jetson-optimize-memory

Memory is reserved across four layers ordered by boot chronology (higher row = earlier in boot, closer to hardware):

Critical rules:

• Only the scenarios in Scenario recipes are validated. Refuse any request to disable a carveout/cluster/node not in that table.
• Zeroing a carveout requires both: disabling the cluster's loading controls AND removing the AST that references it.
• Emit explicit overrides for every cluster in the recipe, regardless of how the source #ifdef/#else looks. Verify the merged binary.
• GPU SW stack must match the chip: T234 -> nvgpu, T264 and later -> OpenRM. Follow the shared derived-platform rule in target-platform-contract.md; stop on mismatches instead of guessing.
• Do not set SWIOTLB to 0 — some peripherals can't use the IOMMU.

Scenario recipes

Chip-specific carveouts

Post-boot: headless → sudo systemctl set-default multi-user.target.

MB1 BCT carveout overrides

File: Linux_for_Tegra/bootloader/generic/BCT/tegra<chip>-mb1-bct-misc-<module>.dts (e.g. tegra234-mb1-bct-misc-p3767-0000.dts for Orin Nano, tegra264-mb1-bct-misc-p3834-0008-p4071-0000.dts for Thor).

For each carveout, add inside the existing carveout node:

aux_info@<CARVEOUT_NAME> {
    pref_base = <0x0 0x0>;
    size      = <0x0 0x0>;
    alignment = <0x0 0x0>;
};

MB2 BCT cluster + AST overrides

File: Linux_for_Tegra/bootloader/generic/BCT/tegra<chip>-mb2-bct-misc-<module>.dts (includes tegra<chip>-mb2-bct-common.dtsi).

For each target cluster:

1. Override auxp_controls@<index>:

   auxp_controls@<index> {
       enable_init    = <0>;
       enable_fw_load = <0>;
       enable_unhalt  = <0>;
   };
   

2. /delete-node/ auxp_ast_config@<idx>;

Look up indices in common.dtsi: auxp_controls@N carries a comment naming its cluster; auxp_ast_config@N has ast_region children whose carveout = <CARVEOUT_…>; lines identify the owner.

Kernel DT reserved-memory

DTB=Linux_for_Tegra/kernel/dtb/<platform-dtb-name>.dtb
dtc -I dtb -O dts -o /tmp/platform.dts $DTB
# edit: status = "disabled" on target nodes
dtc -I dts -O dtb -o $DTB /tmp/platform.dts

Display — disable display@<addr>, plus dce@<addr> if exposed as a separate kernel node.

Camera — under host1x@<addr>, disable whichever of vi* / isp* / nvcsi exist on the BSP (only emit present nodes):

Locate the display controller node in the decompiled DTS and disable it. The node's unit address is chip-specific — find it by compatible string (e.g. nvidia,tegra234-display) rather than hard-coding the address.

host1x@<addr> {
    vi0@<addr>   { status = "disabled"; };
    vi1@<addr>   { status = "disabled"; };
    isp@<addr>   { status = "disabled"; };
    isp1@<addr>  { status = "disabled"; };
    nvcsi@<addr> { status = "disabled"; };
};

SWIOTLB DMA bounce pool

The NVIDIA IOMMU covers peripheral DMA, so SWIOTLB is rarely used. Edit CMDLINE_ADD (never CMDLINE) in Linux_for_Tegra/<module>.conf.common:

# Total bytes = swiotlb_value × 2048; 4 MiB pool:
CMDLINE_ADD="... swiotlb=2048"

Override verification (mandatory)

After every patched MB1/MB2 BCT .dts, reproduce the BSP's compile + decompile using the same -D… flags from bct_flags.append(...) in bootloader/tegraflash_impl_t<chip>.py:

gcc -E -nostdinc -x assembler-with-cpp \
    -DENABLE_<FLAG_1> -DENABLE_<FLAG_2> \
    -I bootloader -I bootloader/generic/BCT \
    -o /tmp/cpp.dts <patched-bct.dts>
dtc -q -I dts -O dtb -o /tmp/cpp.dtb /tmp/cpp.dts
dtc -q -I dtb -O dts /tmp/cpp.dtb | less

Confirm in the merged output:

• Each zeroed aux_info@<NAME> (or aux_info@<id>U post macro expansion) has size = <0x0 0x0> and pref_base = <0x0 0x0>.
• Each disabled auxp_controls@<idx> has all three enable_* fields <0>.
• Each /delete-node/'d auxp_ast_config@<idx> is absent.

Verification (on booted target)

sudo cat /proc/iomem | grep -iE 'nv-reserved|cma|fb|carveout'
ls /proc/device-tree/reserved-memory/
dmesg | grep -iE 'firmware|carveout|bpmp|reserved|fail|error' | head -20
free -m

For SWIOTLB: /proc/cmdline must contain swiotlb=<value>, and watch -n5 cat /sys/kernel/debug/swiotlb/io_tlb_used must stay under io_tlb_nslabs during full workload — if exceeded, restore original CMDLINE_ADD and re-flash kernel-dtb.

Purpose

Cut the unused DRAM carveouts that ship enabled in the reference BSP when a Jetson deployment skips display, camera, or other peripherals, freeing the freed bytes for the application. Always edits the four layers in boot order so an early-stage carveout never outranks a later-stage shrink.

Prerequisites

• Active target profile resolved per ../../context/target-platform-contract.md.
• BSP image extracted and source tree initialized (/jetson-init-image, /jetson-init-source complete).
• For headless / no-camera recipes: confirm the workload truly does not need display or camera.

Limitations

• Only the validated recipes (headless, no-camera, swiotlb) are exposed; ad-hoc subsystem disables outside the recipe set are refused.
• SWIOTLB shrink is bounded by peak in-flight DMA — exceeding the new io_tlb_nslabs requires reverting the change.
• BPMP-DTB edits land in the overlay tracker only after Customize + Build + Deploy run; this skill does not flash on its own.

Troubleshooting

• Boot fails after MB1 BCT carveout disable — restore the pristine misc DTS and re-flash; the missing carveout is mandatory for the active SoC.
• io_tlb_used exceeds io_tlb_nslabs — revert swiotlb= in CMDLINE_ADD and re-flash the kernel DTB partition.
• Reclaimed delta smaller than expected — verify the recipe truly matched the deployment (e.g. display still attached); use the dmesg | grep -iE 'firmware|carveout' check in this file to confirm.
• Validation dmesg shows the disabled subsystem still probing — the change probably did not promote through to bsp_image; re-run /jetson-promote-image.