GPU memory split behavior change on 64-bit Raspberry Pi OS

Answer the exact question first

On 64‑bit Raspberry Pi OS the bootloader limits the GPU allocation to a hard 128 MB ceiling unless the gpu_mem_256 option is present in /boot/config.txt. The legacy gpu_mem=256 setting is ignored (or capped) on 64‑bit kernels that use firmware 2022.02 or newer.

Why this happens

Firmware 2022.02 introduced a new gpu_mem_256 flag that explicitly tells the bootloader to request 256 MB of GPU memory. Without it, the bootloader applies the default 128 MB limit for 64‑bit builds, regardless of the gpu_mem value. Overlays such as dtoverlay=vc4-kms-v3d run inside the GPU memory window but do not influence the split itself.

Confirmed facts

• On 64‑bit OS, gpu_mem=256 alone does not increase the reported GPU memory – vcgencmd get_mem gpu will still show 128 MB.
• Adding gpu_mem_256=1 (or simply gpu_mem_256) instructs the bootloader to allocate 256 MB; the kernel then reports the correct amount.
• Firmware older than 2022.02 does not recognize gpu_mem_256 and will keep the 128 MB ceiling.

Practical steps for a reliable 256 MB request

1. Open /boot/config.txt as root.

   sudo nano /boot/config.txt
   

2. Add or edit the following lines:

   # Request 256 MB GPU memory on 64-bit OS
   gpu_mem_256=1
   # Optional: keep legacy value for 32-bit builds
   # gpu_mem=256
   

3. Save the file and reboot:

   sudo reboot
   

4. After boot, verify the allocation:

   vcgencmd get_mem gpu
   # Expected output: gpu=256M
   

What to check if it still shows 128 MB

• Confirm the firmware version: vcgencmd version should report a bootloader 2022.02 or newer.
• Ensure the config line is correctly spelled and not overridden by a later file or overlay.
• If the firmware is older, update it with sudo rpi-update and reboot.

Diagnostic question

What bootloader firmware version is currently running on your Pi (output of vcgencmd version)?