# --- T2-COPYRIGHT-BEGIN ---
# t2/package/*/xf86-video-rendition/02-underclock.patch
# Copyright (C) 2026 The T2 SDE Project
# SPDX-License-Identifier: GPL-2.0 or patched project license
# --- T2-COPYRIGHT-END ---

Saner V2x00 system/memory clocking:

- run the documented 50 MHz system clock by default: the historic
  SClkPLL value 0xa484d programs mclk=110/sclk=55 despite its comment
  claiming sclk=50; use 0xa4846 (M/N/P/P = 70/5/2/4, VCO 200 MHz ->
  mclk=100/sclk=50) instead
- add a new "Underclock_Mem" option programming 0xa483f (63/5/2/4
  -> mclk=90/sclk=45) for testing marginal boards/memory; evaluated
  already during PreInit so the initial mode setup uses it

--- a/src/vtypes.h
+++ b/src/vtypes.h
@@ -160,6 +160,7 @@
 
     vu32 fbOffset;          /* Currently busy fb-memory marker */
     Bool overclock_mem;     /* Memory overclock ? */
+    Bool underclock_mem;    /* Memory underclock ? */
 
     Bool shadowfb;          /* Use ShadowFB ? */
     vu8 *shadowPtr;
--- a/src/rendition.c
+++ b/src/rendition.c
@@ -139,6 +139,7 @@
     { OPTION_SW_CURSOR, "SW_Cursor", OPTV_BOOLEAN, {0}, FALSE },
     { OPTION_NOACCEL,   "NoAccel",  OPTV_BOOLEAN, {0}, FALSE },
     { OPTION_OVERCLOCK_MEM,"Overclock_Mem",  OPTV_BOOLEAN, {0}, FALSE },
+    { OPTION_UNDERCLOCK_MEM,"Underclock_Mem",  OPTV_BOOLEAN, {0}, FALSE },
     { OPTION_NO_DDC,    "NoDDC",    OPTV_BOOLEAN, {0}, FALSE },
     { OPTION_SHADOW_FB, "ShadowFB", OPTV_BOOLEAN, {0}, FALSE },
     { OPTION_ROTATE,    "Rotate",   OPTV_ANYSTR,  {0}, FALSE },
@@ -594,9 +595,14 @@
     if (!(pRendition->Options = malloc(sizeof(renditionOptions))))
 	return FALSE;
     memcpy(pRendition->Options, renditionOptions, sizeof(renditionOptions));
-    xf86ProcessOptions(pScreenInfo->scrnIndex, pScreenInfo->options, 
+    xf86ProcessOptions(pScreenInfo->scrnIndex, pScreenInfo->options,
         pRendition->Options);
 
+    /* evaluated here (unlike Overclock_Mem) so that already the
+     * initial mode setup uses the lower clocks */
+    if (xf86ReturnOptValBool(pRendition->Options, OPTION_UNDERCLOCK_MEM, 0))
+	pRendition->board.underclock_mem=TRUE;
+
 
     /* Load fb */
     if (!xf86LoadSubModule(pScreenInfo, "fb"))
--- a/src/rendition_options.h
+++ b/src/rendition_options.h
@@ -8,6 +8,7 @@
     OPTION_SW_CURSOR,
     OPTION_NOACCEL,
     OPTION_OVERCLOCK_MEM,
+    OPTION_UNDERCLOCK_MEM,
     OPTION_NO_DDC,
     OPTION_SHADOW_FB,
     OPTION_ROTATE
--- a/src/vmodes.c
+++ b/src/vmodes.c
@@ -244,9 +244,22 @@
     }
 
     if (pRendition->board.chip != V1000_DEVICE) {
-      if(!pRendition->board.overclock_mem){
-	verite_out32(iob+SCLKPLL, 0xa484d);  /* mclk=110 sclk=50 */
-                                        /* M/N/P/P = 77/5/2/4 */
+      /* SClkPLL: M bits[8:0], SClkP bits[12:9], MClkP bits[16:13],
+       * N bits[22:17]; VCO = 14.31818 * M / N (125..250 MHz),
+       * MClk = VCO / MClkP, SClk = VCO / SClkP, MClk >= SClk. */
+      if(pRendition->board.underclock_mem){
+	xf86DrvMsg(pScreenInfo->scrnIndex, X_CONFIG,
+		   (" *** UNDERCLOCKING MEM/CLK mclk=90 sclk=45 ***\n"));
+	/* decrease Mem/Sys clock on request */
+	verite_out32(iob+SCLKPLL, 0xa483f);  /* mclk=90 sclk=45 */
+                                        /* M/N/P/P = 63/5/2/4 */
+      }
+      else if(!pRendition->board.overclock_mem){
+	/* the historic default 0xa484d programmed mclk=110 sclk=55,
+	 * despite its comment claiming sclk=50; run the documented
+	 * sclk=50 for stability */
+	verite_out32(iob+SCLKPLL, 0xa4846);  /* mclk=100 sclk=50 */
+                                        /* M/N/P/P = 70/5/2/4 */
       }
       else{
 	xf86DrvMsg(pScreenInfo->scrnIndex, X_CONFIG,
@@ -255,6 +268,7 @@
 	verite_out32(iob+SCLKPLL, 0xa4854);  /* mclk=125 sclk=60 */
                                         /* M/N/P/P = 84/5/2/4 */
       }
+      /* PLL needs 200us to stabilize after M/N change */
       usleep(500);
     }