drm/i915: Make data/link N value power of two

The BIOS uses power of two values for the data/link N value.

Follow suit to make the Zotac DP to dual-HDMI dongle work.

v2: Clean up the magic numbers and defines
    Change the N clamping to be a bit easier on the eye
    Rename intel_reduce_ratio to intel_reduce_m_n_ratio

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=49402Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=59810Tested-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_reg.h

@@ -2652,14 +2652,14 @@
 #define _PIPEB_GMCH_DATA_M			0x71050
 
 /* Transfer unit size for display port - 1, default is 0x3f (for TU size 64) */
-#define   PIPE_GMCH_DATA_M_TU_SIZE_MASK		(0x3f << 25)
-#define   PIPE_GMCH_DATA_M_TU_SIZE_SHIFT	25
+#define  TU_SIZE(x)             (((x)-1) << 25) /* default size 64 */
+#define  TU_SIZE_MASK           (0x3f << 25)
 
-#define   PIPE_GMCH_DATA_M_MASK			(0xffffff)
+#define  DATA_LINK_M_N_MASK	(0xffffff)
+#define  DATA_LINK_N_MAX	(0x800000)
 
 #define _PIPEA_GMCH_DATA_N			0x70054
 #define _PIPEB_GMCH_DATA_N			0x71054
-#define   PIPE_GMCH_DATA_N_MASK			(0xffffff)
 
 /*
  * Computing Link M and N values for the Display Port link
@@ -2674,11 +2674,9 @@
 
 #define _PIPEA_DP_LINK_M				0x70060
 #define _PIPEB_DP_LINK_M				0x71060
-#define   PIPEA_DP_LINK_M_MASK			(0xffffff)
 
 #define _PIPEA_DP_LINK_N				0x70064
 #define _PIPEB_DP_LINK_N				0x71064
-#define   PIPEA_DP_LINK_N_MASK			(0xffffff)
 
 #define PIPE_GMCH_DATA_M(pipe) _PIPE(pipe, _PIPEA_GMCH_DATA_M, _PIPEB_GMCH_DATA_M)
 #define PIPE_GMCH_DATA_N(pipe) _PIPE(pipe, _PIPEA_GMCH_DATA_N, _PIPEB_GMCH_DATA_N)
@@ -3404,8 +3402,6 @@
 
 
 #define _PIPEA_DATA_M1           (dev_priv->info->display_mmio_offset + 0x60030)
-#define  TU_SIZE(x)             (((x)-1) << 25) /* default size 64 */
-#define  TU_SIZE_MASK           0x7e000000
 #define  PIPE_DATA_M1_OFFSET    0
 #define _PIPEA_DATA_N1           (dev_priv->info->display_mmio_offset + 0x60034)
 #define  PIPE_DATA_N1_OFFSET    0

drivers/gpu/drm/i915/intel_display.c

@@ -4084,26 +4084,36 @@ static int i830_get_display_clock_speed(struct drm_device *dev)
 }
 
 static void
-intel_reduce_ratio(uint32_t *num, uint32_t *den)
+intel_reduce_m_n_ratio(uint32_t *num, uint32_t *den)
 {
-	while (*num > 0xffffff || *den > 0xffffff) {
+	while (*num > DATA_LINK_M_N_MASK ||
+	       *den > DATA_LINK_M_N_MASK) {
 		*num >>= 1;
 		*den >>= 1;
 	}
 }
 
+static void compute_m_n(unsigned int m, unsigned int n,
+			uint32_t *ret_m, uint32_t *ret_n)
+{
+	*ret_n = min_t(unsigned int, roundup_pow_of_two(n), DATA_LINK_N_MAX);
+	*ret_m = div_u64((uint64_t) m * *ret_n, n);
+	intel_reduce_m_n_ratio(ret_m, ret_n);
+}
+
 void
 intel_link_compute_m_n(int bits_per_pixel, int nlanes,
 		       int pixel_clock, int link_clock,
 		       struct intel_link_m_n *m_n)
 {
 	m_n->tu = 64;
-	m_n->gmch_m = bits_per_pixel * pixel_clock;
-	m_n->gmch_n = link_clock * nlanes * 8;
-	intel_reduce_ratio(&m_n->gmch_m, &m_n->gmch_n);
-	m_n->link_m = pixel_clock;
-	m_n->link_n = link_clock;
-	intel_reduce_ratio(&m_n->link_m, &m_n->link_n);
+
+	compute_m_n(bits_per_pixel * pixel_clock,
+		    link_clock * nlanes * 8,
+		    &m_n->gmch_m, &m_n->gmch_n);
+
+	compute_m_n(pixel_clock, link_clock,
+		    &m_n->link_m, &m_n->link_n);
 }
 
 static inline bool intel_panel_use_ssc(struct drm_i915_private *dev_priv)