linux: configure aarch64 contiguous memory allocator via kernel config

As per the in-line comment, this is where distros should configure it.
Not via kernel command line parameters.

As found by looking at the implementation, while exploring the cause of
a bug on the Raspberry Pi 4, it was found that `cma=` on the command
line parameters will overwrite the values a device tree will have
configured for a given platform.

With this, the more recent 5.4 vendor kernel boots just fine on the
Raspberry Pi 4 using our common configuration.

nixos/modules/installer/cd-dvd/sd-image-aarch64.nix

@@ -17,8 +17,7 @@
   # The serial ports listed here are:
   # - ttyS0: for Tegra (Jetson TX1)
   # - ttyAMA0: for QEMU's -machine virt
-  # Also increase the amount of CMA to ensure the virtual console on the RPi3 works.
-  boot.kernelParams = ["cma=32M" "console=ttyS0,115200n8" "console=ttyAMA0,115200n8" "console=tty0"];
+  boot.kernelParams = ["console=ttyS0,115200n8" "console=ttyAMA0,115200n8" "console=tty0"];
 
   boot.initrd.availableKernelModules = [
     # Allows early (earlier) modesetting for the Raspberry Pi

pkgs/os-specific/linux/kernel/common-config.nix

@@ -819,6 +819,12 @@ let
 
       # See comments on https://github.com/NixOS/nixpkgs/commit/9b67ea9106102d882f53d62890468071900b9647
       CRYPTO_AEGIS128_SIMD = whenAtLeast "5.4" no;
+
+      # Distros should configure the default as a kernel option.
+      # We previously defined it on the kernel command line as cma=
+      # The kernel command line will override a platform-specific configuration from its device tree.
+      # https://github.com/torvalds/linux/blob/856deb866d16e29bd65952e0289066f6078af773/kernel/dma/contiguous.c#L35-L44
+      CMA_SIZE_MBYTES = freeform "32";
     };
   };
 in