[PATCH] drm/meson: add support for higher bandwidth HDMI DTDs

[Martin Blumenstingl]

There's a report from a user with a DELL U3415W monitor and a Khadas
VIM3 stating that 3440x1440 at 50Hz works but 3440x1440 at 60Hz doesn't.
edid-decode Detailed Timing Descriptor (DTD) extract of that mode:
  DTD 1:  3440x1440   59.972616 Hz  43:18    88.819 kHz    319.750000 MHz
               Hfront   80 Hsync  32 Hback   48 Hpol P
               Vfront   28 Vsync  10 Vback    3 Vpol N

Those high clock rates are currently not supported by the meson_vclk
code as it limits the PLL OD (output divider) to a minimum of 2. The
formula to calculate the clock internally is (ignoring fractional
dividers):
  319.750MHz * 10 * od = 24MHz * m

With od = 2 this results in m = 266, which is outside the supported
range of the PLL on G12A SoCs (the driver limits m to 106..247).
Using od = 2 results in m = 133, which is supported.

Allow the driver to pick OD = 1 to gain support for modes such as
3440x1440 at 60Hz.

Link: https://forum.khadas.com/t/can-not-set-display-resolution-to-3440x1440-for-dell-monitor/26271/12
Signed-off-by: Martin Blumenstingl <martin.blumenstingl at googlemail.com>
---
 drivers/gpu/drm/meson/meson_vclk.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/meson/meson_vclk.c b/drivers/gpu/drm/meson/meson_vclk.c
index dfe0c28a0f05..c0be0b7655bf 100644
--- a/drivers/gpu/drm/meson/meson_vclk.c
+++ b/drivers/gpu/drm/meson/meson_vclk.c
@@ -700,8 +700,8 @@ static bool meson_hdmi_pll_find_params(struct meson_drm *priv,
 				       unsigned int *frac,
 				       unsigned int *od)
 {
-	/* Cycle from /16 to /2 */
-	for (*od = 16 ; *od > 1 ; *od >>= 1) {
+	/* Cycle from /16 to /1 */
+	for (*od = 16 ; *od > 0 ; *od >>= 1) {
 		*m = meson_hdmi_pll_get_m(priv, freq * *od);
 		if (!*m)
 			continue;
@@ -750,7 +750,7 @@ static void meson_hdmi_pll_generic_set(struct meson_drm *priv,
 		/* OD2 goes to the PHY, and needs to be *10, so keep OD3=1 */
 		od3 = 1;
 		if (od < 4) {
-			od1 = 2;
+			od1 = od;
 			od2 = 1;
 		} else {
 			od2 = od / 4;
-- 
2.52.0