drm/i915/dsc: Allow DSC only with fractional bpp when forced from debugfs

If force_dsc_fractional_bpp_en is set through debugfs allow DSC iff
compressed bpp is fractional. Continue if the computed compressed bpp
turns out to be a integer.

v2:
-Use helpers for fractional, integral bits of bits_per_pixel. (Suraj)
-Fix comment (Suraj)
Signed-off-by: Swati Sharma <swati2.sharma@intel.com>
Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Sui Jingfeng <suijingfeng@loongson.cn>
Link: https://patchwork.freedesktop.org/patch/msgid/20231110101020.4067342-9-ankit.k.nautiyal@intel.com

drivers/gpu/drm/i915/display/intel_dp.c

@@ -1928,6 +1928,9 @@ xelpd_dsc_compute_link_config(struct intel_dp *intel_dp,
 	for (compressed_bppx16 = dsc_max_bpp;
 	     compressed_bppx16 >= dsc_min_bpp;
 	     compressed_bppx16 -= bppx16_step) {
+		if (intel_dp->force_dsc_fractional_bpp_en &&
+		    !to_bpp_frac(compressed_bppx16))
+			continue;
 		ret = dsc_compute_link_config(intel_dp,
 					      pipe_config,
 					      limits,
@@ -1935,6 +1938,10 @@ xelpd_dsc_compute_link_config(struct intel_dp *intel_dp,
 					      timeslots);
 		if (ret == 0) {
 			pipe_config->dsc.compressed_bpp_x16 = compressed_bppx16;
+			if (intel_dp->force_dsc_fractional_bpp_en &&
+			    to_bpp_frac(compressed_bppx16))
+				drm_dbg_kms(&i915->drm, "Forcing DSC fractional bpp\n");
+
 			return 0;
 		}
 	}