From 141de8316e99602050893997b6364d394ba1bbab Mon Sep 17 00:00:00 2001
From: Philipp Zabel <philipp.zabel@gmail.com>
Date: Sun, 26 Oct 2014 12:58:19 +0100
Subject: [PATCH] mensactrl: Add some comments to the fullscreen blit function
---
src/mensactrl.c | 27 +++++++++++++++++++++++++++
1 file changed, 27 insertions(+)
diff --git a/src/mensactrl.c b/src/mensactrl.c
index 817ae54..018e622 100644
--- a/src/mensactrl.c
+++ b/src/mensactrl.c
@@ -76,15 +76,42 @@ static int blit_fullscreen(struct mensa_fb *mensafb)
uint16_t val;
uint16_t *fbline;
+ /*
+ * fbline always points to the beginning of a contiguous run of 40 5-bit
+ * values in the output framebuffer, at the same time corresponding to
+ * the rightmost pixel of 5 contiguous runs of 40 pixels in the input
+ * framebuffer, multiples of 14 rows apart (the same line of each of the
+ * 5 module chains that are fed pixels in parallel).
+ */
fbline = mensafb->fbmem;
for (b = 0; b < BRIGHT_LEVELS - 1; b++) {
+ /*
+ * 17 brightness levels are achieved by 16 consecutive framebuffers that
+ * are shown one after the other. The brightness of each individual pixel
+ * depends on the number of frames in which it is lit up.
+ */
thresh = 255 * (b + 1) / BRIGHT_LEVELS;
for (l = 0; l < mensafb->hmodules * LINES_PER_MODULE * ROWS_PER_LINE; l++) {
+ /*
+ * Each pair of 40-pixel runs share the same x-coordinates in the
+ * input framebuffer, with the even run 7 rows below the odd run,
+ * in the same module.
+ */
r = l / (mensafb->hmodules * LINES_PER_MODULE) + ((l & 1) ? 0 : ROWS_PER_LINE);
+ /*
+ * Since the LCD controller data pins [4:0] are output into shift
+ * registers, the values shifted out first appear as the rightmost
+ * pixels. Thus the column in the input framebuffer is inverted.
+ */
c = mensafb->x_res - 1 - ((l % (mensafb->hmodules * LINES_PER_MODULE)) / LINES_PER_MODULE * COLS_PER_MODULE);
+ /*
+ * The row address selects one of the 7 (sets of two) lines in each
+ * module. It is clocked out on the LCD controller data pins [7:5].
+ */
row_addr = ((6 + (l * COLS_PER_MODULE) / (mensafb->x_res * LINES_PER_MODULE)) % ROWS_PER_LINE) << 5;
for (pos = 0; pos < COLS_PER_MODULE; pos++, c--) {
val = row_addr;
+ /* Iterate over all 5 module chains */
for (i = 0; i < mensafb->vmodules; i++) {
if (mensafb->inputfb[c + (r + LINES_PER_MODULE * ROWS_PER_LINE * i) * mensafb->x_res] >= thresh)
val |= (1 << (mensafb->vmodules - 1 - i));
--
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