drm/vc4: hdmi: Set a default HSM rate

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>
Reviewed-by: Nicolas Saenz Julienne <nsaenz@kernel.org>
Tested-by: Nicolas Saenz Julienne <nsaenz@kernel.org>
Tested-by: Michael Stapelberg <michael@stapelberg.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210922125419.4125779-4-maxime@cerno.tech

diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.c b/drivers/gpu/drm/vc4/vc4_hdmi.c
index 1bc675d727b61278838454e480034a81827f378d..a3813a6137fb045808a8a02c23c76c21097e1402 100644 (file)
--- 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)
@@ -2170,6 +2171,19 @@ 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);