drm/dp/mst: Calculate MST PBN with 31.32 fixed point

commit a9ebb3e46c7ef6112c0da466ef0954673ad36832 upstream.

Our PBN value overflows the 20 bits integer part of the 20.12
fixed point. We need to use 31.32 fixed point to avoid this.

This happens with display clocks larger than 293122 (at 24 bpp),
which we see with the Sharp (and similar) 4k tiled displays.

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index b97f057d69b3166968cbdfb30c3556832f8aeda7..e77e96aeeb20badd816c14b3546f63d2df209bfe 100644 (file)
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -2571,32 +2571,31 @@ EXPORT_SYMBOL(drm_dp_check_act_status);
  */
 int drm_dp_calc_pbn_mode(int clock, int bpp)
 {
-       fixed20_12 pix_bw;
-       fixed20_12 fbpp;
-       fixed20_12 result;
-       fixed20_12 margin, tmp;
-       u32 res;
-
-       pix_bw.full = dfixed_const(clock);
-       fbpp.full = dfixed_const(bpp);
-       tmp.full = dfixed_const(8);
-       fbpp.full = dfixed_div(fbpp, tmp);
-
-       result.full = dfixed_mul(pix_bw, fbpp);
-       margin.full = dfixed_const(54);
-       tmp.full = dfixed_const(64);
-       margin.full = dfixed_div(margin, tmp);
-       result.full = dfixed_div(result, margin);
-
-       margin.full = dfixed_const(1006);
-       tmp.full = dfixed_const(1000);
-       margin.full = dfixed_div(margin, tmp);
-       result.full = dfixed_mul(result, margin);
-
-       result.full = dfixed_div(result, tmp);
-       result.full = dfixed_ceil(result);
-       res = dfixed_trunc(result);
-       return res;
+       u64 kbps;
+       s64 peak_kbps;
+       u32 numerator;
+       u32 denominator;
+
+       kbps = clock * bpp;
+
+       /*
+        * margin 5300ppm + 300ppm ~ 0.6% as per spec, factor is 1.006
+        * The unit of 54/64Mbytes/sec is an arbitrary unit chosen based on
+        * common multiplier to render an integer PBN for all link rate/lane
+        * counts combinations
+        * calculate
+        * peak_kbps *= (1006/1000)
+        * peak_kbps *= (64/54)
+        * peak_kbps *= 8    convert to bytes
+        */
+
+       numerator = 64 * 1006;
+       denominator = 54 * 8 * 1000 * 1000;
+
+       kbps *= numerator;
+       peak_kbps = drm_fixp_from_fraction(kbps, denominator);
+
+       return drm_fixp2int_ceil(peak_kbps);
 }
 EXPORT_SYMBOL(drm_dp_calc_pbn_mode);
 
@@ -2604,11 +2603,23 @@ static int test_calc_pbn_mode(void)
 {
        int ret;
        ret = drm_dp_calc_pbn_mode(154000, 30);
-       if (ret != 689)
+       if (ret != 689) {
+               DRM_ERROR("PBN calculation test failed - clock %d, bpp %d, expected PBN %d, actual PBN %d.\n",
+                               154000, 30, 689, ret);
                return -EINVAL;
+       }
        ret = drm_dp_calc_pbn_mode(234000, 30);
-       if (ret != 1047)
+       if (ret != 1047) {
+               DRM_ERROR("PBN calculation test failed - clock %d, bpp %d, expected PBN %d, actual PBN %d.\n",
+                               234000, 30, 1047, ret);
                return -EINVAL;
+       }
+       ret = drm_dp_calc_pbn_mode(297000, 24);
+       if (ret != 1063) {
+               DRM_ERROR("PBN calculation test failed - clock %d, bpp %d, expected PBN %d, actual PBN %d.\n",
+                               297000, 24, 1063, ret);
+               return -EINVAL;
+       }
        return 0;
 }