Simplify swi for bus_dma.

When a DMA request using bounce pages completes, a swi is triggered to
schedule pending DMA requests using the just-freed bounce pages.  For
a long time this bus_dma swi has been tied to a "virtual memory" swi
(swi_vm).  However, all of the swi_vm implementations are the same and
consist of checking a flag (busdma_swi_pending) which is always true
and if set calling busdma_swi.  I suspect this dates back to the
pre-SMPng days and that the intention was for swi_vm to serve as a
mux.  However, in the current scheme there's no need for the mux.

Instead, remove swi_vm and vm_ih.  Each bus_dma implementation that
uses bounce pages is responsible for creating its own swi (busdma_ih)
which it now schedules directly.  This swi invokes busdma_swi directly
removing the need for busdma_swi_pending.

One consequence is that the swi now works on RISC-V which had previously
failed to invoke busdma_swi from swi_vm.

Reviewed by:	imp, kib
Sponsored by:	Netflix
Differential Revision:	https://reviews.freebsd.org/D33447

sys/arm/arm/busdma_machdep.c

@@ -114,8 +114,6 @@ struct sync_list {
 	bus_size_t	datacount;	/* client data count */
 };
 
-int busdma_swi_pending;
-
 struct bounce_zone {
 	STAILQ_ENTRY(bounce_zone) links;
 	STAILQ_HEAD(bp_list, bounce_page) bounce_page_list;
@@ -151,6 +149,7 @@ static counter_u64_t maploads_physmem;
 #endif
 
 static STAILQ_HEAD(, bounce_zone) bounce_zone_list;
+static void *busdma_ih;
 
 SYSCTL_NODE(_hw, OID_AUTO, busdma, CTLFLAG_RD | CTLFLAG_MPSAFE, 0,
     "Busdma parameters");
@@ -1714,6 +1713,7 @@ free_bounce_page(bus_dma_tag_t dmat, struct bounce_page *bpage)
 {
 	struct bus_dmamap *map;
 	struct bounce_zone *bz;
+	bool schedule_swi;
 
 	bz = dmat->bounce_zone;
 	bpage->datavaddr = 0;
@@ -1728,6 +1728,7 @@ free_bounce_page(bus_dma_tag_t dmat, struct bounce_page *bpage)
 		bpage->busaddr &= ~PAGE_MASK;
 	}
 
+	schedule_swi = false;
 	mtx_lock(&bounce_lock);
 	STAILQ_INSERT_HEAD(&bz->bounce_page_list, bpage, links);
 	bz->free_bpages++;
@@ -1737,16 +1738,17 @@ free_bounce_page(bus_dma_tag_t dmat, struct bounce_page *bpage)
 			STAILQ_REMOVE_HEAD(&bounce_map_waitinglist, links);
 			STAILQ_INSERT_TAIL(&bounce_map_callbacklist,
 			    map, links);
-			busdma_swi_pending = 1;
 			bz->total_deferred++;
-			swi_sched(vm_ih, 0);
+			schedule_swi = true;
 		}
 	}
 	mtx_unlock(&bounce_lock);
+	if (schedule_swi)
+		swi_sched(busdma_ih, 0);
 }
 
-void
-busdma_swi(void)
+static void
+busdma_swi(void *dummy __unused)
 {
 	bus_dma_tag_t dmat;
 	struct bus_dmamap *map;
@@ -1764,3 +1766,13 @@ busdma_swi(void)
 	}
 	mtx_unlock(&bounce_lock);
 }
+
+static void
+start_busdma_swi(void *dummy __unused)
+{
+	if (swi_add(NULL, "busdma", busdma_swi, NULL, SWI_BUSDMA, INTR_MPSAFE,
+	    &busdma_ih))
+		panic("died while creating busdma swi ithread");
+}
+SYSINIT(start_busdma_swi, SI_SUB_SOFTINTR, SI_ORDER_ANY, start_busdma_swi,
+    NULL);

sys/arm/arm/vm_machdep.c

@@ -290,17 +290,6 @@ cpu_fork_kthread_handler(struct thread *td, void (*func)(void *), void *arg)
 	td->td_pcb->pcb_regs.sf_r5 = (register_t)arg;	/* first arg */
 }
 
-/*
- * Software interrupt handler for queued VM system processing.
- */
-void
-swi_vm(void *dummy)
-{
-
-	if (busdma_swi_pending)
-		busdma_swi();
-}
-
 void
 cpu_exit(struct thread *td)
 {

sys/arm/include/cpu.h

@@ -8,7 +8,6 @@
 #include <machine/frame.h>
 
 void	cpu_halt(void);
-void	swi_vm(void *);
 
 #ifdef _KERNEL
 #include <machine/cpu-v6.h>

sys/arm/include/md_var.h

@@ -53,8 +53,6 @@ extern enum cpu_class cpu_class;
 
 struct dumperinfo;
 struct minidumpstate;
-extern int busdma_swi_pending;
-void busdma_swi(void);
 int cpu_minidumpsys(struct dumperinfo *, const struct minidumpstate *);
 
 extern uint32_t initial_fpscr;