drm/i915/dsi: add support for DSI PLL N1 divisor values

Currently DSI PLL N1 is hardcoded off. Make it possible to use it
later. This should have no functional changes for now.

v2: s/ffz(~(n))/ffs(n) - 1/ (Ville)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_dsi_pll.c

@@ -165,7 +165,7 @@ static u32 dsi_clk_from_pclk(u32 pclk, int pixel_format, int lane_count)
 static int dsi_calc_mnp(int target_dsi_clk, struct dsi_mnp *dsi_mnp)
 {
 	unsigned int calc_m = 0, calc_p = 0;
-	unsigned int m, n, p;
+	unsigned int m, n = 1, p;
 	int ref_clk = 25000;
 	int delta = target_dsi_clk;
 	u32 m_seed;
@@ -182,7 +182,7 @@ static int dsi_calc_mnp(int target_dsi_clk, struct dsi_mnp *dsi_mnp)
 			 * Find the optimal m and p divisors with minimal delta
 			 * +/- the required clock
 			 */
-			int calc_dsi_clk = (m * ref_clk) / p;
+			int calc_dsi_clk = (m * ref_clk) / (p * n);
 			int d = abs(target_dsi_clk - calc_dsi_clk);
 			if (d < delta) {
 				delta = d;
@@ -192,10 +192,11 @@ static int dsi_calc_mnp(int target_dsi_clk, struct dsi_mnp *dsi_mnp)
 		}
 	}
 
+	/* register has log2(N1), this works fine for powers of two */
+	n = ffs(n) - 1;
 	m_seed = lfsr_converts[calc_m - 62];
-	n = 1;
 	dsi_mnp->dsi_pll_ctrl = 1 << (DSI_PLL_P1_POST_DIV_SHIFT + calc_p - 2);
-	dsi_mnp->dsi_pll_div = (n - 1) << DSI_PLL_N1_DIV_SHIFT |
+	dsi_mnp->dsi_pll_div = n << DSI_PLL_N1_DIV_SHIFT |
 		m_seed << DSI_PLL_M1_DIV_SHIFT;
 
 	return 0;
@@ -312,7 +313,7 @@ u32 vlv_get_dsi_pclk(struct intel_encoder *encoder, int pipe_bpp)
 	struct intel_dsi *intel_dsi = enc_to_intel_dsi(&encoder->base);
 	u32 dsi_clock, pclk;
 	u32 pll_ctl, pll_div;
-	u32 m = 0, p = 0;
+	u32 m = 0, p = 0, n;
 	int refclk = 25000;
 	int i;
 
@@ -327,6 +328,10 @@ u32 vlv_get_dsi_pclk(struct intel_encoder *encoder, int pipe_bpp)
 	pll_ctl &= DSI_PLL_P1_POST_DIV_MASK;
 	pll_ctl = pll_ctl >> (DSI_PLL_P1_POST_DIV_SHIFT - 2);
 
+	/* N1 divisor */
+	n = (pll_div & DSI_PLL_N1_DIV_MASK) >> DSI_PLL_N1_DIV_SHIFT;
+	n = 1 << n; /* register has log2(N1) */
+
 	/* mask out the other bits and extract the M1 divisor */
 	pll_div &= DSI_PLL_M1_DIV_MASK;
 	pll_div = pll_div >> DSI_PLL_M1_DIV_SHIFT;
@@ -354,7 +359,7 @@ u32 vlv_get_dsi_pclk(struct intel_encoder *encoder, int pipe_bpp)
 
 	m = i + 62;
 
-	dsi_clock = (m * refclk) / p;
+	dsi_clock = (m * refclk) / (p * n);
 
 	/* pixel_format and pipe_bpp should agree */
 	assert_bpp_mismatch(intel_dsi->pixel_format, pipe_bpp);