>> +			device_type = "cpu";
> >>>>> +			reg = <0>;
> >>>>> +			enable-method = "psci";
> >>>>> +			clocks = <&ccu CLK_CPUX>;
> >>>>> +		};
> >>>>> +
> >>>>> +		cpu1: cpu@1 {
> >>>>> +			compatible = "arm,cortex-a53";
> >>>>> +			device_type = "cpu";
> >>>>> +			reg = <1>;
> >>>>> +			enable-method = "psci";
> >>>>> +			clocks = <&ccu CLK_CPUX>;
> >>>>> +		};
> >>>>> +
> >>>>> +		cpu2: cpu@2 {
> >>>>> +			compatible = "arm,cortex-a53";
> >>>>> +			device_type = "cpu";
> >>>>> +			reg = <2>;
> >>>>> +			enable-method = "psci";
> >>>>> +			clocks = <&ccu CLK_CPUX>;
> >>>>> +		};
> >>>>> +
> >>>>> +		cpu3: cpu@3 {
> >>>>> +			compatible = "arm,cortex-a53";
> >>>>> +			device_type = "cpu";
> >>>>> +			reg = <3>;
> >>>>> +			enable-method = "psci";
> >>>>> +			clocks = <&ccu CLK_CPUX>;
> >>>>> +		};
> >>>>> +	};
> >>>>> +
> >>>>> +	reserved-memory {
> >>>>> +		#address-cells = <2>;
> >>>>> +		#size-cells = <2>;
> >>>>> +		ranges;
> >>>>> +
> >>>>> +		/* 512KiB reserved for ARM Trusted Firmware (BL31) */
> >>>>> +		secmon_reserved: secmon@...00000 {
> >>>>> +			reg = <0x0 0x40000000 0x0 0x80000>;
> >>>>> +			no-map;
> >>>>> +		};
> >>>>> +	};    
> >>>>
> >>>> I'm not a fan of above. If anything changes in future in BL31, U-Boot
> >>>> would
> >>>> need to reconfigure it anyway. Can we just skip it?    
> >>>
> >>> I am not a fan neither, but last time I checked this is needed to boot.
> >>> Indeed TF-A inserts this node, with the right values, into U-Boot's DT.
> >>> And that's nicely preserved if you use that DT ($fdtcontroladdr) for
> >>> the kernel as well.
> >>> But if someone *loads* a DTB into U-Boot (to $fdt_addr_r), then
> >>> U-Boot fails to propagate the /reserved-memory node into that copy.
> >>> There does not seem to be a global notion of reserved memory in U-Boot.
> >>> Some commands (like tftp) explicitly parse the control DT to find and
> >>> respect reserved memory regions. bootm does that also, but only to
> >>> avoid placing the ramdisk or DTB into reserved memory. The information
> >>> ends up in images->lmb, but is not used to generate or amend nodes in
> >>> the target DT.
> >>> So the bits and pieces are there, but it will require some code to be
> >>> added to the generic U-Boot code.
> >>>
> >>> So what do you think? Leaving this out will prevent loading DTBs into
> >>> U-Boot, at the moment, which sounds bad. I suggest we keep it in, for
> >>> now, it should not really hurt. U-Boot will hopefully start to do the
> >>> right thing soon, then we can either phase it out here (maybe when we
> >>> actually change something in TF-A), or let U-Boot fix it.    
> >>
> >> TBH, if "soon" is really soon, I would rather wait with H616 DT until U-Boot 
> >> supports carrying over reserved memory nodes.  
> > 
> > But this also carries compatibility issues. U-Boot support the H616 for
> > more than a year now, and the earliest possible U-Boot release having that
> > propagation code would be the one released in October. And then people
> > would still need to update first, so that's quite some months out.
> > And I was actually hoping to get at least the H616 DT patches off my
> > plate, and get them into the tree to have a stable and agreed upon base
> > (before this series turns into a teenager ;-)
> > Then we could for instance update the U-Boot H616 support.  
> 
> There is no compatibility issue here if people are using $fdtcontroladdr.
> 
> >> Whatever we do now, it will have 
> >> compatibility issues. If we introduce reserved memory node now, we can't 
> >> easily drop it later. Bootloaders are not very often updated, but kernels and 
> >> DTB files are, at least in my experience. So when we decide to drop the node?  
> > 
> > I think of the three possibilities:
> > - Drop the node now, and ask people to not load DTBs explicitly  
> 
> This is my preferred solution. My position has always been that the devicetree
> is provided by platform firmware, not the OS. The only reason for even
> submitting the devicetree to Linux is because that is the location of the
> bindings and validation tooling.

Well, you are barking at the wrong tree here ;-)
I am 100% behind the $fdtcontroladdr and "firmware ships DT" idea.
However this relies on one thing: that there will never be an
incompatible change to the DT. So at any point in time there must be
exactly one best DT for that board, and that DT must be able to boot
every kernel: older ones, current ones, FreeBSD ones, you name it.
Because otherwise you cannot update your DT, or you lose the ability to
boot a stable distro, or the stable fallback kernel that your distro
installed, for instance.
And this is not theoretical: Debian 11 ships with v5.10, which does not
boot with a DT from >= v5.13 (r_intc binding change for most AW SoCs).

And yes, this means we have to live with decisions we once made, and
have to make compromises, so cannot get the DT "exactly right by the
book", since we need to maintain compatibility. I strongly believe
there are solutions that allow this, even if we spot mistakes later or
need to amend something to make a new feature work. At the cost of
being potentially somewhat "hacky".

Some years ago I have been explicitly told that mainline sunxi Linux
does not have the resources to pull this off, and we don't guarantee
forward compatibility, which in my view renders this $fdtcontroladdr
approach moot. And the r_intc binding change, also the upcoming A23
clock change tell me that this is still the position among the
maintainers?

Or has this position changed for new SoCs? So do we promise to never
break compatibility for D1 and H616, and other new SoCs? Or even for
older SoCs, from now on?

> I have been using this approach for D1, and so far there have been no unsolvable
> problems. Yes, most image builders assume you want to load a DTB from disk, but
> teaching them not to do that is fairly simple. And yes, it means we have to do
> better about keeping the U-Boot DTSs in sync, but I think we can manage that.

Yes, I am very happy to update them much more regularly, and even have
some prototype tool to update the DTB directly in the FIT image on SPI
flash/eMMC boot/SD card/etc. Or I guess we rather explore the EFI
capsule update path more.
However this is all rather pointless (and actually counterproductive) if
that new DT does not boot all kernels.

> > - Drop the node when U-Boot learned to propagate the reservation
> > - Keep the node
> > the last one is the least painful: having this node in does not really
> > hurt, so we can be very relaxed with this removal decision:  
> 
> Supporting explicitly-loaded DTBs is the most painful option going forward,
> because that means U-Boot has to know about and propagate _every_ runtime change
> made by any firmware component (not just changes made by it) to the control FDT.

Yes, you have some point there.

> For example, consider TF-A patching in information about idle states, or SID
> contents in secure mode, or marking nodes as "reserved" because it delegates
> those devices to a secure enclave. This problem is solved if we say "we support
> $fdtcontroladdr, and we support overlays; but you are on your own if you load a
> DTB from disk."
> 
> Distros will continue loading DTBs from disk as long as it sorta-kinda-mostly
> works, and then years later users will complain that their phone uses 4 watts at
> idle, because their explicitly-loaded DTB contained idle states that their
> firmware did not support (this is a true story).
> 
> "Don't load DTBs" is a much easier story to tell if doing so is obviously broken
> out of the box from day 1.

I mostly agree, from a developer's point of view. My suggestion was
just trying to embrace the (current) reality, and giving the user the
ability to load a new DTB. But yes, they should not do this.

And I actually believe distros don't really like this approach either,
it's just how mainline Linux worked for most platforms: you better run
with the latest DTB, and the one matching the kernel (whatever that
means). So I think they will be quite happy to use U-Boot's DTB, though
this needs to be the universal one.

So I think the whole discussion boils down to the question of how we
are going to deal with upcoming DT fixes? Do we allow them only in a
compatible way?
And this could be a simple thing, like adding a previously unknown
regulator to a device: older kernels might not support that regulator,
so the device now fails probing. This is a prominent problem for
initial (minimal) DTs, where we only get PMIC support later. Sure, we
could demand having PMIC support in from day one, but there might be
other things we miss (like a clock gated by the RTC).

Cheers,
Andre


  

> 
> Regards,
> Samuel
> 
> > - If U-Boot does not add the reserved node, we are covered.
> > - If U-Boot adds the node, it will do so in a way where it deals with
> > existing reservations. So either it doesn't actually change anything, or
> > it extends the reservation.
> > - Should the TF-A location actually move (and we have no plans or needs to
> > do that), people would only get this by updating the firmware, at which
> > point the U-Boot part would surely be in place already. We don't really
> > support updating just BL31 in an existing binary firmware image, so you
> > would get an updated U-Boot as well.
> > 
> > I think the worst case scenario is that users end up with an unneeded 512K
> > reservation. If they care, a firmware update should solve this problem.
> > 
> > As for the time to remove that node: we could do that at the time when
> > (or rather: if) we actually change the TF-A reservation. At the moment
> > there are no plans to do this, and the size reservation is more than
> > generous (the current debug build is actually 77 KB or so only). If there
> > is no change, and the node stays in the .dtsi, it doesn't really hurt, see
> > above.
> >   
> >> After 10 years? Alternatively, reserved memory node can be just dropped and 
> >> anyone loading DTB file from outside would need to make sure it's patched. But 
> >> that's unexpected from user perspective, although patching DT files is done by 
> >> some distros.  
> > 
> > Yeah, let's not go there. As you know, I already dislike the idea of
> > explicitly loading DTBs at all, but I understand this is what people, and
> > distributions, do, so I'd rather have them covered. Hence the node to
> > work with existing firmware.
> > 
> > Does that make sense?
> > 
> > Cheers,
> > Andre
> >   
> 
> 

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