TC358762 can generate the DPI output's timings in two ways, either Video
Timings Generator (VTG) on or off:
- VTG off: Duplicate the timings coming from the DSI. This requires DSI
  pulse mode.
- VTG on: Sync frame on DSI VSync Start, but the exact output timings
  are defined in TC358762 registers. This can be used with DSI
  event/burst mode.

We are currently using VTG off in the driver.

I observe that the hsync signal, on my HW setup, is not 100% stable with
VTG off, and it seems to lengthen by a single clock every now and then.
However, it then stabilizes later. To me the DSI input looks solid, but
that is more challenging to measure exactly. So I have not found the
root cause for this.

Turning VTG on removes that instability. As I dont' see any downsides
with enabling VTG (and it would allow extending the driver to use
event/burst mode in the future), let's always enable the VTG.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
---
 drivers/gpu/drm/bridge/tc358762.c | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/drivers/gpu/drm/bridge/tc358762.c b/drivers/gpu/drm/bridge/tc358762.c
index 9fb921b3fa0d..d3bcdf8af804 100644
--- a/drivers/gpu/drm/bridge/tc358762.c
+++ b/drivers/gpu/drm/bridge/tc358762.c
@@ -18,6 +18,7 @@
 #include <linux/regulator/consumer.h>
 
 #include <video/mipi_display.h>
+#include <video/videomode.h>
 
 #include <drm/drm_atomic_helper.h>
 #include <drm/drm_bridge.h>
@@ -63,6 +64,12 @@
 #define LCDCTRL_VSYNC_POL	BIT(19) /* Polarity of VSYNC signal */
 #define LCDCTRL_DCLK_POL	BIT(20) /* Polarity of pixel clock */
 
+#define LCDC_HSR_HBPR		0x0424
+#define LCDC_HDISPR_HFPR	0x0428
+#define LCDC_VSR_VBPR		0x042C
+#define LCDC_VDISPR_VFPR	0x0430
+#define LCDC_VFUEN		0x0434
+
 /* SPI Master Registers */
 #define SPICMR			0x0450
 #define SPITCR			0x0454
@@ -94,6 +101,7 @@ struct tc358762 {
 	struct drm_display_mode mode;
 	bool pre_enabled;
 	int error;
+	bool use_vtg;
 };
 
 static int tc358762_clear_error(struct tc358762 *ctx)
@@ -155,9 +163,31 @@ static int tc358762_init(struct tc358762 *ctx)
 	tc358762_write(ctx, PPI_D1S_ATMR, 0);
 	tc358762_write(ctx, PPI_LPTXTIMECNT, LPX_PERIOD);
 
+	if (ctx->use_vtg) {
+		struct videomode vm = { 0 };
+
+		drm_display_mode_to_videomode(&ctx->mode, &vm);
+
+		tc358762_write(ctx, LCDC_HSR_HBPR,
+			       vm.hsync_len | (vm.hback_porch << 16));
+		tc358762_write(ctx, LCDC_HDISPR_HFPR,
+			       vm.hactive | (vm.hfront_porch << 16));
+
+		tc358762_write(ctx, LCDC_VSR_VBPR,
+			       vm.vsync_len | (vm.vback_porch << 16));
+		tc358762_write(ctx, LCDC_VDISPR_VFPR,
+			       vm.vactive | (vm.vfront_porch << 16));
+
+		/* Upload VTG timings */
+		tc358762_write(ctx, LCDC_VFUEN, BIT(0));
+	}
+
 	lcdctrl = FIELD_PREP(LCDCTRL_PXLFORM, LCDCTRL_PXLFORM_RGB888) |
 		  LCDCTRL_DPI_EN;
 
+	if (ctx->use_vtg)
+		lcdctrl |= LCDCTRL_VTGEN;
+
 	lcdctrl |= LCDCTRL_DCLK_POL;
 
 	if (ctx->mode.flags & DRM_MODE_FLAG_NHSYNC)
@@ -305,6 +335,9 @@ static int tc358762_probe(struct mipi_dsi_device *dsi)
 	ctx->dev = dev;
 	ctx->pre_enabled = false;
 
+	/* Always use VTG */
+	ctx->use_vtg = true;
+
 	/*
 	 * When using DSI clk for pixel clock (only mode supported in the driver),
 	 * the pclk is derived directly from the DSI byteclk via simple divider,

-- 
2.43.0