[src/trunk]: src/sys/dev/pci gffb(4): Nix membar_sync and explain what's goin...

[riastradh <riastradh%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/63da72f480fb
branches:  trunk
changeset: 363405:63da72f480fb
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Thu Mar 10 00:14:33 2022 +0000

description:
gffb(4): Nix membar_sync and explain what's going on here.

diffstat:

 sys/dev/pci/gffb.c |  17 ++++++++++++++---
 1 files changed, 14 insertions(+), 3 deletions(-)

diffs (43 lines):

diff -r cc3fe35fd0b9 -r 63da72f480fb sys/dev/pci/gffb.c
--- a/sys/dev/pci/gffb.c        Thu Mar 10 00:14:25 2022 +0000
+++ b/sys/dev/pci/gffb.c        Thu Mar 10 00:14:33 2022 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: gffb.c,v 1.18 2022/03/10 00:14:25 riastradh Exp $      */
+/*     $NetBSD: gffb.c,v 1.19 2022/03/10 00:14:33 riastradh Exp $      */
 
 /*
  * Copyright (c) 2013 Michael Lorenz
@@ -35,7 +35,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: gffb.c,v 1.18 2022/03/10 00:14:25 riastradh Exp $");
+__KERNEL_RCSID(0, "$NetBSD: gffb.c,v 1.19 2022/03/10 00:14:33 riastradh Exp $");
 
 #include <sys/param.h>
 #include <sys/systm.h>
@@ -1133,12 +1133,23 @@
        if (rv == GC_OK)
                return;
 
+       /*
+        * Use gffb_sync to wait for the engine to become idle before
+        * we start scribbling into VRAM -- we woudn't want to stomp on
+        * a scroll in progress or a prior glyphcache_add that hasn't
+        * completed yet on the GPU.
+        */
        mutex_enter(&sc->sc_lock);
        gffb_sync(sc);
        sc->sc_putchar(cookie, row, col, c, attr);
-       membar_sync();
        mutex_exit(&sc->sc_lock);
 
+       /*
+        * If glyphcache_try asked us to, cache the newly written
+        * character.  This will issue a gffb_bitblt which will wait
+        * for our CPU writes to the framebuffer in VRAM to complete
+        * before triggering GPU reads from the framebuffer in VRAM.
+        */
        if (rv == GC_ADD) {
                glyphcache_add(&sc->sc_gc, c, x, y);
        }