[PATCH 16/42] drm/mediatek: mtk_crtc: Minimize spinlocked time in cmdq callback

[AngeloGioacchino Del Regno]

In ddp_cmdq_cb (the CMDQ mailbox callback), the config_lock spin
lock is being locked to read the config_updating variable in the
mtk_crtc structure, as, if configuration is updating, the callback
function shall not run (or shall run at a later time).

It is therefore unnecessary to keep the spinlock until the end of
the function, in case pending config, planes, or async planes
bools need to be updated.

Though the cmdq callback is not a performance path in this case,
this may be running for each frame that is pushed to the display,
so from 30 (or less) up to 144 times (or more) per second.

Add a temporary "cfg_updating" variable to aid with readability,
and unlock the spinlock as soon as the config_updating variable
read operation is done as a micro-optimization.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno at collabora.com>
---
 drivers/gpu/drm/mediatek/mtk_crtc.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/mediatek/mtk_crtc.c b/drivers/gpu/drm/mediatek/mtk_crtc.c
index e543fae9c580..35e78ece899a 100644
--- a/drivers/gpu/drm/mediatek/mtk_crtc.c
+++ b/drivers/gpu/drm/mediatek/mtk_crtc.c
@@ -287,6 +287,7 @@ static void ddp_cmdq_cb(struct mbox_client *cl, void *mssg)
 	struct mtk_crtc_state *state;
 	unsigned int i;
 	unsigned long flags;
+	bool cfg_updating;
 
 	/* release GCE HW usage and start autosuspend */
 	pm_runtime_mark_last_busy(cmdq_cl->chan->mbox->dev);
@@ -298,7 +299,10 @@ static void ddp_cmdq_cb(struct mbox_client *cl, void *mssg)
 	state = to_mtk_crtc_state(mtk_crtc->base.state);
 
 	spin_lock_irqsave(&mtk_crtc->config_lock, flags);
-	if (mtk_crtc->config_updating)
+	cfg_updating = mtk_crtc->config_updating;
+	spin_unlock_irqrestore(&mtk_crtc->config_lock, flags);
+
+	if (cfg_updating)
 		goto ddp_cmdq_cb_out;
 
 	state->pending_config = false;
@@ -328,14 +332,11 @@ static void ddp_cmdq_cb(struct mbox_client *cl, void *mssg)
 	}
 
 ddp_cmdq_cb_out:
-
 	if (mtk_crtc->pending_needs_vblank) {
 		mtk_crtc_finish_page_flip(mtk_crtc);
 		mtk_crtc->pending_needs_vblank = false;
 	}
 
-	spin_unlock_irqrestore(&mtk_crtc->config_lock, flags);
-
 	mtk_crtc->cmdq_vblank_cnt = 0;
 	wake_up(&mtk_crtc->cb_blocking_queue);
 }
-- 
2.54.0