Some Notes on using Pixel 4a (Japan version)

I have recently purchased a Pixel 4a from Japan, shipped all the way here to China via EMS. Unfortunately, unlike people living in “the outside world”, doing so means that I have to hack around to bypass some limitations to make it even work like a “normal” Pixel 4a (as sold and used in the US, Canada, or the EU). Specifically,

These two problems are the most serious blockers for using the phone as daily drive. Thankfully, both of these issues can be worked around (at least to some extent) with custom Magisk modules.

Installing Magisk

There is (at the time of writing) no proper TWRP for the phone yet, because it is released with Android 10 and has dynamic partitions. TWRP would also not work very well with A/B devices without a standalone recovery partition.

Like other devices in similar situations, we install Magisk by first installing Magisk Manager, then extracting boot.img from factory image and patching it with Magisk Manager. After that, just flash it into the boot partition using fastboot as usual.

Note that at the time of writing, using Magisk on Android 11 requires opting in the Canary channel.

Global Carrier Support

The core issue of carrier support on Pixels is that there are very few carrier configurations available in RFS (on Pixels, RFS is located in /vendor/rfs, unlike other QCOM devices). I do not know what the rationale behind it is, but this fact makes Pixels basically useless outside the region where they are sold.

One simple fix is to just kang RFS images from devices with the same or similar SoCs and has global carrier support. As Pixel 4a uses Snapdragon 730G, a natural choice of the donor device is the Redmi K30, sporting the same SoC. The RFS image on K30 can be extracted from NON-HLOS.bin inside its factory image, by simply mounting the file using the mount command on Linux (it is actually a VFAT filesystem). What is useful to us, however, is the mcfg_sw directory located somewhere deep in the directory tree. Take that directory and make a Magisk module to override that directory to the path system/vendor/rfs/msm/mpss/readonly/vendor/mbn/ where a directory of the same name can be found, and voila, now the Pixel 4a works perfectly with basically every carrier that it can physically support.

Remember to fix the SELinux contexts and permissions by using the following script

ui_print "-- Setting permissions for modem config files"

find $MODPATH/system/vendor/rfs/msm/mpss/readonly/vendor/mbn/mcfg_sw | while IFS= read -r f; do
  set_perm $f root root 755 u:object_r:vendor_file:s0

(Note: for some reason, even though my Pixel 4a seems to work fine with China Telecom with pure LTE (JPN version has no CDMA support) after the hack, it somehow fails to output audio during outgoing calls over VoLTE. Incoming calls are fine and other carriers seem to be fine, too.)

Remove Screenshot Shutter

This part is easy: the shutter sound is located in /product/media/audio/ui/camera_click.ogg. Just make a Magisk module to override that file with an empty ogg file created using ffmpeg or something similar. Also remember to fix the SELinux context properly.

I tried to remove the camera shutter but failed. Normally one would expect the camera to use the same audio resource file in /product, but apparently for Google Camera this is not true. It has its own shutter sound file packaged inside its APK, and I cannot think of a sane way to remove it without using things like Xposed. I also tried to edit its preference XML file in /data but that also did not work. I figured that camera shutter is not as annoying as screenshot shutter, so I decided to just live with it at last.

Upgrading Android OS

After installing Magisk, the automatic system upgrade functionality no longer works, and the old trick of restoring boot image and re-flashing Magisk to the other slot also stopped working as of Android 11. What I do instead is:

  1. Download latest factory image from Google
  2. IMPORTANT: Edit flash-all.{bat,sh} and to remove all -w flags after fastboot. This prevents wiping data.
  3. Extract boot.img from image-<codename>-<build_number>.zip
  4. Patch that boot.img with Magisk Manager on the phone
  5. Fetch the patched image back to PC and replace the one in the zip with the patched one
  6. Reboot to fastboot and execute flash_all.{bat,sh}