drm/bridge: tc358768: fix PLL target frequency

Correctly compute the PLL target frequency, the current formula works
correctly only when the input bus width is 24bit, actually to properly
compute the PLL target frequency what is relevant is the bits-per-pixel
on the DSI link.

No regression expected since the DSI format is currently hard-coded as
MIPI_DSI_FMT_RGB888.

Fixes: ff1ca639 ("drm/bridge: Add tc358768 driver")
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Reviewed-by: Robert Foss <rfoss@kernel.org>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230427142934.55435-4-francesco@dolcini.it

drivers/gpu/drm/bridge/tc358768.c

@@ -147,6 +147,7 @@ struct tc358768_priv {
 
 	u32 pd_lines; /* number of Parallel Port Input Data Lines */
 	u32 dsi_lanes; /* number of DSI Lanes */
+	u32 dsi_bpp; /* number of Bits Per Pixel over DSI */
 
 	/* Parameters for PLL programming */
 	u32 fbd;	/* PLL feedback divider */
@@ -285,12 +286,12 @@ static void tc358768_hw_disable(struct tc358768_priv *priv)
 
 static u32 tc358768_pll_to_pclk(struct tc358768_priv *priv, u32 pll_clk)
 {
-	return (u32)div_u64((u64)pll_clk * priv->dsi_lanes, priv->pd_lines);
+	return (u32)div_u64((u64)pll_clk * priv->dsi_lanes, priv->dsi_bpp);
 }
 
 static u32 tc358768_pclk_to_pll(struct tc358768_priv *priv, u32 pclk)
 {
-	return (u32)div_u64((u64)pclk * priv->pd_lines, priv->dsi_lanes);
+	return (u32)div_u64((u64)pclk * priv->dsi_bpp, priv->dsi_lanes);
 }
 
 static int tc358768_calc_pll(struct tc358768_priv *priv,
@@ -427,6 +428,7 @@ static int tc358768_dsi_host_attach(struct mipi_dsi_host *host,
 	priv->output.panel = panel;
 
 	priv->dsi_lanes = dev->lanes;
+	priv->dsi_bpp = mipi_dsi_pixel_format_to_bpp(dev->format);
 
 	/* get input ep (port0/endpoint0) */
 	ret = -EINVAL;
@@ -438,7 +440,7 @@ static int tc358768_dsi_host_attach(struct mipi_dsi_host *host,
 	}
 
 	if (ret)
-		priv->pd_lines = mipi_dsi_pixel_format_to_bpp(dev->format);
+		priv->pd_lines = priv->dsi_bpp;
 
 	drm_bridge_add(&priv->bridge);