[PATCH 3/3] drm/i915: add i915.dp_link_train_policy option

[Paulo Zanoni <przanoni@gmail.com>]

From: Paulo Zanoni <paulo.r.zanoni@intel.com>

We still have way too many bugs with DP link training. We keep
switching between "narrow and fast" and "wide and slow", we recently
added 5GHz support, and whenever there's a bug report, we have to ask
people to apply patches and test them.

Wouldn't it be so much better if we could just ask them to boot with
some specific Kernel boot option instead of applying a patch? This
will move the situation from "i915.ko is completely broken!" to
"i915.ko's default values are broken, but there's an option I can set
to fix it, so I won't need to learn how to compile a Kernel!".

Some useful values:
 - i915.dp_link_train_policy=1 for "wide and slow"
 - i915.dp_link_train_policy=0x120 for DP_LINK_BW_2_7 and 2 lanes,
   which should be able to fit 1920x1080@60Hz and 24bpp
 - i915.dp_link_train_policy=0x210 to force DP 5GHz testing on
   not-so-huge modes

The default behavior is still the same.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
---
 drivers/gpu/drm/i915/i915_drv.h    |  1 +
 drivers/gpu/drm/i915/i915_params.c |  8 ++++++++
 drivers/gpu/drm/i915/intel_dp.c    | 39 ++++++++++++++++++++++++++++++++++----
 3 files changed, 44 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 7d6acb4..d5ae8dc 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1941,6 +1941,7 @@ struct i915_params {
 	bool reset;
 	bool disable_display;
 	bool disable_vtd_wa;
+	int dp_link_train_policy;
 };
 extern struct i915_params i915 __read_mostly;
 
diff --git a/drivers/gpu/drm/i915/i915_params.c b/drivers/gpu/drm/i915/i915_params.c
index d05a2af..8c358e7 100644
--- a/drivers/gpu/drm/i915/i915_params.c
+++ b/drivers/gpu/drm/i915/i915_params.c
@@ -48,6 +48,7 @@ struct i915_params i915 __read_mostly = {
 	.disable_display = 0,
 	.enable_cmd_parser = 1,
 	.disable_vtd_wa = 0,
+	.dp_link_train_policy = 0,
 };
 
 module_param_named(modeset, i915.modeset, int, 0400);
@@ -156,3 +157,10 @@ MODULE_PARM_DESC(disable_vtd_wa, "Disable all VT-d workarounds (default: false)"
 module_param_named(enable_cmd_parser, i915.enable_cmd_parser, int, 0600);
 MODULE_PARM_DESC(enable_cmd_parser,
 		 "Enable command parsing (1=enabled [default], 0=disabled)");
+
+module_param_named(dp_link_train_policy, i915.dp_link_train_policy, int, 0600);
+MODULE_PARM_DESC(dp_link_train_policy,
+	"Choose strategy for DP link training "
+	"(0=narrow and fast [default], 1=wide and slow. For other values, "
+	"bits 11:8 are for the BW and bits 7:4 are for the nubmer of lanes, "
+	"check intel_dp_compute_link_config() for more details.)");
diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index 884166b..25f7e1c 100644
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -800,11 +800,42 @@ static bool intel_dp_compute_link_config(int mode_clock, int bpp,
 
 	mode_rate = intel_dp_link_required(mode_clock, bpp);
 
-	for (lanes = min_lanes; lanes <= max_lanes; lanes <<= 1)
+	switch (i915.dp_link_train_policy) {
+	case 0: /* Narrow and fast. */
+		for (lanes = min_lanes; lanes <= max_lanes; lanes <<= 1)
+			for (bw_it = min_bw_index; bw_it <= max_bw_index;
+			     bw_it++)
+				if (link_config_is_possible(mode_rate, lanes,
+							    bws[bw_it]))
+					goto found;
+	case 1: /* Wide and slow. */
 		for (bw_it = min_bw_index; bw_it <= max_bw_index; bw_it++)
-			if (link_config_is_possible(mode_rate, lanes,
-						    bws[bw_it]))
-				goto found;
+			for (lanes = min_lanes; lanes <= max_lanes; lanes <<= 1)
+				if (link_config_is_possible(mode_rate, lanes,
+							    bws[bw_it]))
+					goto found;
+	default: /* Bits 11:8 contain the index of the bws array.
+		  * Bits 7:4 contain the number of lanes.
+		  * Example: i915.dp_link_train_policy=0x140 uses
+		  * DP_LINK_BW_2_7 and 4 lanes. */
+		bw_it = (i915.dp_link_train_policy >> 8) & 0xF;
+		lanes = (i915.dp_link_train_policy >> 4) & 0xF;
+
+		if (bw_it < min_bw_index || bw_it > max_bw_index) {
+			DRM_ERROR("Invalid BW index\n");
+			return false;
+		}
+
+		if (lanes < min_lanes || lanes > max_lanes ||
+		    /* Not a power of two. */
+		    !(lanes != 0 && (lanes & (lanes - 1)) == 0)) {
+			DRM_ERROR("Invalid number of lanes.\n");
+			return false;
+		}
+
+		if (link_config_is_possible(mode_rate, lanes, bws[bw_it]))
+			goto found;
+	}
 
 	return false;
 
-- 
1.9.0