Re: [RFC PATCH v1 0/3] efi_loader: Introduce platform hook for FDT selection

[Sumit Garg <sumit.garg@kernel.org>]

On Sat, Jan 10, 2026 at 02:00:03PM +0530, Aswin Murugan wrote:
> Thanks everyone for all the valuable feedback. I appreciate the detailed
> reviews and want to address the concerns raised.
> 
> I should have been clearer upfront about what makes Qualcomm platforms
> different here. Qualcomm has its own combined-dtb format that's been used
> across Linux products. The format is documented in [1] and the combined dtb
> file is placed in a separate dtb partition
> 
> The combined-dtb file contains multiple DTBs concatenated together as a
> single dtb file, each with Qualcomm-specific properties:
> - qcom,msm-id (platform ID and SoC revision)
> - qcom,board-id (board variant and subtype)
> - qcom,pmic-id (PMIC models - up to 4)

As discussed in the thread here [1], combined DTB is a legacy format
which is superceeded by the FIT based [2]. So rather work on FIT based
DTB packaging. Along with that extend mkimage tooling support in order
to build that FIT based multi DTB image.

[1] https://github.com/qualcomm-linux/meta-qcom/pull/1373#discussion_r2684555441
[2] https://github.com/qualcomm-linux/qcom-dtb-metadata/blob/main/Documentation.md

-Sumit

> 
> At runtime, we read hardware info from SMEM (shared memory interface with
> the firmware) and match it against these properties to pick the right DTB.
> 
> *Why Existing Approaches Don't Quite Fit*
> 
> After reviewing the feedback from Heinrich, Ilias, Simon & Sumith, I realize
> the core issue is that all the existing U-Boot mechanisms assume separate
> DTB files on the ESP:
> 
> VisionFive2 approach: Uses EEPROM detection in SPL and sets `$fdtfile` to
> point to individual files like `dtb/starfive/jh7110-milkv-mars.dtb`,
> `dtb/starfive/jh7110-starfive-visionfive-2-v1.3b.dtb`
> 
> eficonfig and distro boot: These mechanisms work great for selecting between
> multiple DTB files.
> 
> The common thread is they all expect individual DTB files, while Qualcomm's
> deployment model uses a single combined-dtb.dtb file that requires parsing
> and extraction. This is the fundamental gap we're trying to bridge.
> 
>  I agree with Simon that using the weak function approach is not an ideal
> solution. we should be avoiding adding hooks in the EFI loader. Instead, we
> need to handle this at the board level and work with the existing `$fdtfile`
> mechanism.
> 
> *Proposed Approach*
> 
> Based on all the feedback, here's the revised plan:
> For v2:
> 1. In board code: Parse combined-dtb.dtb, extract the matching DTB, write to
> ESP as `dtb/qcom-<board>.dtb`
> 3. Set `$fdtfile` to point to the extracted file
> 4. Let standard EFI mechanisms handle the rest
> 5. Drop the weak function approach entirely
> 
> Does this approach address the concerns raised, or suggest a different
> direction? I'm particularly interested in feedback on runtime DTB extraction
> to ESP is acceptable, or if there's a better way to bridge between our
> combined-dtb format and U-Boot's expectations.
> 
> [1] https://docs.qualcomm.com/doc/80-70023-27/topic/configure_and_secure_boot_with_systemd_boot_and_uki.html#multi-dtb-support
> 
> Thanks,
> Aswin
> 
> On 1/6/2026 5:51 PM, Aswin Murugan wrote:
> > This RFC patch series introduces a weak function hook to allow platforms to
> > provide custom device tree selection logic while keeping common EFI
> > loader code generic.
> > 
> > Background:
> > Currently, EFI loader supports loading a single DTB file.
> > Qualcomm platforms require special multi-DTB selection logic that:
> > - Reads hardware information from SMEM (Shared Memory)
> > - Selects the appropriate device tree from a combined DTB file based on:
> >    * Platform ID (SoC variant)
> >    * Board variant and subtype
> >    * PMIC configuration
> >    * SoC revision.
> > 
> > Solution:
> > Introduce `efi_load_platform_fdt()` as a weak function:
> > - Weak implementation in lib/efi_loader/efi_fdt.c (defaults to no-op)
> > - Called from efi_bootmgr_run() after efi_load_distro_fdt()
> > - Qualcomm override in arch/arm/mach-snapdragon/efi_fdt_qcom.c
> > 
> > Aswin Murugan (3):
> >    efi_loader: Add platform hook for FDT loading
> >    soc: qcom: smem: Added socinfo header file
> >    mach-snapdragon: Implement Qualcomm multi-DTB selection
> > 
> >   arch/arm/mach-snapdragon/Makefile       |   1 +
> >   arch/arm/mach-snapdragon/efi_fdt_qcom.c | 339 ++++++++++++++++++++++++
> >   include/soc/qcom/socinfo.h              | 114 ++++++++
> >   lib/efi_loader/efi_fdt.c                |  34 +++
> >   4 files changed, 488 insertions(+)
> >   create mode 100644 arch/arm/mach-snapdragon/efi_fdt_qcom.c
> >   create mode 100644 include/soc/qcom/socinfo.h
> >