Tweak/unbreak my CPUFreq experiment (#4119)

* Only switch to ondemand when we actually can, and when it's better than the current governor...
  This potentially leaves Mk.5 in the lurch, but there's no perfect solution there :/.

* Switch to UI for the unmark event of navigation hints markers
Might help on non-REAGL devices, and doesn't hurt there (re #3983).

* Switch SQLite DBs to WAL
Sounds nice in theory, behaves fine in practice.

frontend/apps/reader/modules/readerrolling.lua

@@ -497,7 +497,7 @@ function ReaderRolling:onGotoXPointer(xp, marker_xp)
                     -- documents): we drew our black marker in the margin, we
                     -- can just draw a white one to make it disappear
                     Screen.bb:paintRect(0, screen_y, marker_w, marker_h, Blitbuffer.COLOR_WHITE)
-                    Screen["refreshFast"](Screen, 0, screen_y, marker_w, marker_h)
+                    Screen["refreshUI"](Screen, 0, screen_y, marker_w, marker_h)
                 end
                 UIManager:scheduleIn(marker_setting, self.unmark_func)
             end

platform/kobo/koreader.sh

@@ -7,9 +7,21 @@ KOREADER_DIR="${0%/*}"
 # we're always starting from our working directory
 cd "${KOREADER_DIR}" || exit
 
-# Switch to a sensible CPUFreq governor, even if the HW appears not to give an actual fuck about this...
-ORIG_CPUFREQ_GOV="$(cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor)"
-echo "ondemand" >"/sys/devices/system/cpu/cpu0/cpufreq/scaling_governor"
+# Attempt to switch to a sensible CPUFreq governor when that's not already the case...
+current_cpufreq_gov="$(cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor)"
+# NOTE: We're being fairly conservative here, because what's used and what's available varies depending on HW...
+if [ "${current_cpufreq_gov}" != "ondemand" ] && [ "${current_cpufreq_gov}" != "interactive" ]; then
+    # NOTE: Go with ondemand, because it's likely to be the lowest common denominator.
+    #       Plus, interactive is hard to tune right, and only really interesting when it's a recent version,
+    #       which I somehow doubt is the case anywhere here...
+    if grep -q ondemand /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_governors; then
+        ORIG_CPUFREQ_GOV="${current_cpufreq_gov}"
+        echo "ondemand" >"/sys/devices/system/cpu/cpu0/cpufreq/scaling_governor"
+    fi
+fi
+# NOTE: That doesn't actually help us poor userspace plebs, but, short of switching to performance,
+#       I don't really have a golden bullet here... (conservative's rubberbanding is terrible, so that's a hard pass).
+#       All I can say is that userspace is a terrible idea and behaves *very* strangely (c.f., #4114).
 
 # update to new version from OTA directory
 ko_update_check() {
@@ -136,8 +148,10 @@ while [ $RETURN_VALUE -eq 85 ]; do
     RETURN_VALUE=$?
 done
 
-# Restore original CPUFreq governor...
-echo "${ORIG_CPUFREQ_GOV}" >"/sys/devices/system/cpu/cpu0/cpufreq/scaling_governor"
+# Restore original CPUFreq governor if need be...
+if [ -n "${ORIG_CPUFREQ_GOV}" ]; then
+    echo "${ORIG_CPUFREQ_GOV}" >"/sys/devices/system/cpu/cpu0/cpufreq/scaling_governor"
+fi
 
 if [ "${FROM_NICKEL}" = "true" ]; then
     if [ "${FROM_KFMON}" != "true" ]; then

plugins/coverbrowser.koplugin/bookinfomanager.lua

@@ -138,6 +138,8 @@ end
 
 function BookInfoManager:createDB()
     local db_conn = SQ3.open(self.db_location)
+    -- Make it WAL
+    db_conn:exec("PRAGMA journal_mode=WAL;")
     -- Less error cases to check if we do it that way
     -- Create it (noop if already there)
     db_conn:exec(BOOKINFO_DB_SCHEMA)

plugins/statistics.koplugin/main.lua

@@ -266,6 +266,8 @@ function ReaderStatistics:partialMd5(file)
 end
 
 function ReaderStatistics:createDB(conn)
+    -- Make it WAL
+    conn:exec("PRAGMA journal_mode=WAL;")
     local sql_stmt = [[
         CREATE TABLE IF NOT EXISTS book
             (