[PATCH v2 4/5] drm/i915/display: Add Nearest-neighbor based integer scaling support

[Pankaj Bharadiya]

Integer scaling (IS) is a nearest-neighbor upscaling technique that
simply scales up the existing pixels by an integer
(i.e., whole number) multiplier.Nearest-neighbor (NN) interpolation
works by filling in the missing color values in the upscaled image
with that of the coordinate-mapped nearest source pixel value.

Both IS and NN preserve the clarity of the original image. Integer
scaling is particularly useful for pixel art games that rely on
sharp, blocky images to deliver their distinctive look.

Introduce skl_scaler_setup_nearest_neighbor_filter() function which
configures the scaler filter coefficients to enable nearest-neighbor
filtering.

Bspec: 49247

changes since v1:
* Rearrange skl_scaler_setup_nearest_neighbor_filter() to iterate the
  registers directly instead of the phases and taps (Ville)

changes since RFC:
* Refine the skl_scaler_setup_nearest_neighbor_filter() logic (Ville)

Signed-off-by: Shashank Sharma <shashank.sharma at intel.com>
Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal at intel.com>
Signed-off-by: Pankaj Bharadiya <pankaj.laxminarayan.bharadiya at intel.com>
---
 drivers/gpu/drm/i915/display/intel_display.c | 72 ++++++++++++++++++++
 drivers/gpu/drm/i915/display/intel_display.h |  2 +
 2 files changed, 74 insertions(+)

diff --git a/drivers/gpu/drm/i915/display/intel_display.c b/drivers/gpu/drm/i915/display/intel_display.c
index 8f23c4d51c33..791dd908aa89 100644
--- a/drivers/gpu/drm/i915/display/intel_display.c
+++ b/drivers/gpu/drm/i915/display/intel_display.c
@@ -6237,6 +6237,78 @@ void skl_scaler_disable(const struct intel_crtc_state *old_crtc_state)
 		skl_detach_scaler(crtc, i);
 }
 
+static int skl_coef_tap(int i)
+{
+	return i % 7;
+}
+
+static u16 skl_nearest_filter_coef(int t)
+{
+	return t == 3 ? 0x0800 : 0x3000;
+}
+
+/**
+ *  Theory behind setting nearest-neighbor integer scaling:
+ *
+ *  17 phase of 7 taps requires 119 coefficients in 60 dwords per set.
+ *  The letter represents the filter tap (D is the center tap) and the number
+ *  represents the coefficient set for a phase (0-16).
+ *
+ *         +------------+------------------------+------------------------+
+ *         |Index value | Data value coeffient 1 | Data value coeffient 2 |
+ *         +------------+------------------------+------------------------+
+ *         |   00h      |          B0            |          A0            |
+ *         +------------+------------------------+------------------------+
+ *         |   01h      |          D0            |          C0            |
+ *         +------------+------------------------+------------------------+
+ *         |   02h      |          F0            |          E0            |
+ *         +------------+------------------------+------------------------+
+ *         |   03h      |          A1            |          G0            |
+ *         +------------+------------------------+------------------------+
+ *         |   04h      |          C1            |          B1            |
+ *         +------------+------------------------+------------------------+
+ *         |   ...      |          ...           |          ...           |
+ *         +------------+------------------------+------------------------+
+ *         |   38h      |          B16           |          A16           |
+ *         +------------+------------------------+------------------------+
+ *         |   39h      |          D16           |          C16           |
+ *         +------------+------------------------+------------------------+
+ *         |   3Ah      |          F16           |          C16           |
+ *         +------------+------------------------+------------------------+
+ *         |   3Bh      |        Reserved        |          G16           |
+ *         +------------+------------------------+------------------------+
+ *
+ *  To enable nearest-neighbor scaling:  program scaler coefficents with
+ *  the center tap (Dxx) values set to 1 and all other values set to 0 as per
+ *  SCALER_COEFFICIENT_FORMAT
+ *
+ */
+
+void skl_scaler_setup_nearest_neighbor_filter(struct drm_i915_private *dev_priv,
+					      enum pipe pipe, int id, int set)
+{
+	int i;
+
+	/*enable the index auto increment.*/
+	intel_de_write_fw(dev_priv,
+			  SKL_PS_COEF_INDEX_SET(pipe, id, set),
+			  PS_COEE_INDEX_AUTO_INC);
+
+	for (i = 0; i < 17 * 7; i += 2) {
+		u32 tmp;
+		int t;
+
+		t = skl_coef_tap(i);
+		tmp = skl_nearest_filter_coef(t);
+
+		t = skl_coef_tap(i+1);
+		tmp |= skl_nearest_filter_coef(t)<<16;
+
+		intel_de_write_fw(dev_priv, SKL_PS_COEF_DATA_SET(pipe, id, set),
+				  tmp);
+	}
+}
+
 static void skl_pfit_enable(const struct intel_crtc_state *crtc_state)
 {
 	struct intel_crtc *crtc = to_intel_crtc(crtc_state->uapi.crtc);
diff --git a/drivers/gpu/drm/i915/display/intel_display.h b/drivers/gpu/drm/i915/display/intel_display.h
index adb1225a3480..88f3c77f6806 100644
--- a/drivers/gpu/drm/i915/display/intel_display.h
+++ b/drivers/gpu/drm/i915/display/intel_display.h
@@ -587,6 +587,8 @@ void intel_crtc_arm_fifo_underrun(struct intel_crtc *crtc,
 u16 skl_scaler_calc_phase(int sub, int scale, bool chroma_center);
 int skl_update_scaler_crtc(struct intel_crtc_state *crtc_state);
 void skl_scaler_disable(const struct intel_crtc_state *old_crtc_state);
+void skl_scaler_setup_nearest_neighbor_filter(struct drm_i915_private *dev_priv,
+					      enum pipe pipe, int id, int set);
 void ilk_pfit_disable(const struct intel_crtc_state *old_crtc_state);
 u32 glk_plane_color_ctl(const struct intel_crtc_state *crtc_state,
 			const struct intel_plane_state *plane_state);
-- 
2.23.0