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    Subject[PATCH 2/2] drm/vc4: hdmi: Check the HSM rate at runtime_resume
    If our HSM clock has not been properly initialized, any register access
    will silently lock up the system.

    Let's check that this can't happen by adding a check for the rate before
    any register access, and error out otherwise.

    Link: https://lore.kernel.org/dri-devel/20220922145448.w3xfywkn5ecak2et@pengutronix.de/
    Signed-off-by: Maxime Ripard <maxime@cerno.tech>
    ---
    drivers/gpu/drm/vc4/vc4_hdmi.c | 20 ++++++++++++++++++++
    1 file changed, 20 insertions(+)

    diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.c b/drivers/gpu/drm/vc4/vc4_hdmi.c
    index 2e28fe16ed5e..eb3aaaca2b80 100644
    --- a/drivers/gpu/drm/vc4/vc4_hdmi.c
    +++ b/drivers/gpu/drm/vc4/vc4_hdmi.c
    @@ -2889,6 +2889,7 @@ static int vc4_hdmi_runtime_resume(struct device *dev)
    struct vc4_hdmi *vc4_hdmi = dev_get_drvdata(dev);
    unsigned long __maybe_unused flags;
    u32 __maybe_unused value;
    + unsigned long rate;
    int ret;

    /*
    @@ -2904,6 +2905,21 @@ static int vc4_hdmi_runtime_resume(struct device *dev)
    if (ret)
    return ret;

    + /*
    + * Whenever the RaspberryPi boots without an HDMI monitor
    + * plugged in, the firmware won't have initialized the HSM clock
    + * rate and it will be reported as 0.
    + *
    + * If we try to access a register of the controller in such a
    + * case, it will lead to a silent CPU stall. Let's make sure we
    + * prevent such a case.
    + */
    + rate = clk_get_rate(vc4_hdmi->hsm_clock);
    + if (!rate) {
    + ret = -EINVAL;
    + goto err_disable_clk;
    + }
    +
    if (vc4_hdmi->variant->reset)
    vc4_hdmi->variant->reset(vc4_hdmi);

    @@ -2925,6 +2941,10 @@ static int vc4_hdmi_runtime_resume(struct device *dev)
    #endif

    return 0;
    +
    +err_disable_clk:
    + clk_disable_unprepare(vc4_hdmi->hsm_clock);
    + return ret;
    }

    static int vc4_hdmi_bind(struct device *dev, struct device *master, void *data)
    --
    b4 0.10.1

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