Re: [PATCH 07/12] riscv: dts: allwinner: Add Allwinner D1 Nezha devicetree

[Jessica Clarke <jrtc27@jrtc27.com>]

On 9 Sept 2022, at 09:11, Heiko Stübner <heiko@sntech.de> wrote:
> 
> Am Freitag, 9. September 2022, 09:18:40 CEST schrieb Conor.Dooley@microchip.com:
>> On 09/09/2022 05:37, Samuel Holland wrote:
>>> EXTERNAL EMAIL: Do not click links or open attachments unless you
>>> know the content is safe
>>> 
>>> Hi Conor,
>>> 
>>> On 8/19/22 5:10 PM, Conor.Dooley@microchip.com wrote:
>>>> Finally got around to giving this a go with the fix for loading 
>>>> modules which is mostly what was blocking me before..
>>>> 
>>>> On 15/08/2022 06:08, Samuel Holland wrote:
>>>>> "D1 Nezha" is Allwinner's first-party development board for the
>>>>> D1 SoC. It was shipped with 512M, 1G, or 2G of DDR3. It supports
>>>>> onboard audio,
>>>> 
>>>> I am really not keen on the way you have things, with the memory 
>>>> nodes removed from the device tree. I know your preferred flow for
>>>> booting these things might be to pass the dtb up from U-Boot, but I
>>>> think the devicetree in the kernel should be usable in a standalone
>>>> manner, even if that is the barest-minimum memory config.
>>> 
>>> That is simply not possible to guarantee. As an obvious example,
>>> consider the MangoPi MQ-Pro board with socketed DRAM:
>> 
>> Yeah, I knew in my heart-of-hearts that this probably was a non
>> runner.
>> 
>>> 
>>> https://twitter.com/mangopi_sbc/status/1516225559214583808
>>> 
>>> But focusing on the /memory node misses the bigger picture. The DTB
>>> is passed through _all_ of the firmware stages, and gets patched by
>>> every one of them:
>>> 
>>> - SPL/boot0 adds the /memory node with the detected DRAM size. If the
>>> in-tree DTS has a "minimum memory config" (which for a board with
>>> socketed DRAM means the smallest possible die), I guarantee people
>>> will use it and complain about missing DRAM.
>> 
>> True, but they are also complaining about missing DRAM as is ;)
>> No possibility of winning here unfortunately.
>> 
>>> - The SBI implementation reserves memory for itself and any possible
>>> secure partitions. Right now, booting happens to work without the
>>> reserved-memory node because the SBI implementation is loaded at the
>>> beginning of RAM, and Linux ignores RAM below the kernel load
>>> address.
>> 
>> Surely this sort of thing is a common problem though, it's not like
>> any of us are doing something unique here are we? At least if there
>> was a arch wide policy about the validity of the upstream DTS in the
>> face of the SBI etc inflicting changes there'd be something to point
>> to. I am just looking at this from a "everyone else has a usable dts
>> in the kernel, but the D1 boards wont have" point of view.
> 
> Not sure if this would fly, but what about having an sbi call for
> "modify this dtb for me as well"?
> 
> I'll just assume that spl/boot0 + main uboot come in some sort
> of package so moving the memory node over should be in uboot's
> scope, but for the sbi part just have a call pointing to the
> new dtb in memory and have it modify it in the same way as the
> original one?

There’s an EFI protocol for it.

Jess

> Heiko
> 
> 
> 
>>> However, memory-constrained devices (e.g. D1s) will need to get those
>>> 2 MiB back by loading the kernel at the start of DRAM and SBI at the
>>> end of DRAM. Then the reserved-memory node becomes quite important.
>>> 
>>> It also adds nodes for CPU idle states, since the available states
>>> and their latencies depend on the SBI implementation.
>>> 
>>> It also reserves devices used by it or by a secure partition. And it
>>> is responsible for extracting data (e.g. MAC addresses) from "secure"
>>> eFuses which the OS may not have access to.
>>> 
>>> - U-Boot adds other information, like boot arguments, the address of
>>> the initramfs and framebuffer, etc. These are less of a concern
>>> because of course U-Boot can patch these in to a DTB loaded from
>>> disk, but they are relevant if you want to load a DTB from a later
>>> bootloader like GRUB.
>>> 
>>> If you load a DTB from disk, you lose all of the changes made by the
>>> earlier firmware stages. On ARM, U-Boot tries to work around this by
>>> copying a few specific bits of information from the firmware DTB to
>>> the DTB loaded from disk. But this misses the point that the SBI
>>> implementation can modify *any* part of the DTB. (So in practice
>>> U-Boot on ARM already loses CPU idle states and reserved memory nodes
>>> that were added by the PSCI implementation.)
>> 
>> All of these things are valid, but they are reasons why your flow in
>> your bootloaders etc are the way they are more than a reason why the
>> upstream dts will not work for someone who is not interested in that
>> flow. At the end of the day, I only care so much about this as it is
>> not me that has to deal with any confusion from either approach. I'll
>> continue to modify my dts in U-Boot so I can test things without me
>> having to re-program the world. /shrug
>> 
>>> As an extreme example, consider paravirtualization, where only a
>>> small subset of DRAM and peripherals may be made available to any one
>>> OS partition.
>> 
>> Or AMP - though not likely that that is a problem for the D1..
>> 
>>> Fundamentally, I reserve the right to make arbitrary changes to the
>>> DTB in the SBI implementation, and thus I cannot condone using the
>>> DTBs generated from the Linux source tree for any purpose other than
>>> validation.
>> 
>> Fundamentally, I reserve the right to complain that the upstream dts
>> cannot be entirely validated as it does not work out-of-the-box ;)
>> 
>> Either way, I am only going to complain so much about something that
>> triggers my OCD about keeping things the same, you have a
>> Tested-by: Conor Dooley <conor.dooley@microchip.com>
>> already and once the other issues are cleaned up an R-b too. Not trying
>> make an issue out of this, just expressing my dislike for the
>> inconsistency between the D1 stuff and vendors - partly in the hopes
>> that the "higher powers" get involved. I can't imagine that this is
>> the last time something like this comes up.
>> 
>> Thanks,
>> Conor.
>> 
>> 
>> 
>> 
>> 
>> 
> 
> 
> 
> 
> 
> _______________________________________________
> linux-riscv mailing list
> linux-riscv@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-riscv