From 26164df08976b31ea7d28dfbe3c2e18e67bcac87 Mon Sep 17 00:00:00 2001
From: nyanpasu64 <nyanpasu64@tuta.io>
Date: Sat, 22 Apr 2023 02:48:06 -0700
Subject: [PATCH 1/2] Fix hsync_start computation

The CVT v1.2 specification says, in section 3.4.1 "Standard CRT-based
Timing":

> The Horizontal Sync Pulse duration will in all cases be set as closely
> as possible to 8% of the Total Horizontal time, rounded down to the
> nearest cell width.

The previous code achieved this by first setting mode_info->hsync_end to
a multiple of the cell width, then calculating mode_info->hsync_start
and rounding its position up to a cell width. However, if
mode_info->hsync_start was already a multiple of 8 (for example on
640x480@60), the rounding code would erroneously add 8 to it, shortening
the hsync duration by 8 pixels.

This commit fixes the bug by calculating and rounding the sync width
separately, closer to the specification's wording.

Signed-off-by: nyanpasu64 <nyanpasu64@tuta.io>
Closes: https://gitlab.freedesktop.org/xorg/lib/libxcvt/-/issues/6
Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>
---
 lib/libxcvt.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/lib/libxcvt.c b/lib/libxcvt.c
index 51a61e7..8d0eb53 100644
--- a/lib/libxcvt.c
+++ b/lib/libxcvt.c
@@ -161,7 +161,7 @@ libxcvt_gen_mode_info(int hdisplay, int vdisplay, float vrefresh, bool reduced,
 
         float hblank_percentage;
         int vsync_and_back_porch, vback_porch;
-        int hblank;
+        int hblank, hsync_w;
 
         /* 8. Estimated Horizontal period */
         hperiod = ((float) (1000000.0 / vfield_rate - CVT_MIN_VSYNC_BP)) /
@@ -216,10 +216,9 @@ libxcvt_gen_mode_info(int hdisplay, int vdisplay, float vrefresh, bool reduced,
         /* Fill in HSync values */
         mode_info->hsync_end = mode_info->hdisplay + hblank / 2;
 
-        mode_info->hsync_start = mode_info->hsync_end -
-            (mode_info->htotal * CVT_HSYNC_PERCENTAGE) / 100;
-        mode_info->hsync_start += CVT_H_GRANULARITY -
-            mode_info->hsync_start % CVT_H_GRANULARITY;
+        hsync_w = (mode_info->htotal * CVT_HSYNC_PERCENTAGE) / 100;
+        hsync_w -= hsync_w % CVT_H_GRANULARITY;
+        mode_info->hsync_start = mode_info->hsync_end - hsync_w;
 
         /* Fill in vsync values */
         mode_info->vsync_start = mode_info->vdisplay + CVT_MIN_V_PORCH;
-- 
2.20.1