Move HW dithering alignment fixup to base (#1290)

* Move the HW dithering align fix to base

It was slightly trickier in base because of potentially negative values
and weird interaction with painting.

Doing it here, post checkBounds, makes much more sense.

ffi/framebuffer_mxcfb.lua

@@ -7,9 +7,24 @@ local dummy = require("ffi/posix_h")
 
 local band = bit.band
 local bor = bit.bor
+local bnot = bit.bnot
 
 local function yes() return true end
 
+-- A couple helper functions to compute aligned values...
+-- c.f., <linux/kernel.h> & ffi/framebuffer_linux.lua
+local function ALIGN_DOWN(x, a)
+    -- x & ~(a-1)
+    local mask = a - 1
+    return band(x, bnot(mask))
+end
+
+local function ALIGN_UP(x, a)
+    -- (x + (a-1)) & ~(a-1)
+    local mask = a - 1
+    return band(x + mask, bnot(mask))
+end
+
 local framebuffer = {
     -- pass device object here for proper model detection:
     device = nil,
@@ -111,6 +126,30 @@ function framebuffer:_isFullScreen(w, h)
     end
 end
 
+-- Clamp w & h to the screen's physical dimensions
+function framebuffer:_clampToPhysicalDim(w, h)
+    -- NOTE: fb:getWidth() & fb:getHeight() return the viewport size, but obey rotation, which means we can't rely on them directly.
+    --       fb:getScreenWidth() & fb:getScreenHeight return the full screen size, without the viewport, and in the default rotation, which doesn't help either.
+    -- Settle for getWidth() & getHeight() w/ rotation handling, like what bb:getPhysicalRect() does...
+    local max_w, max_h
+    if band(self:getRotationMode(), 1) == 1 then
+        max_w = self:getHeight()
+        max_h = self:getWidth()
+    else
+        max_w = self:getWidth()
+        max_h = self:getHeight()
+    end
+
+    if w > max_w then
+        w = max_w
+    end
+    if h > max_h then
+        h = max_h
+    end
+
+    return w, h
+end
+
 --[[ handlers for the wait API of the eink driver --]]
 
 -- Kindle's MXCFB_WAIT_FOR_UPDATE_COMPLETE_PEARL == 0x4004462f
@@ -199,7 +238,7 @@ end
 -- Kobo's MXCFB_SEND_UPDATE == 0x4044462e
 -- Pocketbook's MXCFB_SEND_UPDATE == 0x4040462e
 -- Cervantes MXCFB_SEND_UPDATE == 0x4044462e
-local function mxc_update(fb, update_ioctl, refarea, refresh_type, waveform_mode, x, y, w, h)
+local function mxc_update(fb, update_ioctl, refarea, refresh_type, waveform_mode, x, y, w, h, dither)
     local bb = fb.full_bb or fb.bb
     w, x = BB.checkBounds(w or bb:getWidth(), x or 0, 0, bb:getWidth(), 0xFFFF)
     h, y = BB.checkBounds(h or bb:getHeight(), y or 0, 0, bb:getHeight(), 0xFFFF)
@@ -214,6 +253,28 @@ local function mxc_update(fb, update_ioctl, refarea, refresh_type, waveform_mode
         return
     end
 
+    -- NOTE: If we're requesting hardware dithering on a partial update, make sure the rectangle is using
+    --       coordinates aligned to the previous multiple of 8, and dimensions aligned to the next multiple of 8.
+    --       Otherwise, some unlucky coordinates will play badly with the PxP's own alignment constraints,
+    --       leading to a refresh where content appears to have moved a few pixels to the side...
+    --       (Sidebar: this is probably a kernel issue, the EPDC driver is responsible for the alignment fixup,
+    --       c.f., epdc_process_update @ drivers/video/fbdev/mxc/mxc_epdc_v2_fb.c on a Kobo Mk. 7 kernel...).
+    if dither then
+        local x_orig = x
+        x = ALIGN_DOWN(x_orig, 8)
+        local x_fixup = x_orig - x
+        w = ALIGN_UP(w + x_fixup, 8)
+        local y_orig = y
+        y = ALIGN_DOWN(y_orig, 8)
+        local y_fixup = y_orig - y
+        h = ALIGN_UP(h + y_fixup, 8)
+
+        -- NOTE: The Forma is the rare beast with screen dimensions that are themselves a multiple of 8.
+        --       Unfortunately, this may not be true for every device, so,
+        --       make sure we clamp w & h to the physical screen's size, otherwise the ioctl will fail ;).
+        w, h = fb:_clampToPhysicalDim(w, h)
+    end
+
     w = w * fb.refresh_pixel_size
 
     -- NOTE: If we're trying to send a:
@@ -385,7 +446,7 @@ local function refresh_zelda(fb, refreshtype, waveform_mode, x, y, w, h, dither)
     end
     -- TODO: There's also the HW-backed NightMode which should be somewhat accessible...
 
-    return mxc_update(fb, C.MXCFB_SEND_UPDATE_ZELDA, refarea, refreshtype, waveform_mode, x, y, w, h)
+    return mxc_update(fb, C.MXCFB_SEND_UPDATE_ZELDA, refarea, refreshtype, waveform_mode, x, y, w, h, dither)
 end
 
 local function refresh_rex(fb, refreshtype, waveform_mode, x, y, w, h, dither)
@@ -421,7 +482,7 @@ local function refresh_rex(fb, refreshtype, waveform_mode, x, y, w, h, dither)
     end
     -- TODO: There's also the HW-backed NightMode which should be somewhat accessible...
 
-    return mxc_update(fb, C.MXCFB_SEND_UPDATE_REX, refarea, refreshtype, waveform_mode, x, y, w, h)
+    return mxc_update(fb, C.MXCFB_SEND_UPDATE_REX, refarea, refreshtype, waveform_mode, x, y, w, h, dither)
 end
 
 local function refresh_kobo(fb, refreshtype, waveform_mode, x, y, w, h)
@@ -468,7 +529,7 @@ local function refresh_kobo_mk7(fb, refreshtype, waveform_mode, x, y, w, h, dith
         refarea[0].flags = 0
     end
 
-    return mxc_update(fb, C.MXCFB_SEND_UPDATE_V2, refarea, refreshtype, waveform_mode, x, y, w, h)
+    return mxc_update(fb, C.MXCFB_SEND_UPDATE_V2, refarea, refreshtype, waveform_mode, x, y, w, h, dither)
 end
 
 local function refresh_pocketbook(fb, refreshtype, waveform_mode, x, y, w, h)