drm/vc4: hdmi: Disable Wifi Frequencies

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@cerno.tech>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20201029134018.1948636-2-maxime@cerno.tech

drivers/gpu/drm/vc4/vc4_hdmi.c

@@ -760,6 +760,9 @@ static void vc4_hdmi_encoder_enable(struct drm_encoder *encoder)
 {
 }
 
+#define WIFI_2_4GHz_CH1_MIN_FREQ	2400000000ULL
+#define WIFI_2_4GHz_CH1_MAX_FREQ	2422000000ULL
+
 static int vc4_hdmi_encoder_atomic_check(struct drm_encoder *encoder,
 					 struct drm_crtc_state *crtc_state,
 					 struct drm_connector_state *conn_state)
@@ -767,12 +770,27 @@ static int vc4_hdmi_encoder_atomic_check(struct drm_encoder *encoder,
 	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 tmds_rate;
 
 	if (vc4_hdmi->variant->unsupported_odd_h_timings &&
 	    ((mode->hdisplay % 2) || (mode->hsync_start % 2) ||
 	     (mode->hsync_end % 2) || (mode->htotal % 2)))
 		return -EINVAL;
 
+	/*
+	 * The 1440p@60 pixel rate is in the same range than the first
+	 * WiFi channel (between 2.4GHz and 2.422GHz with 22MHz
+	 * bandwidth). Slightly lower the frequency to bring it out of
+	 * the WiFi range.
+	 */
+	tmds_rate = pixel_rate * 10;
+	if (vc4_hdmi->disable_wifi_frequencies &&
+	    (tmds_rate >= WIFI_2_4GHz_CH1_MIN_FREQ &&
+	     tmds_rate <= WIFI_2_4GHz_CH1_MAX_FREQ)) {
+		mode->clock = 238560;
+		pixel_rate = mode->clock * 1000;
+	}
+
 	if (pixel_rate > vc4_hdmi->variant->max_pixel_clock)
 		return -EINVAL;
 
@@ -1719,6 +1737,9 @@ 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, "wifi-2.4ghz-coexistence");
+
 	pm_runtime_enable(dev);
 
 	drm_simple_encoder_init(drm, encoder, DRM_MODE_ENCODER_TMDS);

drivers/gpu/drm/vc4/vc4_hdmi.h

@@ -142,6 +142,14 @@ 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@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;