[PATCH v2 1/3] drm/modes: introduce drm_mode_validate_mode() helper function

[Sean Nyekjaer]

Check if the required pixel clock is in within .5% range of the
desired pixel clock.
This will match the requirement for HDMI where a .5% tolerance is allowed.

Signed-off-by: Sean Nyekjaer <sean at geanix.com>
---
 drivers/gpu/drm/drm_modes.c | 34 ++++++++++++++++++++++++++++++++++
 include/drm/drm_modes.h     |  2 ++
 2 files changed, 36 insertions(+)

diff --git a/drivers/gpu/drm/drm_modes.c b/drivers/gpu/drm/drm_modes.c
index 6ba167a3346134072d100af0adbbe9b49e970769..4068b904759bf80502efde6e4d977b297f5d5359 100644
--- a/drivers/gpu/drm/drm_modes.c
+++ b/drivers/gpu/drm/drm_modes.c
@@ -1623,6 +1623,40 @@ bool drm_mode_equal_no_clocks_no_stereo(const struct drm_display_mode *mode1,
 }
 EXPORT_SYMBOL(drm_mode_equal_no_clocks_no_stereo);
 
+/**
+ * drm_mode_validate_mode
+ * @mode: mode to check
+ * @rounded_rate: output pixel clock
+ *
+ * VESA DMT defines a tolerance of 0.5% on the pixel clock, while the
+ * CVT spec reuses that tolerance in its examples, so it looks to be a
+ * good default tolerance for the EDID-based modes. Define it to 5 per
+ * mille to avoid floating point operations.
+ *
+ * Returns:
+ * The mode status
+ */
+enum drm_mode_status drm_mode_validate_mode(const struct drm_display_mode *mode,
+					    unsigned long long rounded_rate)
+{
+	enum drm_mode_status status;
+	unsigned long long rate = mode->clock * 1000;
+	unsigned long long lowest, highest;
+
+	lowest = rate * (1000 - 5);
+	do_div(lowest, 1000);
+	if (rounded_rate < lowest)
+		return MODE_CLOCK_LOW;
+
+	highest = rate * (1000 + 5);
+	do_div(highest, 1000);
+	if (rounded_rate > highest)
+		return MODE_CLOCK_HIGH;
+
+	return MODE_OK;
+}
+EXPORT_SYMBOL(drm_mode_validate_mode);
+
 static enum drm_mode_status
 drm_mode_validate_basic(const struct drm_display_mode *mode)
 {
diff --git a/include/drm/drm_modes.h b/include/drm/drm_modes.h
index b9bb92e4b0295a5cbe0eb0da13e77449ff04f51d..4b638992f3e50d2aba5088644744457d72dbe10a 100644
--- a/include/drm/drm_modes.h
+++ b/include/drm/drm_modes.h
@@ -549,6 +549,8 @@ bool drm_mode_equal_no_clocks(const struct drm_display_mode *mode1,
 			      const struct drm_display_mode *mode2);
 bool drm_mode_equal_no_clocks_no_stereo(const struct drm_display_mode *mode1,
 					const struct drm_display_mode *mode2);
+enum drm_mode_status drm_mode_validate_mode(const struct drm_display_mode *mode,
+					    unsigned long long rounded_rate);
 
 /* for use by the crtc helper probe functions */
 enum drm_mode_status drm_mode_validate_driver(struct drm_device *dev,

-- 
2.46.2