[PATCH v2 3/4] drm/bridge: nwl-dsi: Correct MIPI DSI horizontal sync timing

The NWL MIPI Host controller registers specifies the horizontal front
porch, sync pulse, and back porch in DSI packet payload size in bytes (ie.
not in pixel clocks).

The calculation for this is (mostly) described in section 13.6.3.5.1.2 of
the i.MX 8M Dual/8M QuadLite/8M Quad Applications Processors Reference
Manual (rev. 3.1). The formula shown there does not take packet header size
into account though.

The formula implemented here converts the hfp, hsa, and hbp to DSI packet
bytes and then subtracts the number of packet header bytes.

It is worth noting that these values only needs to match approximately,
according to the reference manual.

Based on commit in NXP vendor tree
commit f2a61699749d ("LF-7981-4: drm/bridge: nwl-dsi: Correct MIPI DSI horizontal sync timing")

Signed-off-by: Esben Haabendal <[email protected]>
Cc: Robert Chiras <[email protected]>
Cc: Oliver F. Brown <[email protected]>
---
 drivers/gpu/drm/bridge/nwl-dsi.c | 64 ++++++++++++++++++++++++++++++++++++++--
 1 file changed, 61 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/bridge/nwl-dsi.c b/drivers/gpu/drm/bridge/nwl-dsi.c
index c9af6a3d47e1..226036d22e3f 100644
--- a/drivers/gpu/drm/bridge/nwl-dsi.c
+++ b/drivers/gpu/drm/bridge/nwl-dsi.c
@@ -22,6 +22,7 @@
 #include <linux/reset.h>
 #include <linux/sys_soc.h>
 #include <linux/time64.h>
+#include <linux/math64.h>
 
 #include <drm/drm_atomic_state_helper.h>
 #include <drm/drm_bridge.h>
@@ -264,6 +265,10 @@ static int nwl_dsi_config_dpi(struct nwl_dsi *dsi)
 	bool burst_mode;
 	int hfront_porch, hback_porch, vfront_porch, vback_porch;
 	int hsync_len, vsync_len;
+	int hfp, hbp, hsa;
+	unsigned long long pclk_period;
+	unsigned long long hs_period;
+	int h_blank, pkt_hdr_len, pkt_len;
 
 	hfront_porch = dsi->mode.hsync_start - dsi->mode.hdisplay;
 	hsync_len = dsi->mode.hsync_end - dsi->mode.hsync_start;
@@ -317,9 +322,62 @@ static int nwl_dsi_config_dpi(struct nwl_dsi *dsi)
 			      dsi->mode.hdisplay);
 	}
 
-	nwl_dsi_write(dsi, NWL_DSI_HFP, hfront_porch);
-	nwl_dsi_write(dsi, NWL_DSI_HBP, hback_porch);
-	nwl_dsi_write(dsi, NWL_DSI_HSA, hsync_len);
+	pclk_period = DIV_ROUND_UP_ULL(PSEC_PER_SEC, dsi->mode.clock * 1000);
+	DRM_DEV_DEBUG_DRIVER(dsi->dev, "pclk_period: %llu\n", pclk_period);
+
+	hs_period = DIV_ROUND_UP_ULL(PSEC_PER_SEC, dsi->phy_cfg.mipi_dphy.hs_clk_rate);
+	DRM_DEV_DEBUG_DRIVER(dsi->dev, "hs_period: %llu\n", hs_period);
+
+	/*
+	 * Calculate the bytes needed, according to the RM formula:
+	 * Time of DPI event = time to transmit x number of bytes on the DSI
+	 * interface
+	 * dpi_event_size * dpi_pclk_period = dsi_bytes * 8 * hs_bit_period /
+	 * num_lanes
+	 * ===>
+	 * dsi_bytes = dpi_event_size * dpi_pclk_period * num_lanes /
+	 * (8 * hs_bit_period)
+	 */
+	hfp = hfront_porch * pclk_period * dsi->lanes / (8 * hs_period);
+	hbp = hback_porch * pclk_period * dsi->lanes / (8 * hs_period);
+	hsa = hsync_len * pclk_period * dsi->lanes / (8 * hs_period);
+
+	/* Make sure horizontal blankins are even numbers */
+	hfp = roundup(hfp, 2);
+	hbp = roundup(hbp, 2);
+	hsa = roundup(hsa, 2);
+
+	/*
+	 * We need to subtract the packet header length: 32
+	 * In order to make sure we don't get negative values,
+	 * subtract a proportional value to the total length of the
+	 * horizontal blanking duration.
+	 */
+	h_blank = hfp + hbp + hsa;
+
+	if (h_blank) {
+		pkt_len = min(hfp, roundup(((hfp * 100 / h_blank) * 32) / 100, 2));
+		pkt_hdr_len = pkt_len;
+		hfp -= pkt_len;
+
+		pkt_len = min(hbp, roundup(((hbp * 100 / h_blank) * 32) / 100, 2));
+		pkt_hdr_len += pkt_len;
+		hbp -= pkt_len;
+
+		pkt_len = min(hsa, 32 - pkt_hdr_len);
+		pkt_hdr_len += pkt_len;
+		hsa -= pkt_len;
+		if (pkt_hdr_len != 32)
+			dev_warn(dsi->dev,
+				 "Unable to subtract packet header length fully: %d left\n",
+				 32 - pkt_hdr_len);
+	}
+
+	DRM_DEV_DEBUG_DRIVER(dsi->dev, "Register values: hfp=%d hbp=%d hsa=%d\n", hfp, hbp, hsa);
+
+	nwl_dsi_write(dsi, NWL_DSI_HFP, hfp);
+	nwl_dsi_write(dsi, NWL_DSI_HBP, hbp);
+	nwl_dsi_write(dsi, NWL_DSI_HSA, hsa);
 
 	nwl_dsi_write(dsi, NWL_DSI_ENABLE_MULT_PKTS, 0x0);
 	nwl_dsi_write(dsi, NWL_DSI_BLLP_MODE, 0x1);

-- 
2.55.0