drm: Make the bw/link rate calculations more forgiving

Although the DisplayPort spec explicitly calls out the 1.62/2.7/5.4/8.1
link rates, the value of LINK_BW_SET is calculated.  The DisplayPort
spec says "Main-Link Bandwidth Setting = Value x 0.27Gbps/lane".

A bridge that we're looking to upstream uses 6.75Gbps rate (value 0x19)
[1], and that precludes it from using these functions.

This 6.75Gbps rate is defined in the spec as (credit to Ville for posting this):
  A MyDP Source device, upon reading the MAX_LINK_RATE register of the
  downstream DPRX programmed to 19h (which can be the case only for a
  MyDP-to-Legacy or MyDP-to-DP lane count converter) can program the
  LINK_BW_SET register (DPCD Address 00100h) to 19h to enable 6.75Gbps/lane."

So to avoid failing on legitimate rates in the future, this patch calculates thevalues according to spec instead of restricting these values to one of the
DP_LINK_BW_* #defines.

No functional change for the well-defined values, but we lose the
warning (and return the correct value) for ill-defined bw values.
Signed-off-by: Sean Paul <seanpaul@chromium.org>

[1] https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/1689251/2/drivers/gpu/drm/bridge/analogix/anx7625.c#636Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190717160148.256826-1-sean@poorly.run

drivers/gpu/drm/drm_dp_helper.c

@@ -152,38 +152,15 @@ EXPORT_SYMBOL(drm_dp_link_train_channel_eq_delay);
 
 u8 drm_dp_link_rate_to_bw_code(int link_rate)
 {
-	switch (link_rate) {
-	default:
-		WARN(1, "unknown DP link rate %d, using %x\n", link_rate,
-		     DP_LINK_BW_1_62);
-		/* fall through */
-	case 162000:
-		return DP_LINK_BW_1_62;
-	case 270000:
-		return DP_LINK_BW_2_7;
-	case 540000:
-		return DP_LINK_BW_5_4;
-	case 810000:
-		return DP_LINK_BW_8_1;
-	}
+	/* Spec says link_bw = link_rate / 0.27Gbps */
+	return link_rate / 27000;
 }
 EXPORT_SYMBOL(drm_dp_link_rate_to_bw_code);
 
 int drm_dp_bw_code_to_link_rate(u8 link_bw)
 {
-	switch (link_bw) {
-	default:
-		WARN(1, "unknown DP link BW code %x, using 162000\n", link_bw);
-		/* fall through */
-	case DP_LINK_BW_1_62:
-		return 162000;
-	case DP_LINK_BW_2_7:
-		return 270000;
-	case DP_LINK_BW_5_4:
-		return 540000;
-	case DP_LINK_BW_8_1:
-		return 810000;
-	}
+	/* Spec says link_rate = link_bw * 0.27Gbps */
+	return link_bw * 27000;
 }
 EXPORT_SYMBOL(drm_dp_bw_code_to_link_rate);