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

1 files changed, 18 insertions, 8 deletions

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 6e423e04c35e..efe829919755 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/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)