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sdnvme: don't write completion queue doorbell register when nothing has been processed

turns out on real hardware, the front falls off if we write
the completion queue doorbell registers without consuming
an entry. so only write the register when we have processed
something.

--- a/sys/src/9/pc/sdnvme.c

+++ b/sys/src/9/pc/sdnvme.c

@@ -165,7 +165,7 @@

 		if(cq->base == nil)

 			continue;

 		phaseshift = 16 - cq->shift;

-		for(;; cq->head++){

+		for(;;){

 			e = &cq->base[(cq->head & cq->mask)<<2];

 			if(((e[3] ^ (cq->head << phaseshift)) & 0x10000) == 0)

 				break;

@@ -183,11 +183,9 @@

 				*wp = nil;

 				wakeup(z);

 			}

+			ctlr->reg[DBell + ((cq-ctlr->cq)*2+1 << ctlr->dstrd)] = ++cq->head & cq->mask;

 		}

-		ctlr->reg[DBell + ((cq-ctlr->cq)*2+1 << ctlr->dstrd)] = cq->head & cq->mask;

 	}

-	if((ctlr->reg[CSts] & 3) != 1)

-		iprint("nvmeintr: fatal controller error\n");

 	ctlr->reg[IntMc] = ctlr->ints;

 	iunlock(&ctlr->intr);

 }