Don't disable acceleration on 830/845g by default

Run the risk of a GPU hang (it shouldn't endanger the entire machine
normally) and let the user elect to disable it through

  Option "NoAccel" "true"

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

src/intel_driver.c

@@ -390,18 +390,9 @@ static Bool can_accelerate_blt(struct intel_screen_private *intel)
 	if (INTEL_INFO(intel)->gen == -1)
 		return FALSE;
 
-	if (0 && (IS_I830(intel) || IS_845G(intel))) {
-		/* These pair of i8xx chipsets have a crippling erratum
-		 * that prevents the use of a PTE entry by the BLT
-		 * engine immediately following updating that
-		 * entry in the GATT.
-		 *
-		 * As the BLT is fundamental to our 2D acceleration,
-		 * and the workaround is lost in the midst of time,
-		 * fallback.
-		 *
-		 * XXX disabled for release as causes regressions in GL.
-		 */
+	if (xf86ReturnOptValBool(intel->Options, OPTION_ACCEL_DISABLE, FALSE)) {
+		xf86DrvMsg(intel->scrn->scrnIndex, X_CONFIG,
+			   "Disabling hardware acceleration.\n");
 		return FALSE;
 	}
 

src/intel_options.c

@@ -5,6 +5,7 @@
 #include "intel_options.h"
 
 const OptionInfoRec intel_options[] = {
+	{OPTION_ACCEL_DISABLE,	"NoAccel",	OPTV_BOOLEAN,	{0},	0},
 	{OPTION_ACCEL_METHOD,	"AccelMethod",	OPTV_STRING,	{0},	0},
 	{OPTION_DRI,		"DRI",		OPTV_BOOLEAN,	{0},	1},
 	{OPTION_COLOR_KEY,	"ColorKey",	OPTV_INTEGER,	{0},	0},

src/intel_options.h

@@ -11,6 +11,7 @@
  */
 
 enum intel_options {
+	OPTION_ACCEL_DISABLE,
 	OPTION_ACCEL_METHOD,
 	OPTION_DRI,
 	OPTION_VIDEO_KEY,

src/sna/kgem.c

@@ -652,8 +652,10 @@ static bool is_hw_supported(struct kgem *kgem,
 	if (kgem->gen == (unsigned)-1) /* unknown chipset, assume future gen */
 		return kgem->has_blt;
 
-	if (kgem->gen <= 20) /* dynamic GTT is fubar */
-		return false;
+	/* Although pre-855gm the GMCH is fubar, it works mostly. So
+	 * let the user decide through "NoAccel" whether or not to risk
+	 * hw acceleration.
+	 */
 
 	if (kgem->gen == 60 && dev->revision < 8) {
 		/* pre-production SNB with dysfunctional BLT */

src/sna/sna_driver.c

@@ -439,7 +439,14 @@ static Bool sna_pre_init(ScrnInfoPtr scrn, int flags)
 		return FALSE;
 
 	sna->info = intel_detect_chipset(scrn, sna->pEnt, sna->PciInfo);
+
 	kgem_init(&sna->kgem, fd, sna->PciInfo, sna->info->gen);
+	if (xf86ReturnOptValBool(sna->Options, OPTION_ACCEL_DISABLE, FALSE)) {
+		xf86DrvMsg(sna->scrn->scrnIndex, X_CONFIG,
+			   "Disabling hardware acceleration.\n");
+		sna->kgem.wedged = true;
+	}
+
 	if (!xf86ReturnOptValBool(sna->Options,
 				  OPTION_RELAXED_FENCING,
 				  sna->kgem.has_relaxed_fencing)) {