[RFC PATCH 1/3] drm: bridge: samsung-dsim: Fix i.MX8M enable flow to meet spec

[Frieder Schrempf <frieder@fris.de>]

From: Frieder Schrempf <frieder.schrempf@kontron.de>

According to the documentation [1] the proper enable flow is:

1. Enable DSI link and keep data lanes in LP-11 (stop state)
2. Disable stop state to bring data lanes into HS mode

Currently we do this all at once within enable(), which doesn't
allow to meet the requirements of some downstream bridges.

To fix this we now enable the DSI in pre_enable() and force it
into stop state using the FORCE_STOP_STATE bit in the ESCMODE
register until enable() is called where we reset the bit.

We currently do this only for i.MX8M as Exynos uses a different
init flow where samsung_dsim_init() is called from
samsung_dsim_host_transfer().

[1] https://docs.kernel.org/gpu/drm-kms-helpers.html#mipi-dsi-bridge-operation

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
---
 drivers/gpu/drm/bridge/samsung-dsim.c | 25 +++++++++++++++++++++++--
 1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/bridge/samsung-dsim.c b/drivers/gpu/drm/bridge/samsung-dsim.c
index e0a402a85787..9775779721d9 100644
--- a/drivers/gpu/drm/bridge/samsung-dsim.c
+++ b/drivers/gpu/drm/bridge/samsung-dsim.c
@@ -859,6 +859,10 @@ static int samsung_dsim_init_link(struct samsung_dsim *dsi)
 	reg = samsung_dsim_read(dsi, DSIM_ESCMODE_REG);
 	reg &= ~DSIM_STOP_STATE_CNT_MASK;
 	reg |= DSIM_STOP_STATE_CNT(driver_data->reg_values[STOP_STATE_CNT]);
+
+	if (!samsung_dsim_hw_is_exynos(dsi->plat_data->hw_type))
+		reg |= DSIM_FORCE_STOP_STATE;
+
 	samsung_dsim_write(dsi, DSIM_ESCMODE_REG, reg);
 
 	reg = DSIM_BTA_TIMEOUT(0xff) | DSIM_LPDR_TIMEOUT(0xffff);
@@ -1340,6 +1344,9 @@ static void samsung_dsim_atomic_pre_enable(struct drm_bridge *bridge,
 		ret = samsung_dsim_init(dsi);
 		if (ret)
 			return;
+
+		samsung_dsim_set_display_mode(dsi);
+		samsung_dsim_set_display_enable(dsi, true);
 	}
 }
 
@@ -1347,9 +1354,16 @@ static void samsung_dsim_atomic_enable(struct drm_bridge *bridge,
 				       struct drm_bridge_state *old_bridge_state)
 {
 	struct samsung_dsim *dsi = bridge_to_dsi(bridge);
+	u32 reg;
 
-	samsung_dsim_set_display_mode(dsi);
-	samsung_dsim_set_display_enable(dsi, true);
+	if (samsung_dsim_hw_is_exynos(dsi->plat_data->hw_type)) {
+		samsung_dsim_set_display_mode(dsi);
+		samsung_dsim_set_display_enable(dsi, true);
+	} else {
+		reg = samsung_dsim_read(dsi, DSIM_ESCMODE_REG);
+		reg &= ~DSIM_FORCE_STOP_STATE;
+		samsung_dsim_write(dsi, DSIM_ESCMODE_REG, reg);
+	}
 
 	dsi->state |= DSIM_STATE_VIDOUT_AVAILABLE;
 }
@@ -1358,10 +1372,17 @@ static void samsung_dsim_atomic_disable(struct drm_bridge *bridge,
 					struct drm_bridge_state *old_bridge_state)
 {
 	struct samsung_dsim *dsi = bridge_to_dsi(bridge);
+	u32 reg;
 
 	if (!(dsi->state & DSIM_STATE_ENABLED))
 		return;
 
+	if (!samsung_dsim_hw_is_exynos(dsi->plat_data->hw_type)) {
+		reg = samsung_dsim_read(dsi, DSIM_ESCMODE_REG);
+		reg |= DSIM_FORCE_STOP_STATE;
+		samsung_dsim_write(dsi, DSIM_ESCMODE_REG, reg);
+	}
+
 	dsi->state &= ~DSIM_STATE_VIDOUT_AVAILABLE;
 }
 
-- 
2.40.0