Re: [PATCH v2 3/3] arm64: dts: qcom: apq8016: Add Schneider HMIBSC board DTS

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On 14/03/2024 15:20, Konrad Dybcio wrote:
> On 3/14/24 14:50, Sumit Garg wrote:
>> On Thu, 14 Mar 2024 at 18:54, Stephan Gerhold <stephan@xxxxxxxxxxx>
>> wrote:
>>> On Thu, Mar 14, 2024 at 05:26:27PM +0530, Sumit Garg wrote:
>>>> On Thu, 14 Mar 2024 at 16:13, Stephan Gerhold <stephan@xxxxxxxxxxx>
>>>> wrote:
>>>>> On Thu, Mar 14, 2024 at 03:02:31PM +0530, Sumit Garg wrote:
>>>>>> On Thu, 14 Mar 2024 at 14:48, Konrad Dybcio
>>>>>> <konrad.dybcio@xxxxxxxxxx> wrote:
>>>>>>> On 3/14/24 10:04, Sumit Garg wrote:
>>>>>>>> On Wed, 13 Mar 2024 at 18:34, Konrad Dybcio
>>>>>>>> <konrad.dybcio@xxxxxxxxxx> wrote:
>>>>>>>>> On 3/13/24 13:30, Sumit Garg wrote:
>>>>>>>>>> Add Schneider Electric HMIBSC board DTS. The HMIBSC board is
>>>>>>>>>> an IIoT Edge
>>>>>>>>>> Box Core board based on the Qualcomm APQ8016E SoC.
>>>>>>>>>> Support for Schneider Electric HMIBSC. Features:
>>>>>>>>>> - Qualcomm Snapdragon 410C SoC - APQ8016 (4xCortex A53, Adreno
>>>>>>>>>> 306)
>>>>>>>>>> - 1GiB RAM
>>>>>>>>>> - 8GiB eMMC, SD slot
>>>>>>>>>> - WiFi and Bluetooth
>>>>>>>>>> - 2x Host, 1x Device USB port
>>>>>>>>>> - HDMI
>>>>>>>>>> - Discrete TPM2 chip over SPI
>>>>>>>>>> - USB ethernet adaptors (soldered)
>>>>>>>>>> Co-developed-by: Jagdish Gediya <jagdish.gediya@xxxxxxxxxx>
>>>>>>>>>> Signed-off-by: Jagdish Gediya <jagdish.gediya@xxxxxxxxxx>
>>>>>>>>>> Signed-off-by: Sumit Garg <sumit.garg@xxxxxxxxxx>
>>>>>>>>>> ---
>>>>>>>>> [...]
>>>>>>>>>> +     memory@80000000 {
>>>>>>>>>> +             reg = <0 0x80000000 0 0x40000000>;
>>>>>>>>>> +     };
>>>>>>>>> I'm not sure the entirety of DRAM is accessible..
>>>>>>>>> This override should be unnecessary, as bootloaders generally
>>>>>>>>> update
>>>>>>>>> the size field anyway.
>>>>>>>> On this board, U-Boot is used as the first stage bootloader
>>>>>>>> (replacing
>>>>>>>> Little Kernel (LK), thanks to Stephan's work). And U-Boot consumes
>>>>>>>> memory range from DT as Linux does but doesn't require any
>>>>>>>> memory to
>>>>>>>> be reserved for U-Boot itself. So apart from reserved memory nodes
>>>>>>>> explicitly described in DT all the other DRAM regions are
>>>>>>>> accessible.
>>>>>>> Still, u-boot has code to fetch the size dynamically, no?
>>>>>> No U-Boot being the first stage bootloader fetches size from DT which
>>>>>> is bundled into U-Boot binary.
>>>>> Back when I added support for using U-Boot as first stage
>>>>> bootloader on
>>>>> DB410c the way it worked is that U-Boot used a fixed amount of DRAM
>>>>> (originally 968 MiB, later 1 GiB since I fixed this in commit
>>>>> 1d667227ea51 ("board: dragonboard410c: Fix PHYS_SDRAM_1_SIZE") [1]).
>>>>> When booting Linux, the Linux DT was dynamically patched with the
>>>>> right
>>>>> amount of DRAM (obtained from SMEM). So if you had e.g. a Geniatech
>>>>> DB4
>>>>> board with 2 GiB DRAM, U-Boot was only using 1 GiB of DRAM, but Linux
>>>>> later got the full 2 GiB patched into its DTB.
>>>>> I didn't have much time for testing U-Boot myself lately but a quick
>>>>> look at the recent changes suggest that Caleb accidentally removed
>>>>> that
>>>>> functionality in the recent cleanup. Specifically, the SMEM-based DRAM
>>>>> size detection was removed in commit 14868845db54 ("board:
>>>>> dragonboard410c: import board code from mach-snapdragon" [2]), the
>>>>> msm_fixup_memory() function does not seem to exist anymore now. :')
>>>> Ah now I see the reasoning for that particular piece of code. Is SMEM
>>>> based approach the standardized way used by early stage boot-loaders
>>>> on other Qcom SoCs too?
>>> It is definitely used on all the SoCs that were deployed with LK. I am
>>> not entirely sure about the newer ABL/UEFI-based ones. A quick look at
>>> the ABL source code suggests it is abstracted through an EFI protocol
>>> there (so we cannot see where the information comes from with just the
>>> open-source code). However, in my experience SMEM data structures are
>>> usually kept quite stable (or properly versioned), so it is quite likely
>>> that we could use this approach for all Qualcomm SoCs.
>> If the SoCs which support this standardized way to dynamic discover
>> DRAM size via SMEM then why do we need to rely on DT at all for those
>> SoCs? Can't U-Boot and Linux have the same driver to fetch DRAM size
>> via SMEM? I am not sure if it's an appropriate way for U-Boot to fixup
>> DT when that information can be discovered dynamically.

"standardized" I'm not so sure... But yes, smem does offer this. The
definition in DT here is for U-Boot, ABL will always clobber it, and so
does U-Boot before handing over to the kernel. Linux should never see
this without a bootloader having looked at it.

The reason I decided to hardcode this in DT for U-Boot is because SMEM
currently relies on the driver model and isn't available early enough.

Also admittedly I just wasn't that familiar with the U-Boot codebase. I
just wanted to avoid hardcoding this in C code, and given that this was
already supported for the "chainloading from ABL" usecase, just
hardcoding the values was the obvious solution.

I would definitely be open to revisiting this in U-Boot, having an SMEM
framework that we could use without the driver model which would just
take a base address and then let us interact with SMEM and populate the
dram bank data would be a good improvement, and would let us avoid
hardcoding the memory layout in DT. We'd just need to manually find the
SMEM base address in the FDT as part of "dram_init_banksize()" and
retrieve the data there.

That all being said, I don't see any reason not to define the memory
layout in DT, it's a hardware feature, DT describes the hardware. The
whole "bootloader will fill this in" implies that the bootloader isn't
also using DT as the source of truth, but now with U-Boot it actually
is, so it's all the more important that DT be accurate ;P
> You're mixing two things. Linux expects a devicetree where
> /memory/reg[size]
> is valid. Such driver should indeed be (re)implemented in u-boot to provide
> this information.
> As for linux, I am working on making Linux aware of the DDR capabilities
> on Snapdragons, for other reasons, but it's on the back burner, as it
> still needs some broad thinking about integrating it with the interested
> consumers.. Bottom line is, Linux should be fed a devicetree with DRAM size
> filled.
> Konrad

// Caleb (they/them)

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