linux-kernel - [PATCH v6 6/6] drm/msm/dsi: Document DSC related pclk

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Date:   Fri, 9 Jun 2023 15:57:18 -0700
From:   Jessica Zhang <quic_jesszhan@...cinc.com>
To:     Rob Clark <robdclark@...il.com>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Marijn Suijten <marijn.suijten@...ainline.org>
CC:     Konrad Dybcio <konrad.dybcio@...aro.org>,
        <linux-arm-msm@...r.kernel.org>, <dri-devel@...ts.freedesktop.org>,
        <freedreno@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>,
        Jessica Zhang <quic_jesszhan@...cinc.com>
Subject: [PATCH v6 6/6] drm/msm/dsi: Document DSC related pclk_rate and
 hdisplay calculations

Add documentation comments explaining the pclk_rate and hdisplay math
related to DSC.

Signed-off-by: Jessica Zhang <quic_jesszhan@...cinc.com>
---
 drivers/gpu/drm/msm/dsi/dsi_host.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/gpu/drm/msm/dsi/dsi_host.c b/drivers/gpu/drm/msm/dsi/dsi_host.c
index fb1d3a25765f..aeaadc18bc7b 100644
--- a/drivers/gpu/drm/msm/dsi/dsi_host.c
+++ b/drivers/gpu/drm/msm/dsi/dsi_host.c
@@ -564,6 +564,13 @@ void dsi_link_clk_disable_v2(struct msm_dsi_host *msm_host)
 static unsigned long dsi_adjust_pclk_for_compression(const struct drm_display_mode *mode,
 		const struct drm_dsc_config *dsc)
 {
+	/*
+	 * Adjust the pclk rate by calculating a new hdisplay proportional to
+	 * the compression ratio such that:
+	 *     new_hdisplay = old_hdisplay * target_bpp / source_bpp
+	 *
+	 * Porches need not be adjusted during compression.
+	 */
 	int new_hdisplay = DIV_ROUND_UP(mode->hdisplay * drm_dsc_get_bpp_int(dsc),
 			dsc->bits_per_component * 3);
 
@@ -961,6 +968,9 @@ static void dsi_timing_setup(struct msm_dsi_host *msm_host, bool is_bonded_dsi)
 
 		/* Divide the display by 3 but keep back/font porch and
 		 * pulse width same
+		 *
+		 * hdisplay will be divided by 3 here to account for the fact
+		 * that DPU sends 3 bytes per pclk cycle to DSI.
 		 */
 		h_total -= hdisplay;
 		hdisplay = DIV_ROUND_UP(msm_dsc_get_bytes_per_line(msm_host->dsc), 3);

-- 
2.40.1