[PATCH] drm/msm/dsi: Implement reset correctly

[Jeffrey Hugo]

On msm8998, vblank timeouts are observed because the DSI controller is not
reset properly, which ends up stalling the MDP.  This is because the reset
logic is not correct per the hardware documentation.

The documentation states that after asserting reset, software should wait
some time (no indication of how long), or poll the status register until it
returns 0 before deasserting reset.

wmb() is insufficient for this purpose since it just ensures ordering, not
timing between writes.  Since asserting and deasserting reset occurs on the
same register, ordering is already guaranteed by the architecture, making
the wmb extraneous.

Since we would define a timeout for polling the status register to avoid a
possible infinite loop, lets just use a static delay of 20 ms, since 16.666
ms is the time available to process one frame at 60 fps.

Fixes: a689554ba6ed (drm/msm: Initial add DSI connector support)
Signed-off-by: Jeffrey Hugo <jeffrey.l.hugo at gmail.com>
---

Rob et al, is it possible for this to go into a 5.4-rc?

 drivers/gpu/drm/msm/dsi/dsi_host.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/msm/dsi/dsi_host.c b/drivers/gpu/drm/msm/dsi/dsi_host.c
index 663ff9f4fac9..68ded9b4735d 100644
--- a/drivers/gpu/drm/msm/dsi/dsi_host.c
+++ b/drivers/gpu/drm/msm/dsi/dsi_host.c
@@ -986,7 +986,7 @@ static void dsi_sw_reset(struct msm_dsi_host *msm_host)
 	wmb(); /* clocks need to be enabled before reset */
 
 	dsi_write(msm_host, REG_DSI_RESET, 1);
-	wmb(); /* make sure reset happen */
+	msleep(20); /* make sure reset happen */
 	dsi_write(msm_host, REG_DSI_RESET, 0);
 }
 
@@ -1396,7 +1396,7 @@ static void dsi_sw_reset_restore(struct msm_dsi_host *msm_host)
 
 	/* dsi controller can only be reset while clocks are running */
 	dsi_write(msm_host, REG_DSI_RESET, 1);
-	wmb();	/* make sure reset happen */
+	msleep(20);	/* make sure reset happen */
 	dsi_write(msm_host, REG_DSI_RESET, 0);
 	wmb();	/* controller out of reset */
 	dsi_write(msm_host, REG_DSI_CTRL, data0);
-- 
2.17.1