[RFC][PATCH 01/11] drm/i915: Reject modes with linetime > 64 usec

[Ville Syrjala <ville.syrjala@linux.intel.com>]

From: Ville Syrjälä <ville.syrjala@linux.intel.com>

Reject modes whose linetime exceeds 64 usec.

First reason being that WM_LINETIME is limited to (nearly) 64 usec.

Additionally knowing the linetime is bounded will help with
determining whether overflows may be a concern during various
calculations.

I decided to round up, and accept the linetime==64 case. We use
various rounding directions for this in other parts of the code,
so I feel this provides the most consistent result all around.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
---
 drivers/gpu/drm/i915/display/intel_display.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/gpu/drm/i915/display/intel_display.c b/drivers/gpu/drm/i915/display/intel_display.c
index b57efd870774..afa78774eaeb 100644
--- a/drivers/gpu/drm/i915/display/intel_display.c
+++ b/drivers/gpu/drm/i915/display/intel_display.c
@@ -7967,6 +7967,14 @@ enum drm_mode_status intel_mode_valid(struct drm_device *dev,
 	    mode->vtotal > vtotal_max)
 		return MODE_V_ILLEGAL;
 
+	/*
+	 * WM_LINETIME only goes up to (almost) 64 usec, and also
+	 * knowing that the linetime is always bounded will ease the
+	 * mind during various calculations.
+	 */
+	if (DIV_ROUND_UP(mode->htotal * 1000, mode->clock) > 64)
+		return MODE_H_ILLEGAL;
+
 	return MODE_OK;
 }
 
-- 
2.49.1