linux-kernel - Re: [PATCH 1/2] drm/vc4: hdmi: Enforce the minimum rate at runtime

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Date:   Thu, 13 Oct 2022 11:37:57 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Maxime Ripard <maxime@...no.tech>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Maxime Ripard <mripard@...nel.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Emma Anholt <emma@...olt.net>
Cc:     Stefan Wahren <stefan.wahren@...e.com>,
        Marc Kleine-Budde <mkl@...gutronix.de>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH 1/2] drm/vc4: hdmi: Enforce the minimum rate at
 runtime_resume

Hello Maxime,

On 9/29/22 11:21, Maxime Ripard wrote:
> 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@...gutronix.de>
> Signed-off-by: Maxime Ripard <maxime@...no.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;
> +

I'm not familiar with VC4 but your commit message has a great explanation
of the issue and the code is doing what you mention there. So this patch
looks good to me.

Reviewed-by: Javier Martinez Canillas <javierm@...hat.com>

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat