[Qemu-devel] [PATCH 1/3] i8257: rewrite DMA_schedule to avoid hooking into the CPU loop

[Paolo Bonzini <pbonzini@redhat.com>]

The i8257 DMA controller uses an idle bottom half, which by default
does not cause the main loop to exit.  Therefore, the DMA_schedule
function is there to ensure that the CPU relinquishes the iothread
mutex to the iothread.

However, this is not enough since the iothread will call
aio_compute_timeout() and go to sleep again.  In the iothread
world, forcing execution of the idle bottom half is much simpler,
and only requires a call to qemu_notify_event().  Do it, removing
the need for the "cpu_request_exit" pseudo-irq.  The next patch
will remove it.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 hw/block/fdc.c       |  2 +-
 hw/dma/i8257.c       | 18 ++++++++++++------
 hw/sparc/sun4m.c     |  2 +-
 hw/sparc64/sun4u.c   |  2 +-
 include/hw/isa/isa.h |  2 +-
 5 files changed, 16 insertions(+), 10 deletions(-)

diff --git a/hw/block/fdc.c b/hw/block/fdc.c
index 5e1b67e..6686a72 100644
--- a/hw/block/fdc.c
+++ b/hw/block/fdc.c
@@ -1417,7 +1417,7 @@ static void fdctrl_start_transfer(FDCtrl *fdctrl, int direction)
                  * recall us...
                  */
                 DMA_hold_DREQ(fdctrl->dma_chann);
-                DMA_schedule(fdctrl->dma_chann);
+                DMA_schedule();
             } else {
                 /* Start transfer */
                 fdctrl_transfer_handler(fdctrl, fdctrl->dma_chann, 0,
diff --git a/hw/dma/i8257.c b/hw/dma/i8257.c
index a414029..409ba7d 100644
--- a/hw/dma/i8257.c
+++ b/hw/dma/i8257.c
@@ -358,6 +358,7 @@ static void channel_run (int ncont, int ichan)
 }
 
 static QEMUBH *dma_bh;
+static bool dma_bh_scheduled;
 
 static void DMA_run (void)
 {
@@ -390,12 +391,15 @@ static void DMA_run (void)
 
     running = 0;
 out:
-    if (rearm)
+    if (rearm) {
         qemu_bh_schedule_idle(dma_bh);
+        dma_bh_scheduled = true;
+    }
 }
 
 static void DMA_run_bh(void *unused)
 {
+    dma_bh_scheduled = false;
     DMA_run();
 }
 
@@ -458,12 +462,14 @@ int DMA_write_memory (int nchan, void *buf, int pos, int len)
     return len;
 }
 
-/* request the emulator to transfer a new DMA memory block ASAP */
-void DMA_schedule(int nchan)
+/* request the emulator to transfer a new DMA memory block ASAP (even
+ * if the idle bottom half would not have exited the iothread yet).
+ */
+void DMA_schedule(void)
 {
-    struct dma_cont *d = &dma_controllers[nchan > 3];
-
-    qemu_irq_pulse(*d->cpu_request_exit);
+    if (dma_bh_scheduled) {
+        qemu_notify_event();
+    }
 }
 
 static void dma_reset(void *opaque)
diff --git a/hw/sparc/sun4m.c b/hw/sparc/sun4m.c
index 68ac4d8..ebaae9d 100644
--- a/hw/sparc/sun4m.c
+++ b/hw/sparc/sun4m.c
@@ -109,7 +109,7 @@ int DMA_write_memory (int nchan, void *buf, int pos, int size)
 }
 void DMA_hold_DREQ (int nchan) {}
 void DMA_release_DREQ (int nchan) {}
-void DMA_schedule(int nchan) {}
+void DMA_schedule(void) {}
 
 void DMA_init(int high_page_enable, qemu_irq *cpu_request_exit)
 {
diff --git a/hw/sparc64/sun4u.c b/hw/sparc64/sun4u.c
index 30cfa0e..44eb4eb 100644
--- a/hw/sparc64/sun4u.c
+++ b/hw/sparc64/sun4u.c
@@ -112,7 +112,7 @@ int DMA_write_memory (int nchan, void *buf, int pos, int size)
 }
 void DMA_hold_DREQ (int nchan) {}
 void DMA_release_DREQ (int nchan) {}
-void DMA_schedule(int nchan) {}
+void DMA_schedule(void) {}
 
 void DMA_init(int high_page_enable, qemu_irq *cpu_request_exit)
 {
diff --git a/include/hw/isa/isa.h b/include/hw/isa/isa.h
index f21ceaa..81b94ea 100644
--- a/include/hw/isa/isa.h
+++ b/include/hw/isa/isa.h
@@ -112,7 +112,7 @@ int DMA_read_memory (int nchan, void *buf, int pos, int size);
 int DMA_write_memory (int nchan, void *buf, int pos, int size);
 void DMA_hold_DREQ (int nchan);
 void DMA_release_DREQ (int nchan);
-void DMA_schedule(int nchan);
+void DMA_schedule(void);
 void DMA_init(int high_page_enable, qemu_irq *cpu_request_exit);
 void DMA_register_channel (int nchan,
                            DMA_transfer_handler transfer_handler,
-- 
2.4.3