[PATCH 1/2] drm/vc4: hdmi: Disable Wifi Frequencies

[Maxime Ripard]

There's cross-talk on the RPi4 between the 2.4GHz channels used by the WiFi
chip and some resolutions, most notably 1440p at 60Hz.

In such a case, we can either reject entirely the mode, or lower slightly
the pixel frequency to remove the overlap. Let's go for the latter.

Signed-off-by: Maxime Ripard <maxime at cerno.tech>
---
 .../bindings/display/brcm,bcm2711-hdmi.yaml        |  6 ++++++
 drivers/gpu/drm/vc4/vc4_hdmi.c                     | 14 +++++++++++++-
 drivers/gpu/drm/vc4/vc4_hdmi.h                     |  8 ++++++++
 3 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/Documentation/devicetree/bindings/display/brcm,bcm2711-hdmi.yaml b/Documentation/devicetree/bindings/display/brcm,bcm2711-hdmi.yaml
index 03a76729d26c..63e7fe999c0a 100644
--- a/Documentation/devicetree/bindings/display/brcm,bcm2711-hdmi.yaml
+++ b/Documentation/devicetree/bindings/display/brcm,bcm2711-hdmi.yaml
@@ -76,6 +76,12 @@ properties:
   resets:
     maxItems: 1
 
+  raspberrypi,disable-wifi-frequencies:
+    type: boolean
+    description: >
+      Should the pixel frequencies in the WiFi frequencies range be
+      avoided?
+
 required:
   - compatible
   - reg
diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.c b/drivers/gpu/drm/vc4/vc4_hdmi.c
index acfb4e235214..74da7c00ecd0 100644
--- a/drivers/gpu/drm/vc4/vc4_hdmi.c
+++ b/drivers/gpu/drm/vc4/vc4_hdmi.c
@@ -877,13 +877,22 @@ static int vc4_hdmi_encoder_atomic_check(struct drm_encoder *encoder,
 	struct vc4_hdmi_connector_state *vc4_state = conn_state_to_vc4_hdmi_conn_state(conn_state);
 	struct drm_display_mode *mode = &crtc_state->adjusted_mode;
 	struct vc4_hdmi *vc4_hdmi = encoder_to_vc4_hdmi(encoder);
-	unsigned long long pixel_rate = mode->clock * 1000;
+	unsigned long long pixel_rate;
 
 	if (vc4_hdmi->variant->broken_odd_h_timings &&
 	    ((mode->hdisplay % 2) || (mode->hsync_start % 2) ||
 	     (mode->hsync_end % 2) || (mode->htotal % 2)))
 		return -EINVAL;
 
+	/*
+	 * The 1440p at 60 pixel rate is in the same range than the WiFi
+	 * channels. Slightly lower the frequency to bring it out of the
+	 * WiFi range.
+	 */
+	if (vc4_hdmi->disable_wifi_frequencies && mode->clock == 241500)
+		mode->clock = 238560;
+
+	pixel_rate = mode->clock * 1000;
 	if (mode->flags & DRM_MODE_FLAG_DBLCLK)
 		pixel_rate *= 2;
 
@@ -1837,6 +1846,9 @@ static int vc4_hdmi_bind(struct device *dev, struct device *master, void *data)
 		vc4_hdmi->hpd_active_low = hpd_gpio_flags & OF_GPIO_ACTIVE_LOW;
 	}
 
+	vc4_hdmi->disable_wifi_frequencies =
+		of_property_read_bool(dev->of_node, "raspberrypi,disable-wifi-frequencies");
+
 	pm_runtime_enable(dev);
 
 	drm_simple_encoder_init(drm, encoder, DRM_MODE_ENCODER_TMDS);
diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.h b/drivers/gpu/drm/vc4/vc4_hdmi.h
index 40e51ece8efe..6618ee758890 100644
--- a/drivers/gpu/drm/vc4/vc4_hdmi.h
+++ b/drivers/gpu/drm/vc4/vc4_hdmi.h
@@ -143,6 +143,14 @@ struct vc4_hdmi {
 	int hpd_gpio;
 	bool hpd_active_low;
 
+	/*
+	 * On some systems (like the RPi4), some modes are in the same
+	 * frequency range than the WiFi channels (1440p at 60Hz for
+	 * example). Should we take evasive actions because that system
+	 * has a wifi adapter.
+	 */
+	bool disable_wifi_frequencies;
+
 	struct cec_adapter *cec_adap;
 	struct cec_msg cec_rx_msg;
 	bool cec_tx_ok;
-- 
2.26.2