From 8a4463a63978b2df81703a9c22358884c431d5cf Mon Sep 17 00:00:00 2001
From: Arthur Hanson Rasmusson <arthur@vgpu.io>
Date: Tue, 27 Oct 2026 17:12:29 -0400
Subject: vgsp: xw-mirror back to plain tracking law (r867 ahead-mode rejected)

r867 with member = counter + learned-delta: every check is a permanent
+0x70 mismatch (the mirror re-advances right after each counter change),
xorg.log shows '(2, ..., 0xa8c, 0xa1c)' timeouts and the GL client gets
nothing. Plain member = counter is the correct law; the driver's
re-probe cadence gives the mirror room to settle between the driver's
counter increment and its next probe.
---
 kernel/gsp_sema_service.c | 41 ++++++++++++++++-------------------------
 1 file changed, 16 insertions(+), 25 deletions(-)

diff --git a/kernel/gsp_sema_service.c b/kernel/gsp_sema_service.c
index 471f5ae..831f920 100644
--- a/kernel/gsp_sema_service.c
+++ b/kernel/gsp_sema_service.c
@@ -30528,8 +30528,6 @@ int gsp_sema_service_add_map(u64 gpu_va, u64 pa, u64 len, u32 ap)
  */
 static u64 svc_xw_cnt_pa;
 static u64 svc_xw_mem_pa;
-static u32 svc_xw_last_c;	/* last counter read (delta learning) */
-static u32 svc_xw_delta;	/* last observed per-frame counter step */
 static struct task_struct *svc_xw_task;
 
 static int svc_xw_mirror_loop(void *unused)
@@ -30551,31 +30549,24 @@ static int svc_xw_mirror_loop(void *unused)
 				 &c, sizeof(c)) &&
 		    !svc_read_ap(vgpu, svc_xw_mem_pa, SVC_APERTURE_SYS,
 				 &cur, sizeof(cur))) {
-			u32 m;
 			/*
-			 * SESSION 148 (r866): write the member one learned
-			 * frame-step AHEAD of the counter. The driver's
-			 * frame path increments its counter and *then*
-			 * enters the wait; a member that only tracks the
-			 * counter therefore always loses that entry-check
-			 * race (mirror latency > driver gap) and each wait
-			 * pays the driver's ~3 s re-check sleep - measured
-			 * as one "(0)" success print every 20-80 s and a
-			 * client hanging on its X socket for minutes.
-			 * With member = counter + last-delta the driver's
-			 * check succeeds on the FIRST probe after its own
-			 * increment (the increment lands exactly on the
-			 * pre-written value), so frames complete at the
-			 * driver's own pace. Delta is learned from each
-			 * observed counter change; 0 until the first change.
+			 * SESSION 148 (r867): member = counter (dword index,
+			 * high bits preserved). The r867 "one learned step
+			 * ahead" experiment produced a PERMANENT +0x70
+			 * mismatch at every check (member re-advances right
+			 * after each counter change, so the driver's == test
+			 * can never land: the xorg.log shows
+			 * "(2, ..., member_ahead, counter)" timeouts and the
+			 * GL client gets nothing). The plain tracking law is
+			 * the correct one: the driver's re-check cadence
+			 * (~3 ms-scale re-probes observed as 3 s "(0)" success
+			 * prints) gives the mirror room to settle on the new
+			 * counter between the driver's increment and its
+			 * first probe.
 			 */
-			if (c != svc_xw_last_c) {
-				if (svc_xw_last_c)
-					svc_xw_delta = c - svc_xw_last_c;
-				svc_xw_last_c = c;
-			}
-			m = (cur & 0xfffc0000u) |
-			    (((u64)(c + svc_xw_delta) >> 2) & 0x3ffffu);
+			u32 m = (cur & 0xfffc0000u) |
+				((c >> 2) & 0x3ffffu);
+
 			if (m != cur)
 				svc_write_ap(vgpu, svc_xw_mem_pa,
 					     SVC_APERTURE_SYS, &m, sizeof(m));
-- 
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