drm/msm/dsi: add a comment to explain pkt_per_line encoding

Make it clear why the pkt_per_line value is being "divided by 2".
Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Jun Nie <jun.nie@linaro.org>
Tested-by: Neil Armstrong <neil.armstrong@linaro.org> # on SM8550-QRD
Tested-by: Neil Armstrong <neil.armstrong@linaro.org> # on SM8650-QRD
Tested-by: Neil Armstrong <neil.armstrong@linaro.org> # on SM8650-HDK
Reviewed-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/596236/
Link: https://lore.kernel.org/r/20240530-msm-drm-dsc-dsi-video-upstream-4-v6-6-2ab1d334c657@linaro.orgSigned-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>

drivers/gpu/drm/msm/dsi/dsi_host.c

@@ -885,7 +885,11 @@ static void dsi_update_dsc_timing(struct msm_dsi_host *msm_host, bool is_cmd_mod
 	/* DSI_VIDEO_COMPRESSION_MODE & DSI_COMMAND_COMPRESSION_MODE
 	 * registers have similar offsets, so for below common code use
 	 * DSI_VIDEO_COMPRESSION_MODE_XXXX for setting bits
+	 *
+	 * pkt_per_line is log2 encoded, >>1 works for supported values (1,2,4)
 	 */
+	if (pkt_per_line > 4)
+		drm_warn_once(msm_host->dev, "pkt_per_line too big");
 	reg |= DSI_VIDEO_COMPRESSION_MODE_CTRL_PKT_PER_LINE(pkt_per_line >> 1);
 	reg |= DSI_VIDEO_COMPRESSION_MODE_CTRL_EOL_BYTE_NUM(eol_byte_num);
 	reg |= DSI_VIDEO_COMPRESSION_MODE_CTRL_EN;