[PATCH] a lockup in the mga driver

[Mikulas Patocka]

Hi David

Are you maintaining the Xorg mga driver? (you anounced the last update 
1.6.3) Or is someone else maintaining it?

I've had a Xserver lockup in the mga driver, examining it with gdb showed 
this obviously broken loop:
	count = INREG(MGAREG_VCOUNT) + 2;
	while(INREG(MGAREG_VCOUNT) < count);

It reads the line counter and waits until the counter advances by two. The 
cause of the lockup is this - if the kernel reschedules the Xorg process 
and lets it run in such a moment when INREG(MGAREG_VCOUNT) returns the 
maximum (or maximum minus 1) line count, the loop never exits.

I created this patch for the bug, it tries to imitate the old behavior, 
but it also exits the loop when the line counter wraps to zero, preventing 
the lockup.

BTW. did you notice my previous patch for pan_ctl in the mga driver?

Mikulas

---
 src/mga_driver.c |   12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

Index: xf86-video-mga-1.6.3/src/mga_driver.c
===================================================================
--- xf86-video-mga-1.6.3.orig/src/mga_driver.c	2013-12-05 02:15:53.000000000 +0100
+++ xf86-video-mga-1.6.3/src/mga_driver.c	2014-03-13 22:56:10.852221315 +0100
@@ -3618,7 +3618,7 @@ void
 MGAAdjustFrame(ADJUST_FRAME_ARGS_DECL)
 {
     SCRN_INFO_PTR(arg);
-    int Base, tmp, count;
+    int Base, tmp, count, last_vcount;
 
     MGAFBLayout *pLayout;
     MGAPtr pMga;
@@ -3648,8 +3648,14 @@ MGAAdjustFrame(ADJUST_FRAME_ARGS_DECL)
         while (INREG8(0x1FDA) & 0x08);
         while (!(INREG8(0x1FDA) & 0x08));
         /* wait until we're past the start (fixseg.c in the DDK) */
-        count = INREG(MGAREG_VCOUNT) + 2;
-        while(INREG(MGAREG_VCOUNT) < count);
+        last_vcount = INREG(MGAREG_VCOUNT);
+        count = last_vcount + 2;
+        while (1) {
+           int vcount = INREG(MGAREG_VCOUNT);
+           if (vcount >= count) break;
+           if (vcount < last_vcount) break;
+           last_vcount = count;
+        }
 
         OUTREG16(MGAREG_CRTC_INDEX, (Base & 0x00FF00) | 0x0C);
         OUTREG16(MGAREG_CRTC_INDEX, ((Base & 0x0000FF) << 8) | 0x0D);