drm/nv30: Fix PFB init for nv31.

Fixes a regression introduced by 58bbb63720c8997e0136fe1884101e7ca40d68fd
(fdo bug 29324).

Reported-by: Johannes Obermayr <johannesobermayr@gmx.de>
Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nv30_fb.c

@@ -30,15 +30,25 @@
 #include "nouveau_drm.h"
 
 static int
-calc_ref(int b, int l, int i)
+calc_bias(struct drm_device *dev, int k, int i, int j)
+{
+	struct drm_nouveau_private *dev_priv = dev->dev_private;
+	int b = (dev_priv->chipset > 0x30 ?
+		 nv_rd32(dev, 0x122c + 0x10 * k + 0x4 * j) >> (4 * (i ^ 1)) :
+		 0) & 0xf;
+
+	return 2 * (b & 0x8 ? b - 0x10 : b);
+}
+
+static int
+calc_ref(struct drm_device *dev, int l, int k, int i)
 {
 	int j, x = 0;
 
 	for (j = 0; j < 4; j++) {
-		int n = (b >> (8 * j) & 0xf);
-		int m = (l >> (8 * i) & 0xff) + 2 * (n & 0x8 ? n - 0x10 : n);
+		int m = (l >> (8 * i) & 0xff) + calc_bias(dev, k, i, j);
 
-		x |= (0x80 | (m & 0x1f)) << (8 * j);
+		x |= (0x80 | clamp(m, 0, 0x1f)) << (8 * j);
 	}
 
 	return x;
@@ -63,18 +73,16 @@ nv30_fb_init(struct drm_device *dev)
 	    dev_priv->chipset == 0x35) {
 		/* Related to ROP count */
 		int n = (dev_priv->chipset == 0x31 ? 2 : 4);
-		int b = (dev_priv->chipset > 0x30 ?
-			 nv_rd32(dev, 0x122c) & 0xf : 0);
 		int l = nv_rd32(dev, 0x1003d0);
 
 		for (i = 0; i < n; i++) {
 			for (j = 0; j < 3; j++)
 				nv_wr32(dev, 0x10037c + 0xc * i + 0x4 * j,
-					calc_ref(b, l, j));
+					calc_ref(dev, l, 0, j));
 
 			for (j = 0; j < 2; j++)
 				nv_wr32(dev, 0x1003ac + 0x8 * i + 0x4 * j,
-					calc_ref(b, l, j));
+					calc_ref(dev, l, 1, j));
 		}
 	}