[PATCH 3/5] drm/vc4: hdmi: Set a default HSM rate

[Maxime Ripard <maxime@cerno.tech>]

When the firmware doesn't setup the HSM rate (such as when booting
without an HDMI cable plugged in), its rate is 0 and thus any register
access results in a CPU stall, even though HSM is enabled.

Let's enforce a minimum rate at boot to avoid this issue.

Fixes: 4f6e3d66ac52 ("drm/vc4: Add runtime PM support to the HDMI encoder driver")
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
---
 drivers/gpu/drm/vc4/vc4_hdmi.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.c b/drivers/gpu/drm/vc4/vc4_hdmi.c
index b4b4653fe301..74222b12f8c8 100644
--- a/drivers/gpu/drm/vc4/vc4_hdmi.c
+++ b/drivers/gpu/drm/vc4/vc4_hdmi.c
@@ -94,6 +94,7 @@
 # define VC4_HD_M_SW_RST			BIT(2)
 # define VC4_HD_M_ENABLE			BIT(0)
 
+#define HSM_MIN_CLOCK_FREQ	120000000
 #define CEC_CLOCK_FREQ 40000
 
 #define HDMI_14_MAX_TMDS_CLK   (340 * 1000 * 1000)
@@ -2169,6 +2170,19 @@ static int vc4_hdmi_bind(struct device *dev, struct device *master, void *data)
 			vc4_hdmi->disable_4kp60 = true;
 	}
 
+	/*
+	 * If we boot without any cable connected to the HDMI connector,
+	 * the firmware will skip the HSM initialization and leave it
+	 * with a rate of 0, resulting in a bus lockup when we're
+	 * accessing the registers even if it's enabled.
+	 *
+	 * Let's put a sensible default at runtime_resume so that we
+	 * don't end up in this situation.
+	 */
+	ret = clk_set_min_rate(vc4_hdmi->hsm_clock, HSM_MIN_CLOCK_FREQ);
+	if (ret)
+		goto err_put_ddc;
+
 	if (vc4_hdmi->variant->reset)
 		vc4_hdmi->variant->reset(vc4_hdmi);
 
-- 
2.31.1