All of the SoCs that don't have dithering control in the gamma IP
have got a GAMMA_LUT_TYPE bit that tells to the IP if the LUT is
"descending" (bit set) or "rising" (bit cleared): make sure to set
it correctly after programming the LUT.

Reviewed-by: Jason-JH.Lin <jason-jh.lin@mediatek.com>
Reviewed-by: Alexandre Mergnat <amergnat@baylibre.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
---
 drivers/gpu/drm/mediatek/mtk_disp_gamma.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/drivers/gpu/drm/mediatek/mtk_disp_gamma.c b/drivers/gpu/drm/mediatek/mtk_disp_gamma.c
index 6746033615db..0f116c0e51b5 100644
--- a/drivers/gpu/drm/mediatek/mtk_disp_gamma.c
+++ b/drivers/gpu/drm/mediatek/mtk_disp_gamma.c
@@ -22,6 +22,7 @@
 #define GAMMA_RELAY_MODE				BIT(0)
 #define GAMMA_LUT_EN					BIT(1)
 #define GAMMA_DITHERING					BIT(2)
+#define GAMMA_LUT_TYPE					BIT(2)
 #define DISP_GAMMA_SIZE				0x0030
 #define DISP_GAMMA_SIZE_HSIZE				GENMASK(28, 16)
 #define DISP_GAMMA_SIZE_VSIZE				GENMASK(12, 0)
@@ -82,6 +83,17 @@ unsigned int mtk_gamma_get_lut_size(struct device *dev)
 	return 0;
 }
 
+static bool mtk_gamma_lut_is_descending(struct drm_color_lut *lut, u32 lut_size)
+{
+	u64 first, last;
+	int last_entry = lut_size - 1;
+
+	first = lut[0].red + lut[0].green + lut[0].blue;
+	last = lut[last_entry].red + lut[last_entry].green + lut[last_entry].blue;
+
+	return !!(first > last);
+}
+
 /*
  * SoCs supporting 12-bits LUTs are using a new register layout that does
  * always support (by HW) both 12-bits and 10-bits LUT but, on those, we
@@ -173,6 +185,14 @@ void mtk_gamma_set(struct device *dev, struct drm_crtc_state *state)
 
 	cfg_val = readl(gamma->regs + DISP_GAMMA_CFG);
 
+	if (!gamma->data->has_dither) {
+		/* Descending or Rising LUT */
+		if (mtk_gamma_lut_is_descending(lut, gamma->data->lut_size - 1))
+			cfg_val |= FIELD_PREP(GAMMA_LUT_TYPE, 1);
+		else
+			cfg_val &= ~GAMMA_LUT_TYPE;
+	}
+
 	/* Enable the gamma table */
 	cfg_val |= FIELD_PREP(GAMMA_LUT_EN, 1);
 
-- 
2.42.0