基于BeagleBoneBlack的U-Boot HDMI图像加载可行性咨询

Great question—yes, you absolutely can get HDMI output working from U-Boot on your BeagleBone Black (AM335x + TDA19988). It’s definitely feasible, even if examples are scarce because most folks stick to kernel-level display handling. Let’s break down exactly how to pull this off with your specific setup:

First, Confirm Feasibility #

The AM335x’s Display Subsystem (DSS) has native support to drive HDMI via the TDA19988 transceiver on the BBB. U-Boot already includes low-level DSS code for AM335x—you just need to extend it to initialize the TDA19988 and switch the output from your LCD to HDMI.

Step 1: Modify Your U-Boot Source #

Your U-Boot version (2016.11-rc3) has a solid base for AM335x, so here’s what to tweak:

1.1 Add TDA19988 Initialization #

Head to board/ti/am335x/ in your U-Boot tree and edit board.c (or am335x.c, depending on your exact tree structure):

• First, initialize I2C bus 0 (the TDA19988 lives at I2C address 0x70).
• Send the required register setup sequences to put the TDA19988 into HDMI mode. You can grab these from the Linux kernel’s tda19988_probe() function (look in drivers/gpu/drm/bridge/tda19988.c—it’s a tested reference for the BBB’s transceiver).

1.2 Reconfigure AM335x DSS for HDMI #

Edit the DSS driver at drivers/video/ti/omap/dss.c (or the AM335x-specific DSS file):

• Enable the DSS clock and power domains (make sure these are active before configuring anything).
• Set your target HDMI resolution (e.g., 1080p60 or 720p60) with valid CEA-861 timing parameters.
• Route the DSS output to the HDMI transceiver instead of your current LCD path.

1.3 Add a Test Command (Optional but Handy) #

If you want to trigger HDMI output manually, create a custom U-Boot command in the cmd/ directory. Alternatively, integrate the initialization into board_init_r() so it runs automatically on boot.

Step 2: Compile with Your Toolchain #

Use your existing Linaro toolchain to build the modified U-Boot—your setup is perfect for this:

cd u-boot-source
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- am335x_boneblack_defconfig
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- -j$(nproc)

This will spit out u-boot.img and MLO—copy these to your BBB’s boot partition to test.

Step 3: Test HDMI Output #

Once you’ve flashed the new U-Boot:

• Hook up your BBB to an HDMI monitor.
• Power on the board—if you integrated initialization into boot, the monitor should detect a signal immediately.
• To load an image, use loadmmc to pull a raw bitmap into memory, then call your DSS blit function to render it to the HDMI display.

Key Tips to Avoid Headaches #

• Don’t skip TDA19988 setup: The transceiver won’t switch to HDMI mode without the right I2C register writes—follow the kernel driver’s sequence exactly.
• Double-check timing: Invalid resolution timings will cause the monitor to reject the signal. Stick to standard CEA-861 values for your target resolution.
• Memory placement: When loading images, use a memory region that’s not reserved for U-Boot itself (check your U-Boot’s memory map to find free space).

内容的提问来源于stack exchange，提问作者While-E