linux-kernel - [PATCH] drm/nouveau/nvkm/dp: Add hack to fix DP 1.3+ DPCD issues

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Date:   Fri,  7 Jul 2023 17:58:51 -0400
From:   Lyude Paul <lyude@...hat.com>
To:     nouveau@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org
Cc:     Ben Skeggs <bskeggs@...hat.com>, Karol Herbst <kherbst@...hat.com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Dave Airlie <airlied@...hat.com>,
        linux-kernel@...r.kernel.org (open list)
Subject: [PATCH] drm/nouveau/nvkm/dp: Add hack to fix DP 1.3+ DPCD issues

Currently we use the drm_dp_dpcd_read_caps() helper in the DRM side of
nouveau in order to read the DPCD of a DP connector, which makes sure we do
the right thing and also check for extended DPCD caps. However, it turns
out we're not currently doing this on the nvkm side since we don't have
access to the drm_dp_aux structure there - which means that the DRM side of
the driver and the NVKM side can end up with different DPCD capabilities
for the same connector.

Ideally to fix this, we want to start setting up the drm_dp_aux device in
NVKM before we've made contact with the DRM side - which should be pretty
easy to accomplish (I'm already working on it!). Until then however, let's
workaround this problem by porting a copy of drm_dp_read_dpcd_caps() into
NVKM - which should fix this issue.

Issue: https://gitlab.freedesktop.org/drm/nouveau/-/issues/211
Signed-off-by: Lyude Paul <lyude@...hat.com>
---
 drivers/gpu/drm/nouveau/nvkm/engine/disp/dp.c | 48 ++++++++++++++++++-
 1 file changed, 47 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/nouveau/nvkm/engine/disp/dp.c b/drivers/gpu/drm/nouveau/nvkm/engine/disp/dp.c
index 40c8ea43c42f..b8ac66b4a2c4 100644
--- a/drivers/gpu/drm/nouveau/nvkm/engine/disp/dp.c
+++ b/drivers/gpu/drm/nouveau/nvkm/engine/disp/dp.c
@@ -26,6 +26,8 @@
 #include "head.h"
 #include "ior.h"
 
+#include <drm/display/drm_dp.h>
+
 #include <subdev/bios.h>
 #include <subdev/bios/init.h>
 #include <subdev/gpio.h>
@@ -634,6 +636,50 @@ nvkm_dp_enable_supported_link_rates(struct nvkm_outp *outp)
 	return outp->dp.rates != 0;
 }
 
+/* XXX: This is a big fat hack, and this is just drm_dp_read_dpcd_caps()
+ * converted to work inside nvkm. This is a temporary holdover until we start
+ * passing the drm_dp_aux device through NVKM
+ */
+static int
+nvkm_dp_read_dpcd_caps(struct nvkm_outp *outp)
+{
+	struct nvkm_i2c_aux *aux = outp->dp.aux;
+	u8 dpcd_ext[DP_RECEIVER_CAP_SIZE];
+	int ret;
+
+	ret = nvkm_rdaux(aux, DPCD_RC00_DPCD_REV, outp->dp.dpcd, DP_RECEIVER_CAP_SIZE);
+	if (ret < 0)
+		return ret;
+
+	/*
+	 * Prior to DP1.3 the bit represented by
+	 * DP_EXTENDED_RECEIVER_CAP_FIELD_PRESENT was reserved.
+	 * If it is set DP_DPCD_REV at 0000h could be at a value less than
+	 * the true capability of the panel. The only way to check is to
+	 * then compare 0000h and 2200h.
+	 */
+	if (!(outp->dp.dpcd[DP_TRAINING_AUX_RD_INTERVAL] &
+	      DP_EXTENDED_RECEIVER_CAP_FIELD_PRESENT))
+		return 0;
+
+	ret = nvkm_rdaux(aux, DP_DP13_DPCD_REV, dpcd_ext, sizeof(dpcd_ext));
+	if (ret < 0)
+		return ret;
+
+	if (outp->dp.dpcd[DP_DPCD_REV] > dpcd_ext[DP_DPCD_REV]) {
+		OUTP_DBG(outp, "Extended DPCD rev less than base DPCD rev (%d > %d)\n",
+			 outp->dp.dpcd[DP_DPCD_REV], dpcd_ext[DP_DPCD_REV]);
+		return 0;
+	}
+
+	if (!memcmp(outp->dp.dpcd, dpcd_ext, sizeof(dpcd_ext)))
+		return 0;
+
+	memcpy(outp->dp.dpcd, dpcd_ext, sizeof(dpcd_ext));
+
+	return 0;
+}
+
 void
 nvkm_dp_enable(struct nvkm_outp *outp, bool auxpwr)
 {
@@ -689,7 +735,7 @@ nvkm_dp_enable(struct nvkm_outp *outp, bool auxpwr)
 			memset(outp->dp.lttpr, 0x00, sizeof(outp->dp.lttpr));
 		}
 
-		if (!nvkm_rdaux(aux, DPCD_RC00_DPCD_REV, outp->dp.dpcd, sizeof(outp->dp.dpcd))) {
+		if (!nvkm_dp_read_dpcd_caps(outp)) {
 			const u8 rates[] = { 0x1e, 0x14, 0x0a, 0x06, 0 };
 			const u8 *rate;
 			int rate_max;
-- 
2.40.1