drm/vc4: hdmi: Introduce a CEC clock

While the BCM2835 had the CEC clock derived from the HSM clock, the
BCM2711 has a dedicated parent clock for it.

Let's introduce a separate clock for it so that we can handle both
cases.
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Tested-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Link: https://patchwork.freedesktop.org/patch/msgid/20210111142309.193441-9-maxime@cerno.tech

drivers/gpu/drm/vc4/vc4_hdmi.c

@@ -145,7 +145,7 @@ static void vc4_hdmi_cec_update_clk_div(struct vc4_hdmi *vc4_hdmi)
 	 * Set the clock divider: the hsm_clock rate and this divider
 	 * setting will give a 40 kHz CEC clock.
 	 */
-	clk_cnt = clk_get_rate(vc4_hdmi->hsm_clock) / CEC_CLOCK_FREQ;
+	clk_cnt = clk_get_rate(vc4_hdmi->cec_clock) / CEC_CLOCK_FREQ;
 	value |= clk_cnt << VC4_HDMI_CEC_DIV_CLK_CNT_SHIFT;
 	HDMI_WRITE(HDMI_CEC_CNTRL_1, value);
 }
@@ -1752,6 +1752,7 @@ static int vc4_hdmi_init_resources(struct vc4_hdmi *vc4_hdmi)
 		return PTR_ERR(vc4_hdmi->hsm_clock);
 	}
 	vc4_hdmi->audio_clock = vc4_hdmi->hsm_clock;
+	vc4_hdmi->cec_clock = vc4_hdmi->hsm_clock;
 
 	return 0;
 }
@@ -1845,6 +1846,12 @@ static int vc5_hdmi_init_resources(struct vc4_hdmi *vc4_hdmi)
 		return PTR_ERR(vc4_hdmi->audio_clock);
 	}
 
+	vc4_hdmi->cec_clock = devm_clk_get(dev, "cec");
+	if (IS_ERR(vc4_hdmi->cec_clock)) {
+		DRM_ERROR("Failed to get CEC clock\n");
+		return PTR_ERR(vc4_hdmi->cec_clock);
+	}
+
 	vc4_hdmi->reset = devm_reset_control_get(dev, NULL);
 	if (IS_ERR(vc4_hdmi->reset)) {
 		DRM_ERROR("Failed to get HDMI reset line\n");

drivers/gpu/drm/vc4/vc4_hdmi.h

@@ -155,6 +155,7 @@ struct vc4_hdmi {
 	bool cec_tx_ok;
 	bool cec_irq_was_rx;
 
+	struct clk *cec_clock;
 	struct clk *pixel_clock;
 	struct clk *hsm_clock;
 	struct clk *audio_clock;