Hi Everyone
I've recompiled the Debian Backports kernel 5.9.6 (with config changes documented above by
devfra@inventati.org) on the Asus T100TA along with an i915 driver (in the firmware-misc-nonfree package) from Debian buster-backports. I can confirm screen backlight can be adjusted and power consumption is reduced. I also ran dmesg after triggering hibernation (closing the laptop). It suggests the laptop is now moving into idle without a problem, but can't enter into hibernation. It still seems to be unable to find crystalcove, but now can control backlight with PWM.
Hope this helps!
Will
$sudo dmesg
...
[ 0.000000] DMI: ASUSTeK COMPUTER INC. T100TAM/T100TAM, BIOS T100TAM.205 07/25/2014
...
[ 3.972360] i915 0000:00:02.0: vgaarb: deactivate vga console
[ 3.973773] i915 0000:00:02.0: vgaarb: changed VGA decodes: olddecodes=io+mem,decodes=io+mem:owns=io+mem
[ 3.974167] i915 0000:00:02.0: cannot find GPIO chip gpio_crystalcove, deferring
[ 3.974169] i915 0000:00:02.0: [drm] *ERROR* Failed to own gpio for panel control
[ 3.974697] i915 0000:00:02.0: [drm] Using PMIC PWM for LCD backlight control
[ 3.981676] [drm] Initialized i915 1.6.0 20200715 for 0000:00:02.0 on minor 0
...
[ 3196.039364] PM: suspend entry (s2idle)
[ 3196.058281] Filesystems sync: 0.018 seconds
[ 3196.061135] (NULL device *): firmware: direct-loading firmware regulatory.db
[ 3196.068645] (NULL device *): firmware: direct-loading firmware brcm/brcmfmac43241b4-sdio.txt
[ 3196.068741] (NULL device *): firmware: direct-loading firmware brcm/brcmfmac43241b4-sdio.bin
[ 3196.070542] (NULL device *): firmware: direct-loading firmware intel/fw_sst_0f28.bin
[ 3196.071389] (NULL device *): firmware: direct-loading firmware regulatory.db.p7s
[ 3196.071466] Freezing user space processes ... (elapsed 0.031 seconds) done.
[ 3196.103199] OOM killer disabled.
[ 3196.103201] Freezing remaining freezable tasks ... (elapsed 0.001 seconds) done.
[ 3196.105184] printk: Suspending console(s) (use no_console_suspend to debug)
[ 3196.154980] i2c_hid i2c-SIS0817:00: failed to change power setting.
[ 3200.390971] OOM killer enabled.
[ 3200.390973] Restarting tasks ... done.
[ 3200.421981] PM: suspend exit
[ 3217.236676] IPv6: ADDRCONF(NETDEV_CHANGE): wlan0: link becomes ready
$ apt show firmware-misc-nonfree
Package: firmware-misc-nonfree
Version: 20200918-1~bpo10+1
...