install-novatek: keep osmem and mem= equal, with ${osmem} in bootargs

en/install-novatek.md

@@ -23,11 +23,46 @@ run uk; run ur; reset
 After you have finished flashing new firmware, please run `firstboot` command
 to format `jffs2` partition used to store settings.
 
-### Known issues
+### Memory split: `osmem` and `mem=`
 
-Change memory address for some tests.
+On these boards U-Boot divides the RAM using the `osmem` variable: Linux gets
+the first `osmem`, and the video pipeline gets the rest. The kernel, however,
+takes its own size from the `mem=` argument in `bootargs`. The two must always
+be equal:
+
+- `osmem` larger than `mem=` — the camera stops right after the kernel is
+  decompressed and prints nothing more.
+- `mem=` larger than `osmem` — Linux and the video pipeline end up using the
+  same memory; the camera crashes and reboots as soon as the video drivers
+  load, on every boot.
+
+Keep a literal `${osmem}` in `bootargs` instead of a number. The `bootcmd`
+these boards use (`setenv setargs setenv bootargs ${bootargs};run setargs;...`)
+expands it at every boot, so `osmem` becomes the only value you change. From
+Linux, replace just the `mem=` value and leave the rest of your `bootargs`,
+partition layout included, exactly as it is:
+
+```
+fw_printenv -n bootargs > /root/bootargs.orig
+fw_setenv bootargs "$(fw_printenv -n bootargs | sed 's/mem=[0-9]*M/mem=${osmem}/')"
+fw_printenv bootargs
+```
+
+The last command must now show `mem=${osmem}`. Copy `/root/bootargs.orig` off
+the camera as well: `firstboot` wipes `/root`, while the U-Boot environment
+survives. To undo: `fw_setenv bootargs "$(cat /root/bootargs.orig)"`.
+
+If `fw_printenv bootcmd` does not start with
+`setenv setargs setenv bootargs ${bootargs};run setargs`, set it from the
+U-Boot console:
 
 ```
 setenv bootcmd 'setenv setargs setenv bootargs ${bootargs};run setargs;sf probe 0;sf read 0x03100000 0x50000 0x200000;nvt_boot'
 saveenv
 ```
+
+Then change the split with `fw_setenv osmem <size>` and reboot. Choose the size
+for your board: everything `osmem` gives to Linux is taken from video memory, so
+a board with less RAM, or a heavier stream, needs a smaller value. As an
+example, a 128 MB NT98566 with a 5 MP sensor boots and streams at both the
+default 32M and at 54M.