From 6ab4b7f0bc1a49440198302c83f877d66999b5a5 Mon Sep 17 00:00:00 2001
From: IanSB <isb@clara.co.uk>
Date: Tue, 7 Dec 2021 13:52:39 +0000
Subject: [PATCH] Fix display list issue stopping resolutions 1024x768 & lower
 from working at 16bpp

---
 src/rgb_to_hdmi.c | 40 +++++++++++++++++++++++++---------------
 1 file changed, 25 insertions(+), 15 deletions(-)

diff --git a/src/rgb_to_hdmi.c b/src/rgb_to_hdmi.c
index 24b55967..48a5251a 100644
--- a/src/rgb_to_hdmi.c
+++ b/src/rgb_to_hdmi.c
@@ -278,6 +278,7 @@ static int ppm_range_count = 0;
 static int powerup = 1;
 static int hsync_threshold_switch = 0;
 static int resolution_status = 0;
+static int display_list_offset = 5;
 static volatile uint32_t display_list_index = 0;
 volatile uint32_t* display_list;
 volatile uint32_t* pi4_hdmi0_regs;
@@ -536,7 +537,7 @@ unsigned int framebuffer = 0;
       capinfo->fb = (unsigned char *)(framebuffer & 0x3fffffff);
     }
 
-    log_info("Size: %dx%d (req %dx%d). Addr: %8.8X (%8.8X)", width, height, capinfo->width, capinfo->height, (unsigned int)capinfo->fb, framebuffer);
+
     //Initialize the palette
     osd_update_palette();
 
@@ -545,10 +546,6 @@ unsigned int framebuffer = 0;
         //have to wait for field sync for display list to be updated
         wait_for_pi_fieldsync();
         wait_for_pi_fieldsync();
-        //delay_in_arm_cycles_cpu_adjust(30000000); // little extra delay
-        //read the index pointer into the display list RAM
-        display_list_index = (uint32_t) *SCALER_DISPLIST1;
-
 /*
         volatile uint32_t  d;
         volatile uint32_t dlist_start;
@@ -560,15 +557,33 @@ unsigned int framebuffer = 0;
         log_info("%08X", d);
         } while (d != 0x80000000);
 */
-        int dli;
+        //delay_in_arm_cycles_cpu_adjust(30000000); // little extra delay
+        //read the index pointer into the display list RAM
+        display_list_index = (uint32_t) *SCALER_DISPLIST1;
+        unsigned int dli;
+        display_list_offset = 0;
+        for(int i = 6; i > 3; i--) {
+            do {
+                dli = display_list[display_list_index + i];
+            } while (dli == 0xff000000);
+            if (dli == framebuffer) {         //start of frame buffer moves in position depending on scaling and resolution so search for it
+                display_list_offset = i;
+            }
+        }
+        if (display_list_offset == 0) {
+#ifdef RPI4
+            display_list_offset = 6;   //default
+#else
+            display_list_offset = 5;   //default
+#endif
+        }
         do {
             dli = display_list[display_list_index];
         } while (dli == 0xFF000000);
         display_list[display_list_index] = (dli & ~0x600f) | (PIXEL_ORDER << 13) | PIXEL_FORMAT;
         //log_info("Modified display list word at %08X = %08X", display_list_index, display_list[display_list_index]);
-
+        log_info("Size: %dx%d (req %dx%d). Addr: %8.8X (%8.8X) Offset = %d", width, height, capinfo->width, capinfo->height, (unsigned int)capinfo->fb, framebuffer, display_list_offset);
     }
-
 }
 #else
 
@@ -2485,13 +2500,8 @@ void swapBuffer(int buffer) {
      // directly manipulate the display list in 16BPP mode otherwise display list gets reconstructed
      int dli = ((int)capinfo->fb | 0xc0000000) + (buffer * capinfo->height * capinfo->pitch);
         do {
-#ifdef RPI4
-           display_list[display_list_index + 6] = dli;
-        } while (dli != display_list[display_list_index + 6]);
-#else
-           display_list[display_list_index + 5] = dli;
-        } while (dli != display_list[display_list_index + 5]);
-#endif
+           display_list[display_list_index + display_list_offset] = dli;
+        } while (dli != display_list[display_list_index + display_list_offset]);
   } else
   {
      RPI_PropertyInit();