How I Migrated U-Boot to Support a New eMMC Chip After Samsung’s Line Shutdown

How I Migrated U-Boot to Support a New eMMC Chip After Samsung’s Line Shutdown

In early 2025, Samsung discontinued one of its popular eMMC chips used in our PX30-based SBC. This unexpected change triggered a critical migration process — from evaluating alternative chips to rewriting low-level bootloader logic.

This post walks through the complete journey of adapting U-Boot to support Foresee’s eMMC module, including hardware evaluation, debugging, quirk handling, and field testing.

Why eMMC Still Matters in Embedded Systems

eMMC (embedded MultiMediaCard) remains a common storage solution in embedded Linux platforms due to its balance between cost, performance, and board integration simplicity. Many boards — including ours — rely on it not just for data storage, but as the first-stage boot source.

The Samsung KLMAG1JETD had been our go-to choice, thanks to its mature driver support. Its sudden unavailability meant compatibility challenges when switching to new chips — especially during early boot phases handled by U-Boot.

🔍 Step 1: Evaluating Replacement Candidates

Our hardware team shortlisted three replacements:

• Foresee NCEMBS99–32G
• Kingston EMMC32G-M525
• Longsys/AISINO eMMC 5.1 (32GB)

We benchmarked each using mmc-utils under Linux. Foresee won out due to availability, stable performance, and reliable response across voltage variations.

🔧 Step 2: Understanding U-Boot’s MMC Stack

U-Boot interacts with eMMC through its MMC subsystem in drivers/mmc/. This layer abstracts host controllers and cards using standard MMC/SD commands (CMD0, CMD1, CMD8, etc).

The Foresee chip initially failed during boot, hanging at:

mmc_send_op_cond: timeout waiting for OCR

After debugging, we discovered it required additional clock delay and didn’t fully respond to CMD8, which Samsung chips handled gracefully.

🛠️ Step 3: Refactoring the Initialization Path

To accommodate Foresee, we added a new quirk in mmc.c:

if (cid[0] == 0x70) { // Foresee
    host->quirks |= MMC_QUIRK_NEED_CLOCK_DELAY;
}

We also tweaked mmc_set_ios() and mmc_send_op_cond() to include longer retry intervals and Foresee-specific delays via udelay(500).

⚙️ Step 4: Tuning Timing and Clock Settings

Foresee struggled with UHS modes. We downgraded to DDR52 legacy mode:

mmc->timing = MMC_TIMING_LEGACY;

Additionally, we adjusted the host clock divider from 1:4 to 1:8 for improved cold-boot stability.

📄 Step 5: Device Tree Adjustments

Although U-Boot can function with limited DT support, we updated the board’s px30-u-boot.dtsi:

mmc@fe310000 {
    clock-frequency = <100000000>;
    disable-cmd23;
    no-sd;
};

This ensured consistent behavior during Linux handoff post-boot.

🧪 Step 6: Test & Validation Strategy

We validated with the following:

• ✅ 30× cold boot cycles
• ✅ 50× warm reboots
• ✅ 5× OTA update tests
• ✅ Simulated power-loss via GPIO-reset
• ✅ Bonnie++ benchmark
• ✅ Thermal soak at -10°C to +60°C

The Foresee eMMC passed all tests with stable performance.

✅ Outcome

With these changes, U-Boot now supports both Samsung and Foresee eMMC modules seamlessly.

We pushed the migration branch to GitHub for others facing similar transitions.

🔁 Lessons Learned

• Even with standardized specs, vendor-specific quirks matter.
• Clock timing, CMD behaviors, and retry logic all affect early boot success.
• Real-world conditions (cold boot, brown-out) are essential test vectors.
• Bootloader work is invisible but foundational to embedded stability.

🧹 More Projects

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📝 Tags suggestion for Medium:

#u-boot #emmc #embedded-linux #rockchip #sbc #bootloader #firmware