# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1541698327 -3600
# Node ID e83be2917e9a90f6a286cf2bbd1ccda54f5f137c
# Parent  6423c19b840b7bcfee5ee88c45b6f3f2122447df
Fixed pixel positioning for single-channel timed transfers, introducing the line
channel count in the display configuration structure, moving the initialisation
of a default into the appropriate header file.

diff -r 6423c19b840b -r e83be2917e9a examples/vga/main.c
--- a/examples/vga/main.c	Wed Nov 07 00:12:44 2018 +0100
+++ b/examples/vga/main.c	Thu Nov 08 18:32:07 2018 +0100
@@ -39,17 +39,11 @@
 /* CPU-based transfers: no channels. */
 
 #ifdef TRANSFER_CPU
-#define LINE_CHANNELS   0
 #define SCROLL_XSTEP    1
 
 /* DMA-based transfers: single channel by default. */
 
 #else
-
-#ifndef LINE_CHANNELS
-#define LINE_CHANNELS   1
-#endif
-
 #define SCROLL_XSTEP    LINE_CHANNELS
 #endif
 
diff -r 6423c19b840b -r e83be2917e9a include/display.h
--- a/include/display.h	Wed Nov 07 00:12:44 2018 +0100
+++ b/include/display.h	Thu Nov 08 18:32:07 2018 +0100
@@ -66,6 +66,10 @@
 
     int line_multiplier;                /* = scanlines / line_count */
 
+    /* Number of line channels used for pixel data transfers. */
+
+    int line_channels;
+
     /* Number of consecutive pixels provided by a framebuffer region. */
 
     uint32_t cell_size;
diff -r 6423c19b840b -r e83be2917e9a include/display_config.h
--- a/include/display_config.h	Wed Nov 07 00:12:44 2018 +0100
+++ b/include/display_config.h	Thu Nov 08 18:32:07 2018 +0100
@@ -24,6 +24,23 @@
 #include "display.h"
 #include "vga.h"
 
+/* Define DMA channels if not indicated in the build configuration. */
+
+/* CPU-based transfers: no channels. */
+
+#ifdef TRANSFER_CPU
+#define LINE_CHANNELS   0
+
+/* DMA-based transfers: single channel by default. */
+
+#else
+
+#ifndef LINE_CHANNELS
+#define LINE_CHANNELS   1
+#endif
+
+#endif
+
 /* Define a structure containing the display parameters. */
 
 display_config_t display_config = {
@@ -31,6 +48,7 @@
     /* Define display properties. */
 
     .scanlines = SCANLINES,
+    .line_channels = LINE_CHANNELS,
     .cell_size = CELL_SIZE,
     .transfer_cell_size = TRANSFER_CELL_SIZE,
 
diff -r 6423c19b840b -r e83be2917e9a lib/display.c
--- a/lib/display.c	Wed Nov 07 00:12:44 2018 +0100
+++ b/lib/display.c	Thu Nov 08 18:32:07 2018 +0100
@@ -121,8 +121,24 @@
 
 int get_position(display_config_t *cfg, int x)
 {
-    int cell = x / cfg->cell_size, offset = x % cfg->cell_size;
-    int pos = (cell / 2) * cfg->cell_size + offset;
+    int cell, offset, pos;
+
+    if (cfg->line_channels < 2)
+        return x;
+
+    /* Determine which cell is providing the position and the offset of the
+       pixel within the cell. */
+
+    cell = x / cfg->cell_size;
+    offset = x % cfg->cell_size;
+
+    /* Determine the resulting position within the divided-up data. */
+
+    pos = (cell / 2) * cfg->cell_size + offset;
+
+    /* Return the final position within the entire data. All cells in
+       odd-numbered positions occur in the first half, all even-numbered cells
+       in the second half. */
 
     return cell % 2 ? pos + cfg->line_length / 2 : pos;
 }