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[ROM][UNOFFICIAL] LineageOS 18.1 for castor/castor_windy

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allcustom

New member
Sep 11, 2021
1
0
There might be something wrong with castor_windy boot.img? I simply just cannot get Lineage, not 17.1 or 18.1 to boot, both are just blank screen after image flash reboot, even no more access to recovery using vol-, and I have tried perhaps everything. Tried both TWRP and Lineage recovery, wiping all, when fastboot flashing Lineage boot.img it will go wrong. AICP does work just fine after all testing.

When windy gets this stuck state, power button does vibrate shortly, I can get to fastboot vol+ and connect usb. So it never gets full brick, if flash Lineage boot.img by using: fastboot boot boot.img (Lineage version) will get windy to problems.

I am not sure does TWRP flash also boot.img from inside Lineage.zip to boot partition, I did try to swap boot.img inside Lineage.zip to AICP version before flashing it thru recovery, which did not help either.

Any help appreciated!
 

3RW4NFR

Member
Jan 25, 2017
27
5
There might be something wrong with castor_windy boot.img? I simply just cannot get Lineage, not 17.1 or 18.1 to boot, both are just blank screen after image flash reboot, even no more access to recovery using vol-, and I have tried perhaps everything. Tried both TWRP and Lineage recovery, wiping all, when fastboot flashing Lineage boot.img it will go wrong. AICP does work just fine after all testing.

When windy gets this stuck state, power button does vibrate shortly, I can get to fastboot vol+ and connect usb. So it never gets full brick, if flash Lineage boot.img by using: fastboot boot boot.img (Lineage version) will get windy to problems.

I am not sure does TWRP flash also boot.img from inside Lineage.zip to boot partition, I did try to swap boot.img inside Lineage.zip to AICP version before flashing it thru recovery, which did not help either.

Any help appreciated!
I didn't flash any boot.img with fastboot, just flashed TWRP and from TWRP flashed the lineage zip file and works fine
 

earis

New member
Sep 20, 2021
2
1
Gave it a go and tried to install this ROM. TWRP installation went fine, but when rebooting into the system I wasn't able to anymore.

Basically the screen is completely blank (I don't even see the Sony logo). I left it to discharge completely and was able to enter fastboot and flash mode. Tried then to reinstall from scratch an old ftf file via flashtool, but I wasn't able. Then I thought that since I have fastboot access I can flash the Lineage recovery and reinstall everything from there. Command line told me that everything was in order with the flash, but then when rebooting into recovery, nothing happens - same black screen.

Currently I am charging it and the red led is blinking for the last 15 minutes or so. Once there is some power into it, fastboot or flash mode cannot be accessed since as soon as I connect the USB cable it starts blinking red. Turning it off seems impossible as I don't know when it is off as the screen is blank. It does vibrate, but that is not helping that much in the current situation.


Any help or advice would be highly appreciated.
I'm having the same problem with my SGP512JP.
I can't flash any version of TWRP from StockROM.
In addition, "fastboot boot recovery.img" doesn't even show the Sony logo.

Only, I was able to install AICP15 by the following procedure.
1. flash "SGP512_23.0.1.A.4.30".
2. get root with "rootkitXperia".
3. install TWRP with "TabZ2-lockeddualrecovery2.8.26".
4. install AICP15 from TWRP
5. update TWRP with "fastboot flash recovery TWRP-3.4.0-sc-1.img".

In this state, when I flash LineageOS17.1 or 18.1 from TWRP, I can't boot again as the first time, and I have to start over from flashing StockROM.
 

earis

New member
Sep 20, 2021
2
1
Self-solution.
In SGP512JP, I confirmed the startup by the following procedure.

1. flash "SGP512_23.5.A.1.291".
2. fastboot flash:raw boot lineage-17.1.img
3. fastboot flash recovery lineage-17.1-recovery.img
4. fastboot reboot
5. boot Lineage recovery and burn 18.1 and GApps
6. reboot

In castor_windy, "flash:raw" seems to work well.

I found the following post helpful.
Thanks, @creditabricole
 
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Rusty_Gunn

Senior Member
Jun 30, 2012
119
20
anyone else having challenges where this rom has suddenly decided it no longer has an internet connection? it was working fine up to yesterday. rom is great and would like to keep it running!
 

WimDeWinter

New member
Mar 18, 2020
1
0
45
Lokeren
Hi all,

First of all a BIG thank you to all of you for all your efforts to create and maintain this custom ROM.

I have a curious issue though, I don't even know if this is the right place to ask, but here it goes since I'm completely stuck.

I am using a VPN service: AzireVPN, I'm using their official android client: Google Play store link.

I am using the same client on my Sony XZ3C smartphone with Lineage 18.1 and it works flawless. I cannot get it to properly work on my tablet though. I can get it to connect to the VPN service, but there's a handy feature that doesn't work. The app offers the possibility to let applications bypass the VPN tunnel. I use this for instance to connect the kore remote app to my kodi server. This works without issues on my XZ3C, but not on my tablet.

I come from the official 17.1 build and since the app didn't work using that rom, I tried upgrading to 18.1, but to no avail.

Can anybody help me out? Or point me to some place where someone might be able to help? If you're willing to help, what information do you need?

Thanks!
 

Bobbika

Senior Member
Oct 20, 2010
63
8
anyone else having challenges where this rom has suddenly decided it no longer has an internet connection? it was working fine up to yesterday. rom is great and would like to keep it running!
Had the same issue I think, it was connected to the WiFi but it said No internet connection. A reboot fixed it for me.

Apart from this and the NFC flickering, it's a great ROM and I'm grateful that the tablet is still maintained by the dev after all these years.
 

Rusty_Gunn

Senior Member
Jun 30, 2012
119
20
Had the same issue I think, it was connected to the WiFi but it said No internet connection. A reboot fixed it for me.

Apart from this and the NFC flickering, it's a great ROM and I'm grateful that the tablet is still maintained by the dev after all these years.

i feel kind of dumb now, a restart and it is all good now. i spent a couple hours messing around with the route and the tablet.... :/
 
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kapibara

Member
Feb 13, 2010
15
11
Hello. It's much appreciated that development is still happening for these devices, thanks for the work you put in.


I've got a castor_windy/SGP511 that was running CarbonROM 6.1 until yesterday. Installed the Lineage recovery from here, factory reset and installed the latest LineageOS 17.1 nightly starting out, but the device wouldn't boot afterwards.

Figured this was my own fault for not following installation instructions, fair enough. Following the installation guide, I flashed stock 23.5.A.1.291 firmware, but now get stuck at the recovery installation step. After flashing Lineage Recovery with fastboot, I'm not able to boot into it. I've tried flashing TWRP instead of Lineage Recovery as well, but get the same result.


The following is deviating from the installation guide again, but since I was stuck and not getting anywhere: in the CarbonROM installation instructions, one was instructed to extract the boot.img file from the distribution ZIP archive and flash that to the boot partition via fastboot, before flashing the recovery. When I do this the device will just boot loop, while still being unable to access recovery. Have tried with flashing both LineageOS 17.1/18.1 boot.img for that matter.


Can someone please point out where exactly I'm messing this up?
 

Rusty_Gunn

Senior Member
Jun 30, 2012
119
20
which version of twrp? i did the same upgrade path as you, but the first time i had a boot loop too, but then I used the other version of twrp and it worked. specifically twrp 3.2.1 vs 3.4.0.
 

kapibara

Member
Feb 13, 2010
15
11
which version of twrp? i did the same upgrade path as you, but the first time i had a boot loop too, but then I used the other version of twrp and it worked. specifically twrp 3.2.1 vs 3.4.0.
Thanks for the information. I am pretty sure that I've tried with both of these TWRP versions without luck. The 3.4.0 TWRP being the one linked in the OP of this thread, and the 3.2.1 being the one linked in the CarbonROM 6.1 thread. Do you remember which specific recovery build you used and how you managed to flash it?

If I just flash a recovery to the recovery partition (e.g. fastboot flash recovery lineage-17.1-20210929-recovery-castor_windy.img, or fastboot flash recovery twrp_3.2.1-cw.img), nothing happens on reboot. It doesn't make my tablet boot loop, but I am not able to access recovery either.

After reverting to stock firmware, the LED doesn't flash green when the Sony logo shows up in early boot like it used to before, and nothing seems to happen if holding in any combination of the power button and volume buttons.

If I flash the boot.img from lineage-17.1-20210929-nightly-castor_windy-signed.zip to the boot partition and a recovery to recovery, the tablet will no longer boot.

If I flash a recovery and follow it with attempting to boot said recovery (e.g. fastboot flash recovery twrp_3.2.1-cw.img && fastboot boot twrp_3.2.1-cw.img), the tablet seems to hang and will do nothing. I can force shut it off by holding Power Button + Volume Up.


I did another attempt today, ended up soft bricking the tablet again, then used Sony Flash Tool (Ella) to downgrade it to stock 23.1.A.1.28. Had no more luck getting into recovery from there, ended up soft bricking again after attempting to flash LOS boot.img again, and have now flashed back 23.5.A.1.291 in flashmode.

Surely there must be something really basic I'm missing, but on both 23.1.A.1.28 and 23.5.A.1.291 the LED never seemed to light up alongside the Sony logo, and I never managed to boot into either of the three recovery images I've tried to use.
 

Rusty_Gunn

Senior Member
Jun 30, 2012
119
20
those are the two twrp recoveries I used too. i never tired to go back to an old version of the factory image, so I don't know if that changes anything.

i flashed twrp, used the button combo to get into recovery, then flashed zip from an sd card in twrp. even if you have the wrong twrp, you should still be able to get into it, but not the rom.
 

kapibara

Member
Feb 13, 2010
15
11
those are the two twrp recoveries I used too. i never tired to go back to an old version of the factory image, so I don't know if that changes anything.

i flashed twrp, used the button combo to get into recovery, then flashed zip from an sd card in twrp. even if you have the wrong twrp, you should still be able to get into it, but not the rom.
Thank you so much Rusty_Gunn! It was extremely valuable for me to get this confirmation that you're supposed to be able to get into the recovery either way, as I was getting close to giving up. Posting a writeup of my experience here, in case someone else ends up in the same situation.


In my case, the issue I was not getting past (because I quite literally couldn't see it), was that my SGP511's display won't turn on when entering recovery. I suppose I probably entered both TWRP and Lineage Recovery lots of times without realizing it over the past few days.


I used the recovery image twrp_3.2.1-castor_windy.img (SHA256 checksum: 2de0567f65ab3dae6256a36a4cdd57d9cd24b8d69c21fc15a5a0d1a33a3b411e) for this. Since half the download links related to this device were dead when I tried to visit them (the tablet is getting old by now), I uploaded a copy of the recovery image to the Internet Archive. Might need it myself in the future, for that matter.

On stock 23.5.A.1.291 firmware I enabled debugging and rebooted to fastboot mode:
$ adb reboot bootloader

Once the device was ready, I flashed and booted the recovery:
$ fastboot devices
$MY_DEVICE fastboot
$ fastboot flash recovery twrp_3.2.1-castor_windy.img
Sending 'recovery' (13074 KB) OKAY [ 0.420s]
Writing 'recovery' OKAY [ 0.882s]
Finished. Total time: 1.314s
$ fastboot boot twrp_3.2.1-castor_windy.img
Sending 'boot.img' (13074 KB) OKAY [ 0.412s]
Booting OKAY [ 0.054s]
Finished. Total time: 0.487s


I left the device for some minutes after issuing the above command. Expecting it to just be frozen, I ran the following out of curiosity:
$ adb devices
List of devices attached
$MY_DEVICE recovery


...and to my surprise, it had actually booted into recovery, but I didn't realize this as the display wouldn't turn on.

Thankfully TWRP exposes everything needed for installation via adb, so I went right ahead with installing the latest nightly build of LineageOS 17.1:

$ adb shell
~ # twrp wipe cache
Formatting Cache using make_ext4fs...
Done processing script file
~ # twrp wipe dalvik
Wiping Dalvik Cache Directories...
Cleaned: /data/dalvik-cache...
-- Dalvik Cache Directories Wipe Complete!
Done processing script file
~ # twrp wipe system
Formatting System using make_ext4fs...
Done processing script file
~ # twrp wipe data
Formatting cache using make_ext4fs...
Wiping data without wiping /data/media ...
Done.
Done processing script file
~ # twrp sideload
command is: 'sideload' and there is no value
Starting ADB sideload feature...
$ adb sideload lineage-17.1-20211006-nightly-castor_windy-signed.zip
Total xfer: 1.00x
$ adb shell
~ # twrp sideload
$ adb sideload open_gapps-arm-10.0-pico-20211006.zip
Total xfer: 1.22x
$ adb reboot


(In case it's not clear from the above output: switching from recovery to sideload mode kicks you back out from the tablet's shell.)


This got me up and running again!
 

Sacktreter

Member
Aug 24, 2015
33
26
.... Once the device was ready, I flashed and booted the recovery:
$ fastboot devices
$MY_DEVICE fastboot
$ fastboot flash recovery twrp_3.2.1-castor_windy.img
Sending 'recovery' (13074 KB) OKAY [ 0.420s]
Writing 'recovery' OKAY [ 0.882s]
Finished. Total time: 1.314s

$ fastboot boot twrp_3.2.1-castor_windy.img
Sending 'boot.img' (13074 KB) OKAY [ 0.412s]
Booting OKAY [ 0.054s]
Finished. Total time: 0.487s

...
First you flashed the twrp_3.2.1-castor_windy.img file in the boot partition, then the same file in the recovery patition.

So Tablet has to start in TWRP because there is no other solution for the tablet.

Your luck has been: When LineageOS is installed the boot sector became overwritten by boot.img and your tablet got the right way to boot normaly with option to start TWRP.

But a good way to got both. LineageOS and TWRP :)
 
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kapibara

Member
Feb 13, 2010
15
11
First you flashed the twrp_3.2.1-castor_windy.img file in the boot partition, then the same file in the recovery patition.

So Tablet has to start in TWRP because there is no other solution for the tablet.

Your luck has been: When LineageOS is installed the boot sector became overwritten by boot.img and your tablet got the right way to boot normaly with option to start TWRP.

But a good way to got both. LineageOS and TWRP :)
There's a bit of a difference actually.

Notice that I'm running fastboot flash recovery recovery.img first, and then fastboot boot recovery.img afterwards.

The first command, fastboot flash, will write a given file to a partition. The second, fastboot boot, will stream the contents of the file to the device and boot it directly, without writing over a partition.

But I agree that it's probably redundant as presented in my post, and that you could likely get away with only running fastboot flash recovery recovery.img. I decided to post the exact steps I took instead of modifying anything because I wasn't planning on going back and testing the instructions again, rather than potentially cutting out anything important.
 
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rcll

Senior Member
Oct 15, 2008
254
11
Great work! Does anyone have this Lineage 18.1 build running on Castor (SGP521)?
Are there any issues different from the Castor_Windy issues?

It seems there is an official Lineage 17.1 for Castor, but an 18.1 for Castor equally stable to the official 17.1 would be great.
Big thanks for keeping this device updated!
 

Bonpotiron

Member
Oct 27, 2014
11
0
Thank you @kapibara I would never have managed without your instructions!
Now I actually manage to enter TWRP with the button combination. Did we do the combination wrong you think, or was it definitely not working before?
 
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  • 4
    those are the two twrp recoveries I used too. i never tired to go back to an old version of the factory image, so I don't know if that changes anything.

    i flashed twrp, used the button combo to get into recovery, then flashed zip from an sd card in twrp. even if you have the wrong twrp, you should still be able to get into it, but not the rom.
    Thank you so much Rusty_Gunn! It was extremely valuable for me to get this confirmation that you're supposed to be able to get into the recovery either way, as I was getting close to giving up. Posting a writeup of my experience here, in case someone else ends up in the same situation.


    In my case, the issue I was not getting past (because I quite literally couldn't see it), was that my SGP511's display won't turn on when entering recovery. I suppose I probably entered both TWRP and Lineage Recovery lots of times without realizing it over the past few days.


    I used the recovery image twrp_3.2.1-castor_windy.img (SHA256 checksum: 2de0567f65ab3dae6256a36a4cdd57d9cd24b8d69c21fc15a5a0d1a33a3b411e) for this. Since half the download links related to this device were dead when I tried to visit them (the tablet is getting old by now), I uploaded a copy of the recovery image to the Internet Archive. Might need it myself in the future, for that matter.

    On stock 23.5.A.1.291 firmware I enabled debugging and rebooted to fastboot mode:
    $ adb reboot bootloader

    Once the device was ready, I flashed and booted the recovery:
    $ fastboot devices
    $MY_DEVICE fastboot
    $ fastboot flash recovery twrp_3.2.1-castor_windy.img
    Sending 'recovery' (13074 KB) OKAY [ 0.420s]
    Writing 'recovery' OKAY [ 0.882s]
    Finished. Total time: 1.314s
    $ fastboot boot twrp_3.2.1-castor_windy.img
    Sending 'boot.img' (13074 KB) OKAY [ 0.412s]
    Booting OKAY [ 0.054s]
    Finished. Total time: 0.487s


    I left the device for some minutes after issuing the above command. Expecting it to just be frozen, I ran the following out of curiosity:
    $ adb devices
    List of devices attached
    $MY_DEVICE recovery


    ...and to my surprise, it had actually booted into recovery, but I didn't realize this as the display wouldn't turn on.

    Thankfully TWRP exposes everything needed for installation via adb, so I went right ahead with installing the latest nightly build of LineageOS 17.1:

    $ adb shell
    ~ # twrp wipe cache
    Formatting Cache using make_ext4fs...
    Done processing script file
    ~ # twrp wipe dalvik
    Wiping Dalvik Cache Directories...
    Cleaned: /data/dalvik-cache...
    -- Dalvik Cache Directories Wipe Complete!
    Done processing script file
    ~ # twrp wipe system
    Formatting System using make_ext4fs...
    Done processing script file
    ~ # twrp wipe data
    Formatting cache using make_ext4fs...
    Wiping data without wiping /data/media ...
    Done.
    Done processing script file
    ~ # twrp sideload
    command is: 'sideload' and there is no value
    Starting ADB sideload feature...
    $ adb sideload lineage-17.1-20211006-nightly-castor_windy-signed.zip
    Total xfer: 1.00x
    $ adb shell
    ~ # twrp sideload
    $ adb sideload open_gapps-arm-10.0-pico-20211006.zip
    Total xfer: 1.22x
    $ adb reboot


    (In case it's not clear from the above output: switching from recovery to sideload mode kicks you back out from the tablet's shell.)


    This got me up and running again!
    1
    Self-solution.
    In SGP512JP, I confirmed the startup by the following procedure.

    1. flash "SGP512_23.5.A.1.291".
    2. fastboot flash:raw boot lineage-17.1.img
    3. fastboot flash recovery lineage-17.1-recovery.img
    4. fastboot reboot
    5. boot Lineage recovery and burn 18.1 and GApps
    6. reboot

    In castor_windy, "flash:raw" seems to work well.

    I found the following post helpful.
    Thanks, @creditabricole
    1
    Had the same issue I think, it was connected to the WiFi but it said No internet connection. A reboot fixed it for me.

    Apart from this and the NFC flickering, it's a great ROM and I'm grateful that the tablet is still maintained by the dev after all these years.

    i feel kind of dumb now, a restart and it is all good now. i spent a couple hours messing around with the route and the tablet.... :/
    1
    .... Once the device was ready, I flashed and booted the recovery:
    $ fastboot devices
    $MY_DEVICE fastboot
    $ fastboot flash recovery twrp_3.2.1-castor_windy.img
    Sending 'recovery' (13074 KB) OKAY [ 0.420s]
    Writing 'recovery' OKAY [ 0.882s]
    Finished. Total time: 1.314s

    $ fastboot boot twrp_3.2.1-castor_windy.img
    Sending 'boot.img' (13074 KB) OKAY [ 0.412s]
    Booting OKAY [ 0.054s]
    Finished. Total time: 0.487s

    ...
    First you flashed the twrp_3.2.1-castor_windy.img file in the boot partition, then the same file in the recovery patition.

    So Tablet has to start in TWRP because there is no other solution for the tablet.

    Your luck has been: When LineageOS is installed the boot sector became overwritten by boot.img and your tablet got the right way to boot normaly with option to start TWRP.

    But a good way to got both. LineageOS and TWRP :)
    1
    First you flashed the twrp_3.2.1-castor_windy.img file in the boot partition, then the same file in the recovery patition.

    So Tablet has to start in TWRP because there is no other solution for the tablet.

    Your luck has been: When LineageOS is installed the boot sector became overwritten by boot.img and your tablet got the right way to boot normaly with option to start TWRP.

    But a good way to got both. LineageOS and TWRP :)
    There's a bit of a difference actually.

    Notice that I'm running fastboot flash recovery recovery.img first, and then fastboot boot recovery.img afterwards.

    The first command, fastboot flash, will write a given file to a partition. The second, fastboot boot, will stream the contents of the file to the device and boot it directly, without writing over a partition.

    But I agree that it's probably redundant as presented in my post, and that you could likely get away with only running fastboot flash recovery recovery.img. I decided to post the exact steps I took instead of modifying anything because I wasn't planning on going back and testing the instructions again, rather than potentially cutting out anything important.
  • 8
    2okPze5.png



    Introduction
    LineageOS, an open-source Android distribution, is available for several devices,
    with more being continuously added thanks to the biggest, yet ever growing, Android open-source community.
    Join us and breathe new life in your device, be it old or new.
    If you don't know LineageOS and would like to read about it before installing it you can take a look at the official Website.

    Features
    Individuality

    Customization is paramount to productivity.
    That’s why LineageOS promises to push for user personalization and preference.
    Everyone is unique and your device should be too.
    Security
    Your data, your rules. With powerful tools such as Privacy Guard, you are in control of what your apps can do whenever you want.
    Trust will help you understand the security of your device and warn you about possible threats.
    We take security very seriously: that’s why we deliver security updates every month to all our supported devices.
    And to make your device more secure, lock everything behind an enhanced lock screen.
    Longevity
    LineageOS extends the functionality and lifespan of mobile devices from more than 20 different manufacturers thanks to our open-source community of contributors from all around the world.
    Installation:
    If you are on stock OS, you need a custom recovery first. You can get the recommended recovery in the official installation instructions link below.
    If you are coming from stock or other ROMs, you need to make a factory reset.
    As always, make sure to backup before installing this ROM.

    More detailed instructions at:
    Install LineageOS on castor.

    Downloads
    castor: Download LineageOS 18.1 for Xperia Tablet Z2 LTE
    castor_windy: Download LineageOS 18.1 for Xperia Tablet Z2 Wifi
    If you prefer TWRP over Lineage recovery, you can get it here.
    Recommended Google Apps package: Open GApps (Choose the pico package)

    Source code
    All the source code for LineageOS is available in the LineageOS Github repo. And if you would like to contribute to LineageOS, please visit out Gerrit Code Review.

    The device specific source code can be found in the LineageOS Github repo.
    Tablet Z2 LTE device tree
    Tablet Z2 Wi-Fi device tree
    Sony Shinano common device tree
    Sony MSM8974 common device tree
    Sony MSM8974 kernel

    Changelog:
    Changes for castor

    Bug reports:
    Please submit bugs and other errors in this thread
    How to submit a bug report
    LineageOS GitLab


    Donate to support development:
    Donate via PayPal to vware
    Donate via PayPal to LineageOS

    Contributors
    Georg Veichtlbauer
    NeoArian (Common)
    ROM OS Version: Android 11
    ROM Kernel: Linux 3.4.x

    Version Information
    Status:
    Stable

    Created 2021-04-13
    Last Updated 2021-07-19
    4
    those are the two twrp recoveries I used too. i never tired to go back to an old version of the factory image, so I don't know if that changes anything.

    i flashed twrp, used the button combo to get into recovery, then flashed zip from an sd card in twrp. even if you have the wrong twrp, you should still be able to get into it, but not the rom.
    Thank you so much Rusty_Gunn! It was extremely valuable for me to get this confirmation that you're supposed to be able to get into the recovery either way, as I was getting close to giving up. Posting a writeup of my experience here, in case someone else ends up in the same situation.


    In my case, the issue I was not getting past (because I quite literally couldn't see it), was that my SGP511's display won't turn on when entering recovery. I suppose I probably entered both TWRP and Lineage Recovery lots of times without realizing it over the past few days.


    I used the recovery image twrp_3.2.1-castor_windy.img (SHA256 checksum: 2de0567f65ab3dae6256a36a4cdd57d9cd24b8d69c21fc15a5a0d1a33a3b411e) for this. Since half the download links related to this device were dead when I tried to visit them (the tablet is getting old by now), I uploaded a copy of the recovery image to the Internet Archive. Might need it myself in the future, for that matter.

    On stock 23.5.A.1.291 firmware I enabled debugging and rebooted to fastboot mode:
    $ adb reboot bootloader

    Once the device was ready, I flashed and booted the recovery:
    $ fastboot devices
    $MY_DEVICE fastboot
    $ fastboot flash recovery twrp_3.2.1-castor_windy.img
    Sending 'recovery' (13074 KB) OKAY [ 0.420s]
    Writing 'recovery' OKAY [ 0.882s]
    Finished. Total time: 1.314s
    $ fastboot boot twrp_3.2.1-castor_windy.img
    Sending 'boot.img' (13074 KB) OKAY [ 0.412s]
    Booting OKAY [ 0.054s]
    Finished. Total time: 0.487s


    I left the device for some minutes after issuing the above command. Expecting it to just be frozen, I ran the following out of curiosity:
    $ adb devices
    List of devices attached
    $MY_DEVICE recovery


    ...and to my surprise, it had actually booted into recovery, but I didn't realize this as the display wouldn't turn on.

    Thankfully TWRP exposes everything needed for installation via adb, so I went right ahead with installing the latest nightly build of LineageOS 17.1:

    $ adb shell
    ~ # twrp wipe cache
    Formatting Cache using make_ext4fs...
    Done processing script file
    ~ # twrp wipe dalvik
    Wiping Dalvik Cache Directories...
    Cleaned: /data/dalvik-cache...
    -- Dalvik Cache Directories Wipe Complete!
    Done processing script file
    ~ # twrp wipe system
    Formatting System using make_ext4fs...
    Done processing script file
    ~ # twrp wipe data
    Formatting cache using make_ext4fs...
    Wiping data without wiping /data/media ...
    Done.
    Done processing script file
    ~ # twrp sideload
    command is: 'sideload' and there is no value
    Starting ADB sideload feature...
    $ adb sideload lineage-17.1-20211006-nightly-castor_windy-signed.zip
    Total xfer: 1.00x
    $ adb shell
    ~ # twrp sideload
    $ adb sideload open_gapps-arm-10.0-pico-20211006.zip
    Total xfer: 1.22x
    $ adb reboot


    (In case it's not clear from the above output: switching from recovery to sideload mode kicks you back out from the tablet's shell.)


    This got me up and running again!
    4
    new release is up. due to summer, investigating nfc is going a bit slowly, everything else should be working just fine. so please go and check it out :) links in the first post are updated.
    4

    that should solve the problems people have been having with magisk. no support here for such things though guys, so please keep on topic.
    3
    I've just noticed the first official build for castor_windy is now available: https://download.lineageos.org/castor_windy

    Thank you @qsnc!