From 30a9d5907018a578873bff2f7f1eaccfcc4d5a1f Mon Sep 17 00:00:00 2001
From: mischief <mischief@offblast.org>
Date: Sun, 20 Sep 2015 01:56:56 -0700
Subject: [PATCH] pc, pc64: fix sdvirtio descriptor count when sending flush

---
 sys/src/9/pc/sdvirtio.c | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/sys/src/9/pc/sdvirtio.c b/sys/src/9/pc/sdvirtio.c
index 64c43ddf3..6bb6c1162 100644
--- a/sys/src/9/pc/sdvirtio.c
+++ b/sys/src/9/pc/sdvirtio.c
@@ -326,7 +326,7 @@ vqio(Vqueue *q, int head)
 static int
 vioblkreq(Vdev *vd, int typ, void *a, long count, long secsize, uvlong lba)
 {
-	int free, head;
+	int need, free, head;
 	Vqueue *q;
 	Vdesc *d;
 
@@ -337,14 +337,18 @@ vioblkreq(Vdev *vd, int typ, void *a, long count, long secsize, uvlong lba)
 		u64int	lba;
 	} req;
 
-	status = 0;
+	need = 2;
+	if(a != nil)
+		need = 3;
+
+	status = -1;
 	req.typ = typ;
 	req.prio = 0;
 	req.lba = lba;
 
 	q = vd->queue[0];
 	ilock(q);
-	while(q->nfree < 3){
+	while(q->nfree < need){
 		iunlock(q);
 
 		if(!waserror())
@@ -361,10 +365,12 @@ vioblkreq(Vdev *vd, int typ, void *a, long count, long secsize, uvlong lba)
 	d->len = sizeof(req);
 	d->flags = Next;
 
-	d = &q->desc[free]; free = d->next;
-	d->addr = PADDR(a);
-	d->len = secsize*count;
-	d->flags = typ ? Next : (Write|Next);
+	if(a != nil){
+		d = &q->desc[free]; free = d->next;
+		d->addr = PADDR(a);
+		d->len = secsize*count;
+		d->flags = typ ? Next : (Write|Next);
+	}
 
 	d = &q->desc[free]; free = d->next;
 	d->addr = PADDR(&status);
@@ -372,7 +378,7 @@ vioblkreq(Vdev *vd, int typ, void *a, long count, long secsize, uvlong lba)
 	d->flags = Write;
 
 	q->free = free;
-	q->nfree -= 3;
+	q->nfree -= need;
 
 	/* queue io, unlock and wait for completion */
 	vqio(q, head);