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Date: Fri, 15 Mar 2024 12:53:29 +0000
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Subject: Re: [PATCH v2 3/3] arm64: dts: qcom: apq8016: Add Schneider HMIBSC
 board DTS
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To: Sumit Garg <sumit.garg@linaro.org>,
 Konrad Dybcio <konrad.dybcio@linaro.org>
Cc: Stephan Gerhold <stephan@gerhold.net>, linux-arm-msm@vger.kernel.org,
 devicetree@vger.kernel.org, andersson@kernel.org, robh+dt@kernel.org,
 krzysztof.kozlowski+dt@linaro.org, conor+dt@kernel.org,
 neil.armstrong@linaro.org, laetitia.mariottini@se.com,
 pascal.eberhard@se.com, abdou.saker@se.com, jimmy.lalande@se.com,
 benjamin.missey@non.se.com, daniel.thompson@linaro.org,
 linux-kernel@vger.kernel.org, Jagdish Gediya <jagdish.gediya@linaro.org>
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 <20240313123017.362570-4-sumit.garg@linaro.org>
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From: Caleb Connolly <caleb.connolly@linaro.org>
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On 15/03/2024 09:31, Sumit Garg wrote:
> On Thu, 14 Mar 2024 at 21:07, Caleb Connolly <caleb.connolly@linaro.org> wrote:
>>
>>
>>
>> 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@gerhold.net>
>>>> 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@gerhold.net>
>>>>>> 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@linaro.org> wrote:
>>>>>>>>> On 3/14/24 10:04, Sumit Garg wrote:
>>>>>>>>>> On Wed, 13 Mar 2024 at 18:34, Konrad Dybcio
>>>>>>>>>> <konrad.dybcio@linaro.org> 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@linaro.org>
>>>>>>>>>>>> Signed-off-by: Jagdish Gediya <jagdish.gediya@linaro.org>
>>>>>>>>>>>> Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
>>>>>>>>>>>> ---
>>>>>>>>>>>
>>>>>>>>>>> [...]
>>>>>>>>>>>
>>>>>>>>>>>> +     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,
> 
> We should move away from that thinking that U-Boot has its own DT and
> Linux kernel has its own. IMO, that's just the opposite of the true DT
> definition.

I was clarifying here that the memory node being defined with real
values was because U-Boot uses them. Just in case some folks thought
that Linux was using whatever values were here.
> 
>> 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.
> 
> Where does U-Boot clobber SMEM? I would be interested to see if ABL
> clobbers it too?

Not SMEM, the /memory node.
> 
>>
>> 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.
> 
> These are the similar problems Linux has to deal with too but on Qcom
> platforms it is rather offloaded to bootloaders to rather implement
> this. It leads to custom DT modification or board code in U-Boot which
> is hard to maintain. If we want to implement it properly then
> corresponding bindings should be upstreamed too regarding how DRAM
> range is to be discovered via SMEM.
> 
>>
>> 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
> 
> I agree DT should be accurate and not a fan of DT fixups. However,
> when it comes to some hardware configuration being discoverable then
> IMHO DT isn't the appropriate place for that. For the time being I am
> fine with the DRAM range to be specified in DT.

Agreed
> 
>>>
>>> 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.
> 
> No, I don't think so. We should rather start thinking about the
> overall stack rather than just being Linux kernel centric. Do you have
> a generic solution for U-Boot regarding how this should be
> implemented?
> 
> -Sumit
> 
>>>
>>> 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)

-- 
// Caleb (they/them)