firmware: arm_scmi: Simplify spinlocks in virtio transport

Remove unneeded nested irqsave/irqrestore spinlocks.
Add also a few descriptive comments to explain better the system behaviour
at shutdown time.

Link: https://lore.kernel.org/r/20210916103336.7243-2-cristian.marussi@arm.com
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

diff --git a/drivers/firmware/arm_scmi/virtio.c b/drivers/firmware/arm_scmi/virtio.c
index 8941bb4..f3a1ef9 100644 (file)
--- a/drivers/firmware/arm_scmi/virtio.c
+++ b/drivers/firmware/arm_scmi/virtio.c
@@ -110,18 +110,16 @@ static void scmi_finalize_message(struct scmi_vio_channel *vioch,
        if (vioch->is_rx) {
                scmi_vio_feed_vq_rx(vioch, msg);
        } else {
-               unsigned long flags;
-
-               spin_lock_irqsave(&vioch->lock, flags);
+               /* Here IRQs are assumed to be already disabled by the caller */
+               spin_lock(&vioch->lock);
                list_add(&msg->list, &vioch->free_list);
-               spin_unlock_irqrestore(&vioch->lock, flags);
+               spin_unlock(&vioch->lock);
        }
 }
 
 static void scmi_vio_complete_cb(struct virtqueue *vqueue)
 {
        unsigned long ready_flags;
-       unsigned long flags;
        unsigned int length;
        struct scmi_vio_channel *vioch;
        struct scmi_vio_msg *msg;
@@ -140,7 +138,8 @@ static void scmi_vio_complete_cb(struct virtqueue *vqueue)
                        goto unlock_ready_out;
                }
 
-               spin_lock_irqsave(&vioch->lock, flags);
+               /* IRQs already disabled here no need to irqsave */
+               spin_lock(&vioch->lock);
                if (cb_enabled) {
                        virtqueue_disable_cb(vqueue);
                        cb_enabled = false;
@@ -151,7 +150,7 @@ static void scmi_vio_complete_cb(struct virtqueue *vqueue)
                                goto unlock_out;
                        cb_enabled = true;
                }
-               spin_unlock_irqrestore(&vioch->lock, flags);
+               spin_unlock(&vioch->lock);
 
                if (msg) {
                        msg->rx_len = length;
@@ -161,11 +160,18 @@ static void scmi_vio_complete_cb(struct virtqueue *vqueue)
                        scmi_finalize_message(vioch, msg);
                }
 
+               /*
+                * Release ready_lock and re-enable IRQs between loop iterations
+                * to allow virtio_chan_free() to possibly kick in and set the
+                * flag vioch->ready to false even in between processing of
+                * messages, so as to force outstanding messages to be ignored
+                * when system is shutting down.
+                */
                spin_unlock_irqrestore(&vioch->ready_lock, ready_flags);
        }
 
 unlock_out:
-       spin_unlock_irqrestore(&vioch->lock, flags);
+       spin_unlock(&vioch->lock);
 unlock_ready_out:
        spin_unlock_irqrestore(&vioch->ready_lock, ready_flags);
 }
@@ -435,6 +441,13 @@ static int scmi_vio_probe(struct virtio_device *vdev)
 
 static void scmi_vio_remove(struct virtio_device *vdev)
 {
+       /*
+        * Once we get here, virtio_chan_free() will have already been called by
+        * the SCMI core for any existing channel and, as a consequence, all the
+        * virtio channels will have been already marked NOT ready, causing any
+        * outstanding message on any vqueue to be ignored by complete_cb: now
+        * we can just stop processing buffers and destroy the vqueues.
+        */
        vdev->config->reset(vdev);
        vdev->config->del_vqs(vdev);
        scmi_vdev = NULL;