[RFC PATCH 1/3] drm/msm/dsi: select RGB DSC 1.2 rate-control parameters

[Zhang Jiaxi]

From: Jiaxi Zhang <z1529105815@xxxxxxxxxxx>

The DSI host forces RGB conversion but always selects the DSC 1.1 pre-SCR
rate-control defaults. For a DSC 1.2 RGB configuration, select the existing
DRM_DSC_1_2_444 table instead. Its recommended parameters differ from the
pre-SCR table even for the same bpp/bpc. This changes encoder/PPS RC
values, not just the advertised version.

Limit the new selection to version 1.2. Keep the existing defaults for
other versions and retain byte-clock rounding, slice handling, supported
component depths and fractional-bpp rejection. Reject native 4:2:0/4:2:2
rather than combine their flags with this path's forced RGB conversion.
This does not add native YCbCr or future DSC-version support.

The table definitions and mode-specific helper are public Linux DRM code:
drivers/gpu/drm/display/drm_dsc_helper.c. The earlier Warsaw display recovery
is supporting development evidence, not a new hardware test of this target.

Signed-off-by: Jiaxi Zhang <z1529105815@xxxxxxxxxxx>

---
diff --git a/drivers/gpu/drm/msm/dsi/dsi_host.c b/drivers/gpu/drm/msm/dsi/dsi_host.c
--- a/drivers/gpu/drm/msm/dsi/dsi_host.c
+++ b/drivers/gpu/drm/msm/dsi/dsi_host.c
@@ -1877,7 +1877,12 @@
 
 static int dsi_populate_dsc_params(struct msm_dsi_host *msm_host, struct drm_dsc_config *dsc)
 {
+	enum drm_dsc_params_type rc_params_type = DRM_DSC_1_1_PRE_SCR;
 	int ret;
+
+	/* This DSI path configures RGB, not native subsampled YCbCr. */
+	if (dsc->native_420 || dsc->native_422)
+		return -EOPNOTSUPP;
 
 	if (dsc->bits_per_pixel & 0xf) {
 		DRM_DEV_ERROR(&msm_host->pdev->dev, "DSI does not support fractional bits_per_pixel\n");
@@ -1908,8 +1913,10 @@
 	drm_dsc_set_const_params(dsc);
 	drm_dsc_set_rc_buf_thresh(dsc);
 
-	/* DPU supports only pre-SCR panels */
-	ret = drm_dsc_setup_rc_params(dsc, DRM_DSC_1_1_PRE_SCR);
+	if (dsc->dsc_version_major == 1 && dsc->dsc_version_minor == 2)
+		rc_params_type = DRM_DSC_1_2_444;
+
+	ret = drm_dsc_setup_rc_params(dsc, rc_params_type);
 	if (ret) {
 		DRM_DEV_ERROR(&msm_host->pdev->dev, "could not find DSC RC parameters\n");
 		return ret;