drm/stm: dsi: provide the implementation of mode_valid()

The dsi has several constraints on the video modes it can support,
mainly due to the frequencies that can be generated by the PLL
integrated in the DSI device.

Verify that the required HS clock can be generated by the PLL.

The dsi clock from the dsi PLL and the ltdc pixel clock are
asynchronous. The dsi needs to return in LP mode during HFP or HBP
to re-synchronize at each video line.

Verify that the duration of HFP and HBP allows the dsi to enter in
LP mode.
Signed-off-by: Antonio Borneo <antonio.borneo@foss.st.com>
Acked-by: Philippe Cornu <philippe.cornu@foss.st.com>
Reviewed-by: Philippe Cornu <philippe.cornu@foss.st.com>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211218215055.212421-3-antonio.borneo@foss.st.com

drivers/gpu/drm/stm/dw_mipi_dsi-stm.c

@@ -322,6 +322,103 @@ dw_mipi_dsi_phy_get_timing(void *priv_data, unsigned int lane_mbps,
 	return 0;
 }
 
+#define CLK_TOLERANCE_HZ 50
+
+static enum drm_mode_status
+dw_mipi_dsi_stm_mode_valid(void *priv_data,
+			   const struct drm_display_mode *mode,
+			   unsigned long mode_flags, u32 lanes, u32 format)
+{
+	struct dw_mipi_dsi_stm *dsi = priv_data;
+	unsigned int idf, ndiv, odf, pll_in_khz, pll_out_khz;
+	int ret, bpp;
+
+	bpp = mipi_dsi_pixel_format_to_bpp(format);
+	if (bpp < 0)
+		return MODE_BAD;
+
+	/* Compute requested pll out */
+	pll_out_khz = mode->clock * bpp / lanes;
+
+	if (pll_out_khz > dsi->lane_max_kbps)
+		return MODE_CLOCK_HIGH;
+
+	if (mode_flags & MIPI_DSI_MODE_VIDEO_BURST) {
+		/* Add 20% to pll out to be higher than pixel bw */
+		pll_out_khz = (pll_out_khz * 12) / 10;
+	} else {
+		if (pll_out_khz < dsi->lane_min_kbps)
+			return MODE_CLOCK_LOW;
+	}
+
+	/* Compute best pll parameters */
+	idf = 0;
+	ndiv = 0;
+	odf = 0;
+	pll_in_khz = clk_get_rate(dsi->pllref_clk) / 1000;
+	ret = dsi_pll_get_params(dsi, pll_in_khz, pll_out_khz, &idf, &ndiv, &odf);
+	if (ret) {
+		DRM_WARN("Warning dsi_pll_get_params(): bad params\n");
+		return MODE_ERROR;
+	}
+
+	if (!(mode_flags & MIPI_DSI_MODE_VIDEO_BURST)) {
+		unsigned int px_clock_hz, target_px_clock_hz, lane_mbps;
+		int dsi_short_packet_size_px, hfp, hsync, hbp, delay_to_lp;
+		struct dw_mipi_dsi_dphy_timing dphy_timing;
+
+		/* Get the adjusted pll out value */
+		pll_out_khz = dsi_pll_get_clkout_khz(pll_in_khz, idf, ndiv, odf);
+
+		px_clock_hz = DIV_ROUND_CLOSEST_ULL(1000ULL * pll_out_khz * lanes, bpp);
+		target_px_clock_hz = mode->clock * 1000;
+		/*
+		 * Filter modes according to the clock value, particularly useful for
+		 * hdmi modes that require precise pixel clocks.
+		 */
+		if (px_clock_hz < target_px_clock_hz - CLK_TOLERANCE_HZ ||
+		    px_clock_hz > target_px_clock_hz + CLK_TOLERANCE_HZ)
+			return MODE_CLOCK_RANGE;
+
+		/* sync packets are codes as DSI short packets (4 bytes) */
+		dsi_short_packet_size_px = DIV_ROUND_UP(4 * BITS_PER_BYTE, bpp);
+
+		hfp = mode->hsync_start - mode->hdisplay;
+		hsync = mode->hsync_end - mode->hsync_start;
+		hbp = mode->htotal - mode->hsync_end;
+
+		/* hsync must be longer than 4 bytes HSS packets */
+		if (hsync < dsi_short_packet_size_px)
+			return MODE_HSYNC_NARROW;
+
+		if (mode_flags & MIPI_DSI_MODE_VIDEO_SYNC_PULSE) {
+			/* HBP must be longer than 4 bytes HSE packets */
+			if (hbp < dsi_short_packet_size_px)
+				return MODE_HSYNC_NARROW;
+			hbp -= dsi_short_packet_size_px;
+		} else {
+			/* With sync events HBP extends in the hsync */
+			hbp += hsync - dsi_short_packet_size_px;
+		}
+
+		lane_mbps = pll_out_khz / 1000;
+		ret = dw_mipi_dsi_phy_get_timing(priv_data, lane_mbps, &dphy_timing);
+		if (ret)
+			return MODE_ERROR;
+		/*
+		 * In non-burst mode DSI has to enter in LP during HFP
+		 * (horizontal front porch) or HBP (horizontal back porch) to
+		 * resync with LTDC pixel clock.
+		 */
+		delay_to_lp = DIV_ROUND_UP((dphy_timing.data_hs2lp + dphy_timing.data_lp2hs) *
+					   lanes * BITS_PER_BYTE, bpp);
+		if (hfp < delay_to_lp && hbp < delay_to_lp)
+			return MODE_HSYNC;
+	}
+
+	return MODE_OK;
+}
+
 static const struct dw_mipi_dsi_phy_ops dw_mipi_dsi_stm_phy_ops = {
 	.init = dw_mipi_dsi_phy_init,
 	.power_on = dw_mipi_dsi_phy_power_on,
@@ -332,6 +429,7 @@ static const struct dw_mipi_dsi_phy_ops dw_mipi_dsi_stm_phy_ops = {
 
 static struct dw_mipi_dsi_plat_data dw_mipi_dsi_stm_plat_data = {
 	.max_data_lanes = 2,
+	.mode_valid = dw_mipi_dsi_stm_mode_valid,
 	.phy_ops = &dw_mipi_dsi_stm_phy_ops,
 };