[PATCH] drm/i915: Try to re-use GOP / previous M-N-P settings for vlv DSI PLL

[Hans de Goede]

Fastboot is not working on an Asus T100HA, it gives the following
relevant messages / errors:

 dsi pll div 000201e6, ctrl 80080100
 fastset mismatch in dsi_pll.ctrl (expected 0x80100100, found 0x80080100)
 fastset mismatch in dsi_pll.div (expected 0x0002008e, found 0x000201e6)

The problem seems to be that the GOP picks 5 for the P divisor, where as
we end up picking 4.

This commit fixes this by first checking of the currently configured
DSI PLL settings match the desired pclk and if they do, stick with
the currently configured PLL settings.

Note that vlv_dsi_get_pclk() stores the read ctrl and div values inside
config->dsi_pll, so they are set to the GOP / previous values after
calling it.

Signed-off-by: Hans de Goede <hdegoede at redhat.com>
---
 drivers/gpu/drm/i915/display/vlv_dsi_pll.c | 26 +++++++++++++++-------
 1 file changed, 18 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/vlv_dsi_pll.c b/drivers/gpu/drm/i915/display/vlv_dsi_pll.c
index 95f39cd0ce02..4a09edecd597 100644
--- a/drivers/gpu/drm/i915/display/vlv_dsi_pll.c
+++ b/drivers/gpu/drm/i915/display/vlv_dsi_pll.c
@@ -119,15 +119,25 @@ int vlv_dsi_pll_compute(struct intel_encoder *encoder,
 	struct drm_i915_private *dev_priv = to_i915(encoder->base.dev);
 	struct intel_dsi *intel_dsi = enc_to_intel_dsi(&encoder->base);
 	int ret;
-	u32 dsi_clk;
-
-	dsi_clk = dsi_clk_from_pclk(intel_dsi->pclk, intel_dsi->pixel_format,
-				    intel_dsi->lane_count);
+	u32 dsi_clk, current_pclk;
 
-	ret = dsi_calc_mnp(dev_priv, config, dsi_clk);
-	if (ret) {
-		DRM_DEBUG_KMS("dsi_calc_mnp failed\n");
-		return ret;
+	/*
+	 * For exact matches, the GOP may pick another set of divisors
+	 * then we do, if the GOP settings are an exact match keep them.
+	 */
+	current_pclk = vlv_dsi_get_pclk(encoder, config);
+	if (current_pclk == intel_dsi->pclk) {
+		config->dsi_pll.ctrl &= DSI_PLL_P1_POST_DIV_MASK;
+	} else {
+		dsi_clk = dsi_clk_from_pclk(intel_dsi->pclk,
+					    intel_dsi->pixel_format,
+					    intel_dsi->lane_count);
+
+		ret = dsi_calc_mnp(dev_priv, config, dsi_clk);
+		if (ret) {
+			DRM_DEBUG_KMS("dsi_calc_mnp failed\n");
+			return ret;
+		}
 	}
 
 	if (intel_dsi->ports & (1 << PORT_A))
-- 
2.23.0