# --- T2-COPYRIGHT-BEGIN ---
# t2/package/*/xf86-video-sis/hotfix-12bit-stride-noaccel.patch
# Copyright (C) 2026 The T2 SDE Project
# SPDX-License-Identifier: GPL-2.0 or patched project license
# --- T2-COPYRIGHT-END ---

Disable acceleration when the screen stride exceeds the engine's 12 bit limit.

The 1st generation 2D engine (5597/5598/6326) holds the source/destination
pitch (BR(2)) and the blit width (BR(3)) in 12 bit fields, counted in bytes.
sis_accel.h masks both to 16 bit, so a larger stride is silently truncated and
the engine blits with a bogus pitch instead of failing - at depth 24 this
corrupts every mode wider than 1365 pixels (1400, 1600, ...).

Compute the screen pitch in SiSAccelInit() and fall back to software rendering
when it does not fit. Xv keeps working (the XAA path still sets up the fb
memory manager and only skips XAAInit), and DGA already NULL-checks the
FillRect/BlitRect hooks that are left unset. The EXA Prepare hooks program the
pitch per pixmap rather than from the screen pitch, so they get the same check
next to their existing alignment checks.

--- xf86-video-sis/src/sis_accel.h.orig
+++ xf86-video-sis/src/sis_accel.h
@@ -46,6 +46,12 @@
 
 #define BR(x) sisReg32MMIO[x]
 
+/* Source/destination pitch (BR(2)) and blit width (BR(3)) are 12 bit
+ * fields holding byte counts. Both are fed from the screen pitch, hence
+ * the largest pitch (in bytes) the engine can be programmed with.
+ */
+#define SIS_MAX_ACCEL_PITCH 4095
+
 /* These are done using Memory Mapped IO, of the registers */
 /*
  * Modified for Sis by Xavier Ducoin (xavier@rd.lectra.fr)
--- xf86-video-sis/src/sis_accel.c.orig
+++ xf86-video-sis/src/sis_accel.c
@@ -662,6 +671,21 @@
     pSiS->exa_scratch = NULL;
 #endif
 
+    /* The engine's pitch and width registers are 12 bit wide; a screen
+     * pitch beyond that would be truncated silently and make the engine
+     * draw with a bogus stride.
+     */
+    if(!pSiS->NoAccel && pSiS->Chipset == PCI_CHIP_SIS6326) {
+       int pitch = pScrn->displayWidth * ((pScrn->bitsPerPixel + 7) / 8);
+
+       if(pitch > SIS_MAX_ACCEL_PITCH) {
+	  xf86DrvMsg(pScrn->scrnIndex, X_WARNING,
+		"Screen pitch of %d bytes exceeds engine limit of %d bytes, "
+		"disabling acceleration\n", pitch, SIS_MAX_ACCEL_PITCH);
+	  pSiS->NoAccel = TRUE;
+       }
+    }
+
     if(!pSiS->NoAccel) {
 #ifdef SIS_USE_XAA
        if(!pSiS->useEXA) {