[PATCH 1/2] drm/dp: start a DPCD based DP sink/branch device quirk database

[Jani Nikula]

Face the fact, there are Display Port sink and branch devices out there
in the wild that don't follow the Display Port specifications, or they
have bugs, or just otherwise require special treatment. Start a common
quirk database the drivers can query based on OUI (with the option of
expanding to device identification string and hardware/firmware
revisions later). At least for now, we leave the workarounds for the
drivers to implement as they see fit.

For starters, add a branch device that can't handle full 24-bit main
link M and N attributes properly. Naturally, the workaround of reducing
main link attributes for all devices ended up in regressions for other
devices. So here we are.

Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan at intel.com>
Cc: Clint Taylor <clinton.a.taylor at intel.com>
Cc: Adam Jackson <ajax at redhat.com>
Cc: Harry Wentland <harry.wentland at amd.com>
Signed-off-by: Jani Nikula <jani.nikula at intel.com>
---
 drivers/gpu/drm/drm_dp_helper.c | 61 +++++++++++++++++++++++++++++++++++++++++
 include/drm/drm_dp_helper.h     |  7 +++++
 2 files changed, 68 insertions(+)

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index 3e5f52110ea1..58b2ced33151 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -1208,3 +1208,64 @@ int drm_dp_stop_crc(struct drm_dp_aux *aux)
 	return 0;
 }
 EXPORT_SYMBOL(drm_dp_stop_crc);
+
+/*
+ * Display Port sink and branch devices in the wild have a variety of bugs, try
+ * to collect them here. The quirks are shared, but it's up to the drivers to
+ * implement workarounds for them.
+ */
+
+/*
+ * FIXME: Add support for device identification and hardware/firmware revision.
+ */
+struct dpcd_quirk {
+	u8 oui[3];
+	bool is_branch;
+	u32 quirks;
+};
+
+#define OUI(first, second, third) { (first), (second), (third) }
+
+static const struct dpcd_quirk dpcd_quirk_list[] = {
+	/* Analogix 7737 needs reduced M and N at HBR2 link rates */
+	{ OUI(0x00, 0x22, 0xb9), true, DP_DPCD_QUIRK_LIMITED_M_N },
+};
+
+#undef OUI
+
+/**
+ * drm_dp_get_quirks() - get quirks for the sink/branch device
+ * @oui: pointer to len bytes of DPCD at 0x400 (sink) or 0x500 (branch)
+ * @len: 3-12 bytes of DPCD data
+ * @is_branch: true for branch devices, false for sink devices
+ *
+ * Get a bit mask of DPCD quirks for the sink/branch device. The quirk data is
+ * shared but it's up to the drivers to act on the data.
+ *
+ * For now, only the OUI (first three bytes) is used, but this may be extended
+ * to device identification string and hardware/firmware revisions later.
+ */
+u32 drm_dp_get_quirks(const u8 *oui, unsigned int len, bool is_branch)
+{
+	const struct dpcd_quirk *quirk;
+	u32 quirks = 0;
+	int i;
+
+	if (WARN_ON_ONCE(len < 3))
+		return 0;
+
+	for (i = 0; i < ARRAY_SIZE(dpcd_quirk_list); i++) {
+		quirk = &dpcd_quirk_list[i];
+
+		if (quirk->is_branch != is_branch)
+			continue;
+
+		if (memcmp(quirk->oui, oui, 3) != 0)
+			continue;
+
+		quirks |= quirk->quirks;
+	}
+
+	return quirks;
+}
+EXPORT_SYMBOL(drm_dp_get_quirks);
diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
index f7007e544f29..8abe9897fe59 100644
--- a/include/drm/drm_dp_helper.h
+++ b/include/drm/drm_dp_helper.h
@@ -1079,4 +1079,11 @@ void drm_dp_aux_unregister(struct drm_dp_aux *aux);
 int drm_dp_start_crc(struct drm_dp_aux *aux, struct drm_crtc *crtc);
 int drm_dp_stop_crc(struct drm_dp_aux *aux);
 
+/* Display Port sink and branch device quirks. */
+
+/* The sink is limited to small link M and N values. */
+#define DP_DPCD_QUIRK_LIMITED_M_N			BIT(0)
+
+u32 drm_dp_get_quirks(const u8 *oui, unsigned int len, bool is_branch);
+
 #endif /* _DRM_DP_HELPER_H_ */
-- 
2.11.0