[PATCH v2] drm/msm/dsi: Document DSC related pclk_rate and hdisplay calculations

[Dmitry Baryshkov]

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

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov at linaro.org>
---

Changes since v1:
- Converted dsi_adjust_pclk_for_compression() into kerneldoc (Marijn)
- Added a pointer from dsi_timing_setup() docs to
  dsi_adjust_pclk_for_compression() (Marijn)
- Fixed two typo (Marijn)

---
 drivers/gpu/drm/msm/dsi/dsi_host.c | 40 ++++++++++++++++++++++++++++--
 1 file changed, 38 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/msm/dsi/dsi_host.c b/drivers/gpu/drm/msm/dsi/dsi_host.c
index 3f6dfb4f9d5a..a8a31c3dd168 100644
--- a/drivers/gpu/drm/msm/dsi/dsi_host.c
+++ b/drivers/gpu/drm/msm/dsi/dsi_host.c
@@ -528,6 +528,25 @@ 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 the VIDEO mode they are not compressed by DSC and are passed as is.
+ * - For the 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
+ *  refresh rate 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)
 {
@@ -926,8 +945,25 @@ 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 compression is
+		 * not used, a single pixel is transferred at each pulse, no
+		 * matter what bpp or pixel format is used. In case of DSC
+		 * compression this results (due to data alignment
+		 * requirements) in a transfer of 3 compressed pixel per pclk
+		 * cycle.
+		 *
+		 * If widebus is enabled, bus width is extended to 6 bytes.
+		 * This way the DPU can transfer 6 compressed pixels with bpp
+		 * less or equal to 8 or 3 compressed pixels in case bpp is
+		 * greater than 8.
+		 *
+		 * The back/font porch and pulse width are kept intact.  They
+		 * represent timing parameters rather than actual data
+		 * transfer. See the documentation of
+		 * dsi_adjust_pclk_for_compression().
+		 *
+		 * XXX: widebus is not supported by the driver (yet).
 		 */
 		h_total -= hdisplay;
 		hdisplay = DIV_ROUND_UP(msm_dsc_get_bytes_per_line(msm_host->dsc), 3);
-- 
2.39.2