drm/i915/dsi: Use a fuzzy check for burst mode clock check

Prior to this commit we fail to init the DSI panel on the GPD MicroPC:
https://www.indiegogo.com/projects/gpd-micropc-6-inch-handheld-industry-laptop#/

The problem is intel_dsi_vbt_init() failing with the following error:
*ERROR* Burst mode freq is less than computed

The pclk in the VBT panel modeline is 70000, together with 24 bpp and
4 lines this results in a bitrate value of 70000 * 24 / 4 = 420000.
But the target_burst_mode_freq in the VBT is 418000.

This commit works around this problem by adding an intel_fuzzy_clock_check
when target_burst_mode_freq < bitrate and setting target_burst_mode_freq to
bitrate when that checks succeeds, fixing the panel not working.

Cc: stable@vger.kernel.org
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190524174028.21659-2-hdegoede@redhat.com
(cherry picked from commit 2c1c5525)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_display.c

@@ -11942,7 +11942,7 @@ intel_modeset_pipe_config(struct drm_crtc *crtc,
 	return 0;
 }
 
-static bool intel_fuzzy_clock_check(int clock1, int clock2)
+bool intel_fuzzy_clock_check(int clock1, int clock2)
 {
 	int diff;
 

drivers/gpu/drm/i915/intel_drv.h

@@ -1742,6 +1742,7 @@ int vlv_force_pll_on(struct drm_i915_private *dev_priv, enum pipe pipe,
 		     const struct dpll *dpll);
 void vlv_force_pll_off(struct drm_i915_private *dev_priv, enum pipe pipe);
 int lpt_get_iclkip(struct drm_i915_private *dev_priv);
+bool intel_fuzzy_clock_check(int clock1, int clock2);
 
 /* modesetting asserts */
 void assert_panel_unlocked(struct drm_i915_private *dev_priv,

drivers/gpu/drm/i915/intel_dsi_vbt.c

@@ -853,6 +853,17 @@ bool intel_dsi_vbt_init(struct intel_dsi *intel_dsi, u16 panel_id)
 		if (mipi_config->target_burst_mode_freq) {
 			u32 bitrate = intel_dsi_bitrate(intel_dsi);
 
+			/*
+			 * Sometimes the VBT contains a slightly lower clock,
+			 * then the bitrate we have calculated, in this case
+			 * just replace it with the calculated bitrate.
+			 */
+			if (mipi_config->target_burst_mode_freq < bitrate &&
+			    intel_fuzzy_clock_check(
+					mipi_config->target_burst_mode_freq,
+					bitrate))
+				mipi_config->target_burst_mode_freq = bitrate;
+
 			if (mipi_config->target_burst_mode_freq < bitrate) {
 				DRM_ERROR("Burst mode freq is less than computed\n");
 				return false;