drm: Add helpers for q4 fixed point values

Add helpers to convert between q4 fixed point and integer/fraction
values. Also add the format/argument macros required to printk q4 fixed
point variables. The q4 notation is based on the short variant described
by

https://en.wikipedia.org/wiki/Q_(number_format)

where only the number of fraction bits in the fixed point value are
defined, while the full size is deducted from the container type, that
is the size of int for these helpers. Using the fxp_ prefix, which makes
moving these helpers outside of drm to a more generic place easier, if
they prove to be useful.

These are needed by later patches dumping the Display Stream Compression
configuration in DRM core and in the i915 driver to replace the
corresponding bpp_x16 helpers defined locally in the driver.

v2: Use the more generic/descriptive fxp_q4 prefix instead of drm_x16.
   (Jani)

Cc: Jani Nikula <jani.nikula@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240628164451.1177612-2-imre.deak@intel.com

drivers/gpu/drm/display/drm_dp_helper.c

@@ -35,6 +35,7 @@
 #include <drm/display/drm_dp_helper.h>
 #include <drm/display/drm_dp_mst_helper.h>
 #include <drm/drm_edid.h>
+#include <drm/drm_fixed.h>
 #include <drm/drm_print.h>
 #include <drm/drm_vblank.h>
 #include <drm/drm_panel.h>
@@ -4151,9 +4152,9 @@ int drm_dp_bw_overhead(int lane_count, int hactive,
 	int symbol_cycles;
 
 	if (lane_count == 0 || hactive == 0 || bpp_x16 == 0) {
-		DRM_DEBUG_KMS("Invalid BW overhead params: lane_count %d, hactive %d, bpp_x16 %d.%04d\n",
+		DRM_DEBUG_KMS("Invalid BW overhead params: lane_count %d, hactive %d, bpp_x16 " FXP_Q4_FMT "\n",
 			      lane_count, hactive,
-			      bpp_x16 >> 4, (bpp_x16 & 0xf) * 625);
+			      FXP_Q4_ARGS(bpp_x16));
 		return 0;
 	}
 

include/drm/drm_fixed.h

@@ -214,4 +214,27 @@ static inline s64 drm_fixp_exp(s64 x)
 	return sum;
 }
 
+static inline int fxp_q4_from_int(int val_int)
+{
+	return val_int << 4;
+}
+
+static inline int fxp_q4_to_int(int val_q4)
+{
+	return val_q4 >> 4;
+}
+
+static inline int fxp_q4_to_int_roundup(int val_q4)
+{
+	return (val_q4 + 0xf) >> 4;
+}
+
+static inline int fxp_q4_to_frac(int val_q4)
+{
+	return val_q4 & 0xf;
+}
+
+#define FXP_Q4_FMT		"%d.%04d"
+#define FXP_Q4_ARGS(val_q4)	fxp_q4_to_int(val_q4), (fxp_q4_to_frac(val_q4) * 625)
+
 #endif