drm/msm/dsi: Document DSC related pclk_rate and hdisplay calculations

Provide actual documentation for the pclk and hdisplay calculations in
the case of DSC compression being used.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/577534/
Link: https://lore.kernel.org/r/20240208-fd_document_dsc_pclk_rate-v4-1-56fe59d0a2e0@linaro.org

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

@@ -519,6 +519,25 @@ void dsi_link_clk_disable_v2(struct msm_dsi_host *msm_host)
 	clk_disable_unprepare(msm_host->byte_clk);
 }
 
+/**
+ * dsi_adjust_pclk_for_compression() - Adjust the pclk rate for compression case
+ * @mode: The selected mode for the DSI output
+ * @dsc: DRM DSC configuration for this DSI output
+ *
+ * Adjust the pclk rate by calculating a new hdisplay proportional to
+ * the compression ratio such that:
+ *     new_hdisplay = old_hdisplay * compressed_bpp / uncompressed_bpp
+ *
+ * Porches do not need to be adjusted:
+ * - For VIDEO mode they are not compressed by DSC and are passed as is.
+ * - For CMD mode there are no actual porches. Instead these fields
+ *   currently represent the overhead to the image data transfer. As such, they
+ *   are calculated for the final mode parameters (after the compression) and
+ *   are not to be adjusted too.
+ *
+ *  FIXME: Reconsider this if/when CMD mode handling is rewritten to use
+ *  transfer time and data overhead as a starting point of the calculations.
+ */
 static unsigned long dsi_adjust_pclk_for_compression(const struct drm_display_mode *mode,
 		const struct drm_dsc_config *dsc)
 {
@@ -941,8 +960,18 @@ static void dsi_timing_setup(struct msm_dsi_host *msm_host, bool is_bonded_dsi)
 		if (ret)
 			return;
 
-		/* Divide the display by 3 but keep back/font porch and
-		 * pulse width same
+		/*
+		 * DPU sends 3 bytes per pclk cycle to DSI. If widebus is
+		 * enabled, bus width is extended to 6 bytes.
+		 *
+		 * Calculate the number of pclks needed to transmit one line of
+		 * the compressed data.
+
+		 * The back/font porch and pulse width are kept intact. For
+		 * VIDEO mode they represent timing parameters rather than
+		 * actual data transfer, see the documentation for
+		 * dsi_adjust_pclk_for_compression(). For CMD mode they are
+		 * unused anyway.
 		 */
 		h_total -= hdisplay;
 		if (wide_bus_enabled && !(msm_host->mode_flags & MIPI_DSI_MODE_VIDEO))