I'm not sure I'm running the right package, and I'm not sure I should be expecting this to work (i.e. maybe Pinebook Pro is "not supported" by debian).  In any case, here's what happens when I try to do this:

On serial console, output is:
  Enter choice: 12
  12:     Debian-arm64-6.11.2-mmcblk1p6
  Retrieving file: /vmlinuz-6.11.2-arm64
  Retrieving file: /initrd.img-6.11.2-arm64
  append: root=LABEL=internalC1118241 console=ttyS2,1500000n8 console=tty0 ro quiet splash plymouth.ignore-serial-consoles maxcpus=4 coherent_pool=1M
  Retrieving file: /usr/lib/linux-image-6.11.2-arm64/rockchip/rk3399-pinebook-pro.dtb
  Moving Image from 0x2080000 to 0x2200000, end=4320000
  ## Flattened Device Tree blob at f4f0dd30
     Booting using the fdt blob at 0xf4f0dd30
  Working FDT set to f4f0dd30
     Loading Ramdisk to f396a000, end f3ef5150 ... OK
     Loading Device Tree to 00000000f3954000, end 00000000f396905f ... OK
  Working FDT set to f3954000

  Starting kernel ...

  [    1.001897] armv8-pmu pmu_a72: hw perfevents: failed to probe PMU!

and nothing else.
On the built-in screen, it displays less than that.

I have u-boot installed from u-boot-rockchip:arm64 2024.01+dfsg-1

Versions of packages linux-image-6.11.2-arm64-unsigned depends on:
ii  initramfs-tools [linux-initramfs-tool]  0.142+deb12u1
ii  kmod                                    30+20221128-1
ii  linux-base                              4.6

Versions of packages linux-image-6.11.2-arm64-unsigned recommends:
pn  apparmor  <none>

Versions of packages linux-image-6.11.2-arm64-unsigned suggests:
pn  debian-kernel-handbook  <none>
pn  firmware-linux-free     <none>
pn  linux-doc-6.11          <none>

Versions of packages linux-image-6.11.2-arm64-unsigned is related to:
pn  firmware-amd-graphics     <none>
pn  firmware-atheros          <none>
pn  firmware-bnx2             <none>
pn  firmware-bnx2x            <none>
pn  firmware-brcm80211        <none>
pn  firmware-cavium           <none>
pn  firmware-intel-sound      <none>
pn  firmware-intelwimax       <none>
pn  firmware-ipw2x00          <none>
pn  firmware-ivtv             <none>
pn  firmware-iwlwifi          <none>
pn  firmware-libertas         <none>
pn  firmware-linux-nonfree    <none>
ii  firmware-misc-nonfree     20230210-5
pn  firmware-myricom          <none>
pn  firmware-netxen           <none>
pn  firmware-qlogic           <none>
pn  firmware-realtek          <none>
pn  firmware-samsung          <none>
pn  firmware-siano            <none>
pn  firmware-ti-connectivity  <none>
pn  xen-hypervisor            <none>

I have found the same issue on 6.12.9. Upgrading to 6.12.22 has fixed
the issue for me. Please try with that version. You can force u-boot to
boot an older version by running setenv "fk_kvers 6.xx.xx-n-arm64; boot"
from the u-boot shell.

close 1085305 6.12.22-1
thanks

Closing it with that version. Brian if you still have problems with it, do
reopen the bug please.