[Samsung Galaxy Tab A series ROMs, Kernels, Recover] robobenklein:
Thanks, that got me a lot further, now I'm at a problem I don't really know how to solve immediately: the graphics allocator service binary (`android.hardware.graphics.allocator@2.0-service`, aka 'gralloc') doesn't seem to have a 64-bit binary.

I'm assuming that's why surfaceflinger is causing the whole system crash, "Abort message: 'gralloc-mapper is missing'"

Relevant logs:

07-26 21:44:24.915 3826 3826 I ServiceManagement: Removing namespace from process name android.hardware.graphics.allocator@2.0-service to allocator@2.0-service.
07-26 21:44:24.917 3826 3826 I android.hardware.graphics.allocator@2.0-service: Registration complete for android.hardware.graphics.allocator@2.0::IAllocator/default.
...
07-26 21:44:25.039 3827 3827 I ServiceManagement: Removing namespace from process name android.hardware.graphics.composer@2.1-service to composer@2.1-service.
07-26 21:44:25.042 3827 3827 I android.hardware.graphics.composer@2.1-service: Registration complete for android.hardware.graphics.composer@2.1::IComposer/default.
...
07-26 21:44:25.251 3850 3850 I SurfaceFlinger: SurfaceFlinger's main thread ready to run. Initializing graphics H/W...
07-26 21:44:25.498 3850 3850 D libEGL : loaded /vendor/lib64/egl/libGLES_mali.so
07-26 21:44:25.604 3850 3850 I RenderEngine: OpenGL ES informations:
07-26 21:44:25.604 3850 3850 I RenderEngine: vendor : ARM
07-26 21:44:25.604 3850 3850 I RenderEngine: renderer : Mali-G71
07-26 21:44:25.604 3850 3850 I RenderEngine: version : OpenGL ES 3.2 v1.r16p0-01rel0.###other-sha0123456789ABCDEF0###
07-26 21:44:25.604 3850 3850 I RenderEngine: extensions: GL_EXT_debug_marker GL_ARM_rgba8 GL_ARM_mali_shader_binary GL_OES_depth24 GL_OES_depth_texture GL_OES_depth_texture_cube_map GL_OES_packed_depth_stencil GL_OES_rgb8_rgba8 GL_EXT_read_format_bgra GL_OES_compressed_paletted_texture GL_OES_compressed_ETC1_RGB8_texture GL_OES_standard_derivatives GL_OES_EGL_image GL_OES_EGL_image_external GL_OES_EGL_image_external_essl3 GL_OES_EGL_sync GL_OES_texture_npot GL_OES_vertex_half_float GL_OES_required_internalformat GL_OES_vertex_array_object GL_OES_mapbuffer GL_EXT_texture_format_BGRA8888 GL_EXT_texture_rg GL_EXT_texture_type_2_10_10_10_REV GL_OES_fbo_render_mipmap GL_OES_element_index_uint GL_EXT_shadow_samplers GL_OES_texture_compression_astc GL_KHR_texture_compression_astc_ldr GL_KHR_texture_compression_astc_hdr GL_KHR_texture_compression_astc_sliced_3d GL_KHR_debug GL_EXT_occlusion_query_boolean GL_EXT_disjoint_timer_query GL_EXT_blend_minmax GL_EXT_discard_framebuffer GL_OES_get_program_binary GL_OES_texture_3D GL_EXT_texture_storage GL_EXT_multisamp
07-26 21:44:25.604 3850 3850 I RenderEngine: GL_MAX_TEXTURE_SIZE = 8192
07-26 21:44:25.604 3850 3850 I RenderEngine: GL_MAX_VIEWPORT_DIMS = 8192
...
07-26 21:44:25.658 3364 3364 I hwservicemanager: getTransport: Cannot find entry android.hardware.graphics.mapper@3.0::IMapper/default in either framework or device manifest.
07-26 21:44:25.658 3850 3850 W Gralloc3: mapper 3.x is not supported
07-26 21:44:25.663 3850 3850 W Gralloc2: mapper 2.x is not supported
07-26 21:44:25.664 3850 3850 F GraphicBufferMapper: gralloc-mapper is missing
07-26 21:44:25.664 3850 3850 F libc : Fatal signal 6 (SIGABRT), code -1 (SI_QUEUE) in tid 3850 (surfaceflinger), pid 3850 (surfaceflinger)
...
07-26 21:44:25.764 3967 3967 I crash_dump64: performing dump of process 3850 (target tid = 3850)
07-26 21:44:25.769 3967 3967 F DEBUG : *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
07-26 21:44:25.769 3967 3967 F DEBUG : LineageOS Version: '17.1-20200726-UNOFFICIAL-'
07-26 21:44:25.769 3967 3967 F DEBUG : Build fingerprint: 'samsung/gta3xlwifixx/gta3xlwifi:10/QQ3A.200705.002/eng.robo.20200725.212218:userdebug/test-keys'
07-26 21:44:25.769 3967 3967 F DEBUG : Revision: '4'
07-26 21:44:25.769 3967 3967 F DEBUG : ABI: 'arm64'
07-26 21:44:25.771 3967 3967 F DEBUG : Timestamp: 2020-07-26 21:44:25-0400
07-26 21:44:25.771 3967 3967 F DEBUG : pid: 3850, tid: 3850, name: surfaceflinger >>> /system/bin/surfaceflinger <<<
07-26 21:44:25.771 3967 3967 F DEBUG : uid: 1000
07-26 21:44:25.772 3967 3967 F DEBUG : signal 6 (SIGABRT), code -1 (SI_QUEUE), fault addr --------
07-26 21:44:25.772 3967 3967 F DEBUG : Abort message: 'gralloc-mapper is missing'
07-26 21:44:25.772 3967 3967 F DEBUG : x0 0000000000000000 x1 0000000000000f0a x2 0000000000000006 x3 0000007fff826970
07-26 21:44:25.772 3967 3967 F DEBUG : x4 fefeff725af2dfc7 x5 fefeff725af2dfc7 x6 fefeff725af2dfc7 x7 7f7f7f7f7fffffff
07-26 21:44:25.772 3967 3967 F DEBUG : x8 00000000000000f0 x9 e0f9456a55f6fb50 x10 0000000000000001 x11 0000000000000000
07-26 21:44:25.772 3967 3967 F DEBUG : x12 fffffff0ffffffdf x13 ffffffffffffffff x14 0000000000000004 x15 ffffffffffffffff
07-26 21:44:25.772 3967 3967 F DEBUG : x16 000000735c1c08c0 x17 000000735c19e3b0 x18 0000000000000004 x19 0000000000000f0a
07-26 21:44:25.772 3967 3967 F DEBUG : x20 0000000000000f0a x21 00000000ffffffff x22 0000000000000001 x23 0000000000000780
07-26 21:44:25.772 3967 3967 F DEBUG : x24 00000000000004b0 x25 000000735c5f3640 x26 000000735c77a020 x27 000000735c5f3640
07-26 21:44:25.772 3967 3967 F DEBUG : x28 0000000000000000 x29 0000007fff826a10
07-26 21:44:25.772 3967 3967 F DEBUG : sp 0000007fff826950 lr 000000735c151f48 pc 000000735c151f74
07-26 21:44:25.807 3967 3967 F DEBUG :
07-26 21:44:25.807 3967 3967 F DEBUG : backtrace:
07-26 21:44:25.807 3967 3967 F DEBUG : #00 pc 0000000000081f74 /apex/com.android.runtime/lib64/bionic/libc.so (abort+160) (BuildId: bf14cf7a62d1f91755beddd4a937354d)
07-26 21:44:25.807 3967 3967 F DEBUG : #01 pc 00000000000089f8 /system/lib64/liblog.so (__android_log_assert+324) (BuildId: fa7cbba17f5086fc8671eb6757477dd0)
07-26 21:44:25.807 3967 3967 F DEBUG : #02 pc 00000000000235b8 /system/lib64/libui.so (android::GraphicBufferMapper::GraphicBufferMapper()+192) (BuildId: 2745ee4ceba7fa8b8b8ea9c7f8c17f6f)
07-26 21:44:25.807 3967 3967 F DEBUG : #03 pc 0000000000020958 /system/lib64/libui.so (android::GraphicBuffer::GraphicBuffer()+148) (BuildId: 2745ee4ceba7fa8b8b8ea9c7f8c17f6f)
07-26 21:44:25.807 3967 3967 F DEBUG : #04 pc 0000000000020af4 /system/lib64/libui.so (android::GraphicBuffer::GraphicBuffer(unsigned int, unsigned int, int, unsigned int, unsigned long, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>)+68) (BuildId: 2745ee4ceba7fa8b8b8ea9c7f8c17f6f)
07-26 21:44:25.807 3967 3967 F DEBUG : #05 pc 0000000000073fdc /system/lib64/libgui.so (android::BufferQueueProducer::allocateBuffers(unsigned int, unsigned int, int, unsigned long)+204) (BuildId: b6d9aa74796a820d2734edd87ab51b5a)
07-26 21:44:25.808 3967 3967 F DEBUG : #06 pc 00000000000dd840 /system/lib64/libsurfaceflinger.so (android::SurfaceFlinger::processDisplayChangesLocked()+4428) (BuildId: 96b996cfa12bca08accd8031151ec993)
07-26 21:44:25.808 3967 3967 F DEBUG : #07 pc 00000000000d1d08 /system/lib64/libsurfaceflinger.so (android::SurfaceFlinger::processDisplayHotplugEventsLocked()+4756) (BuildId: 96b996cfa12bca08accd8031151ec993)
07-26 21:44:25.808 3967 3967 F DEBUG : #08 pc 00000000000d4db4 /system/lib64/libsurfaceflinger.so (android::SurfaceFlinger::onHotplugReceived(int, unsigned long, HWC2::Connection)+324) (BuildId: 96b996cfa12bca08accd8031151ec993)
07-26 21:44:25.808 3967 3967 F DEBUG : #09 pc 000000000009b48c /system/lib64/libsurfaceflinger.so (_ZN4HWC212_GLOBAL__N_122ComposerCallbackBridge9onHotplugEmN7android8hardware8graphics8composer4V2_117IComposerCallback10ConnectionE$fb84e2fe6d73b02ebb46161904c61b1b+36) (BuildId: 96b996cfa12bca08accd8031151ec993)
07-26 21:44:25.808 3967 3967 F DEBUG : #10 pc 0000000000026420 /system/lib64/android.hardware.graphics.composer@2.1.so (android::hardware::graphics::composer::V2_1::BnHwComposerCallback::_hidl_onHotplug(android::hidl::base::V1_0::BnHwBase*, android::hardware::Parcel const&, android::hardware::Parcel*, std::__1::function<void (android::hardware::Parcel&)>)+208) (BuildId: d08fd29cad9e4b935f9c1d21b7be9455)
07-26 21:44:25.808 3967 3967 F DEBUG : #11 pc 0000000000026f30 /system/lib64/android.hardware.graphics.composer@2.1.so (android::hardware::graphics::composer::V2_1::BnHwComposerCallback::onTransact(unsigned int, android::hardware::Parcel const&, android::hardware::Parcel*, unsigned int, std::__1::function<void (android::hardware::Parcel&)>)+528) (BuildId: d08fd29cad9e4b935f9c1d21b7be9455)
07-26 21:44:25.808 3967 3967 F DEBUG : #12 pc 0000000000095d9c /system/lib64/libhidlbase.so (android::hardware::BHwBinder::transact(unsigned int, android::hardware::Parcel const&, android::hardware::Parcel*, unsigned int, std::__1::function<void (android::hardware::Parcel&)>)+68) (BuildId: 22cf05c5c6786bd59f2bc2b7d4104821)
07-26 21:44:25.808 3967 3967 F DEBUG : #13 pc 000000000009b5d8 /system/lib64/libhidlbase.so (android::hardware::IPCThreadState::transact(int, unsigned int, android::hardware::Parcel const&, android::hardware::Parcel*, unsigned int)+1384) (BuildId: 22cf05c5c6786bd59f2bc2b7d4104821)
07-26 21:44:25.808 3967 3967 F DEBUG : #14 pc 000000000009677c /system/lib64/libhidlbase.so (android::hardware::BpHwBinder::transact(unsigned int, android::hardware::Parcel const&, android::hardware::Parcel*, unsigned int, std::__1::function<void (android::hardware::Parcel&)>)+72) (BuildId: 22cf05c5c6786bd59f2bc2b7d4104821)
07-26 21:44:25.808 3967 3967 F DEBUG : #15 pc 000000000002b50c /system/lib64/android.hardware.graphics.composer@2.1.so (android::hardware::graphics::composer::V2_1::BpHwComposerClient::_hidl_registerCallback(android::hardware::IInterface*, android::hardware::details::HidlInstrumentor*, android::sp<android::hardware::graphics::composer::V2_1::IComposerCallback> const&)+336) (BuildId: d08fd29cad9e4b935f9c1d21b7be9455)
07-26 21:44:25.809 3967 3967 F DEBUG : #16 pc 000000000008ad40 /system/lib64/libsurfaceflinger.so (android::Hwc2::impl::Composer::registerCallback(android::sp<android::hardware::graphics::composer::V2_1::IComposerCallback> const&)+140) (BuildId: 96b996cfa12bca08accd8031151ec993)
07-26 21:44:25.809 3967 3967 F DEBUG : #17 pc 0000000000096b90 /system/lib64/libsurfaceflinger.so (HWC2::Device::registerCallback(HWC2::ComposerCallback*, int)+200) (BuildId: 96b996cfa12bca08accd8031151ec993)
07-26 21:44:25.809 3967 3967 F DEBUG : #18 pc 00000000000d06e8 /system/lib64/libsurfaceflinger.so (android::SurfaceFlinger::init()+308) (BuildId: 96b996cfa12bca08accd8031151ec993)
07-26 21:44:25.809 3967 3967 F DEBUG : #19 pc 00000000000031bc /system/bin/surfaceflinger (main+364) (BuildId: b78b8993428f334aea26a30d4e6545bc)
07-26 21:44:25.809 3967 3967 F DEBUG : #20 pc 000000000007d780 /apex/com.android.runtime/lib64/bionic/libc.so (__libc_init+108) (BuildId: bf14cf7a62d1f91755beddd4a937354d)

That makes me think that the 64-bit libraries are generally working, now we're just missing the accompanying 64-bit service binaries. The repo that gave the 64bit libs didn't have all the same programs in /bin/hw as the sm-t510 /vendor/bin/hw does, so perhaps we need to find another device on the same board with known working 64-bit stock images to copy from.

Through all this trial and error I haven't yet needed to increase the /vendor partition size, so there may be hope that we can fit everything into the stock device partition layout. (Starting with this ROM's vendor.img, adding lib64, replacing some of bin/hw, one prop modification, still 20-30MB of space left)