[PATCH 1/2] drm/vc4: hdmi: Enforce the minimum rate at runtime_resume

[Stefan Wahren]

Hi Maxime,

Am 29.09.22 um 11:21 schrieb Maxime Ripard:
> This is a revert of commit fd5894fa2413 ("drm/vc4: hdmi: Remove clock
> rate initialization"), with the code slightly moved around.
>
> It turns out that we can't downright remove that code from the driver,
> since the Pi0-3 and Pi4 are in different cases, and it only works for
> the Pi4.
>
> Indeed, the commit mentioned above was relying on the RaspberryPi
> firmware clocks driver to initialize the rate if it wasn't done by the
> firmware. However, the Pi0-3 are using the clk-bcm2835 clock driver that
> wasn't doing this initialization. We therefore end up with the clock not
> being assigned a rate, and the CPU stalling when trying to access a
> register.
>
> We can't move that initialization in the clk-bcm2835 driver, since the
> HSM clock we depend on is actually part of the HDMI power domain, so any
> rate setup is only valid when the power domain is enabled. Thus, we
> reinstated the minimum rate setup at runtime_suspend, which should
> address both issues.
>
> Link: https://lore.kernel.org/dri-devel/20220922145448.w3xfywkn5ecak2et@pengutronix.de/
> Fixes: fd5894fa2413 ("drm/vc4: hdmi: Remove clock rate initialization")
> Reported-by: Marc Kleine-Budde <mkl at pengutronix.de>
> Signed-off-by: Maxime Ripard <maxime at cerno.tech>
> ---
>   drivers/gpu/drm/vc4/vc4_hdmi.c | 9 +++++++++
>   1 file changed, 9 insertions(+)
>
> diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.c b/drivers/gpu/drm/vc4/vc4_hdmi.c
> index 199bc398817f..2e28fe16ed5e 100644
> --- a/drivers/gpu/drm/vc4/vc4_hdmi.c
> +++ b/drivers/gpu/drm/vc4/vc4_hdmi.c
> @@ -2891,6 +2891,15 @@ static int vc4_hdmi_runtime_resume(struct device *dev)
>   	u32 __maybe_unused value;
>   	int ret;
>   
> +	/*
> +	 * The HSM clock is in the HDMI power domain, so we need to set
> +	 * its frequency while the power domain is active so that it
> +	 * keeps its rate.
> +	 */
> +	ret = clk_set_min_rate(vc4_hdmi->hsm_clock, HSM_MIN_CLOCK_FREQ);
> +	if (ret)
> +		return ret;
> +

unfortunately this breaks X on Raspberry Pi 4 in Linux 6.0.5 
(multi_v7_defconfig + LPAE). Today i saw this report [1] and bisected 
the issue down to this patch. Shame on me that i only tested this patch 
with Rpi 3B+ :-(

Best regards

[1] - https://bugzilla.suse.com/show_bug.cgi?id=1205259

>   	ret = clk_prepare_enable(vc4_hdmi->hsm_clock);
>   	if (ret)
>   		return ret;
>