Tweak JIT's loopunroll threshold on Kobo 16bpp (#4157)

* Tweak JIT's loopunroll threshold on Kobo 16bpp
    To avoid early blacklist of the blitbuffer code on alpha-blending
    workloads.
    Least terrible workaround we could come up with to "fix" #4137

* gzip kindle/kobo/pb OTA tarballs w/ --rsyncable
    Should make them more zsync friendly.

Makefile

@@ -186,7 +186,7 @@ kindleupdate: all
 	# note that the targz file extension is intended to keep ISP from caching
 	# the file, see koreader#1644.
 	cd $(INSTALL_DIR) && \
-		tar -czah --no-recursion -f ../koreader-$(DIST)-$(MACHINE)-$(VERSION).targz \
+		tar -I"gzip --rsyncable" -cah --no-recursion -f ../koreader-$(DIST)-$(MACHINE)-$(VERSION).targz \
 		-T koreader/ota/package.index
 
 koboupdate: all
@@ -215,7 +215,7 @@ koboupdate: all
 		koreader/ota/package.index koreader.png README_kobo.txt
 	# make gzip koboupdate for zsync OTA update
 	cd $(INSTALL_DIR) && \
-		tar -czah --no-recursion -f ../koreader-kobo-$(MACHINE)-$(VERSION).targz \
+		tar -I"gzip --rsyncable" -cah --no-recursion -f ../koreader-kobo-$(MACHINE)-$(VERSION).targz \
 		-T koreader/ota/package.index
 
 pbupdate: all
@@ -252,7 +252,7 @@ pbupdate: all
 		applications/koreader/ota/package.index system
 	# make gzip pbupdate for zsync OTA update
 	cd $(INSTALL_DIR)/applications && \
-		tar -czah --no-recursion -f ../../koreader-pocketbook-$(MACHINE)-$(VERSION).targz \
+		tar -I"gzip --rsyncable" -cah --no-recursion -f ../../koreader-pocketbook-$(MACHINE)-$(VERSION).targz \
 		-T koreader/ota/package.index
 
 utupdate: all

base

@@ -1 +1 @@
-Subproject commit 08c021af2a1aed63a02c4407c5bbcfd53d15bcec
+Subproject commit 10d3aa43753d36e6449d54609a949524b7bd1959

frontend/device/kobo/device.lua

@@ -23,7 +23,7 @@ local Kobo = Generic:new{
     model = "Kobo",
     isKobo = yes,
     isTouchDevice = yes, -- all of them are
-    hasBGRFrameBuffer = yes, -- has always been the case, even on 16bpp FWs
+    hasBGRFrameBuffer = yes, -- True when >16bpp
 
     -- most Kobos have X/Y switched for the touch screen
     touch_switch_xy = true,
@@ -198,6 +198,18 @@ local KoboNova = Kobo:new{
 
 function Kobo:init()
     self.screen = require("ffi/framebuffer_mxcfb"):new{device = self, debug = logger.dbg}
+    -- NOTE: Something about the extra work needed to handle RGB565 conversions is making the JIT optimizer crazy when doing
+    --       alpha-blending, causing it to very quickly blacklist the entire blitbuffer code, which basically murders performance.
+    --       The least terrible workaround we could come up with is tweaking the optimizer's thresholds a bit,
+    --       to convince it not to give up too early.
+    --       As this may have a detrimental effect on performance, we try to deviate from defaults as little as possible,
+    --       completely avoiding it if possible...
+    --       c.f., #4137 for the gory details.
+    if self.screen.fb_bpp == 16 then
+        logger.info("Enabling Kobo @ 16bpp tweaks")
+        jit.opt.start("loopunroll=45")
+        self.hasBGRFrameBuffer = no
+    end
     self.powerd = require("device/kobo/powerd"):new{device = self}
     self.input = require("device/input"):new{
         device = self,