[PATCH 3/5] drm/bridge: nwl-dsi: Add support for clock-drop-level

["Robert Chiras (OSS)" <robert.chiras@oss.nxp.com>]

From: Robert Chiras <robert.chiras@nxp.com>

The clock-drop-level is needed in order to add more blanking space needed
by DSI panels when sending DSI commands. One level is the equivalent of
phy_ref rate from the PLL rate. Since the PLL rate is targeted as highest
possible, each level should not get the crtc_clock too low, compared to
the actual clock.

Example for a clock of 132M, with "clock-drop-level = <1>" in dts file
will result in a crtc_clock of 129M, using the following logic:
- video_pll rate to provide both phy_ref rate of 24M and pixel-clock
  of 132M is 1056M (divisor /43 for phy_ref and /8 for pixel-clock)
- from this rate, we subtract the equivalent of phy_ref (24M) but
  keep the same divisor. This way, the video_pll rate will be 1056 - 24
= 1032M.
- new pixel-clock will be: 1032 / 8 = 129M

For a "clock-drop-level = <2>", new pixel-clock will be:
(1056 - (24 * 2)) / 8 = 1008 / 8 = 126M

Signed-off-by: Robert Chiras <robert.chiras@nxp.com>
---
 drivers/gpu/drm/bridge/nwl-dsi.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/bridge/nwl-dsi.c b/drivers/gpu/drm/bridge/nwl-dsi.c
index 1228466..ac4aa0a 100644
--- a/drivers/gpu/drm/bridge/nwl-dsi.c
+++ b/drivers/gpu/drm/bridge/nwl-dsi.c
@@ -142,6 +142,7 @@ struct nwl_dsi {
 
 	struct nwl_dsi_transfer *xfer;
 	struct list_head valid_modes;
+	u32 clk_drop_lvl;
 };
 
 static const struct regmap_config nwl_dsi_regmap_config = {
@@ -842,13 +843,14 @@ static unsigned long nwl_dsi_get_lcm(unsigned long a, unsigned long b)
 
 	return ((unsigned long long)a * b) / gcf;
 }
-
 /*
  * This function tries to adjust the crtc_clock for a DSI device in such a way
  * that the video pll will be able to satisfy both Display Controller pixel
  * clock (feeding out DPI interface) and our input phy_ref clock.
+ * Also, the DC pixel clock must be lower than the actual clock in order to
+ * have enough blanking space to send DSI commands, if the device is a panel.
  */
-static void nwl_dsi_setup_pll_config(struct mode_config *config)
+static void nwl_dsi_setup_pll_config(struct mode_config *config, u32 lvl)
 {
 	unsigned long pll_rate;
 	int div;
@@ -908,7 +910,6 @@ static void nwl_dsi_setup_pll_config(struct mode_config *config)
 	}
 }
 
-
 /*
  * This function will try the required phy speed for current mode
  * If the phy speed can be achieved, the phy will save the speed
@@ -1103,7 +1104,7 @@ nwl_dsi_bridge_mode_valid(struct drm_bridge *bridge,
 
 	pll_rate = config->pll_rates[config->phy_rate_idx];
 	if (dsi->pll_clk && !pll_rate)
-		nwl_dsi_setup_pll_config(config);
+		nwl_dsi_setup_pll_config(config, dsi->clk_drop_lvl);
 
 	return MODE_OK;
 }
@@ -1248,6 +1249,7 @@ static const struct drm_bridge_funcs nwl_dsi_bridge_funcs = {
 static int nwl_dsi_parse_dt(struct nwl_dsi *dsi)
 {
 	struct platform_device *pdev = to_platform_device(dsi->dev);
+	struct device_node *np = dsi->dev->of_node;
 	struct clk *clk;
 	void __iomem *base;
 	int ret;
@@ -1364,6 +1366,8 @@ static int nwl_dsi_parse_dt(struct nwl_dsi *dsi)
 		return PTR_ERR(dsi->rst_dpi);
 	}
 
+	of_property_read_u32(np, "fsl,clock-drop-level", &dsi->clk_drop_lvl);
+
 	INIT_LIST_HEAD(&dsi->valid_modes);
 
 	return 0;
-- 
2.7.4