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

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Date:   Mon, 14 Nov 2022 01:48:14 +0100
From:   Stefan Wahren <stefan.wahren@...e.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:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Marc Kleine-Budde <mkl@...gutronix.de>
Subject: Re: [PATCH 1/2] drm/vc4: hdmi: Enforce the minimum rate at
 runtime_resume

Am 11.11.22 um 22:08 schrieb 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@...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;
>> +
>
> 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+ :-(
Looks like "drm/vc4: hdmi: Fix HSM clock too low on Pi4" addresses this 
issue ...
>
> Best regards
>
> [1] - https://bugzilla.suse.com/show_bug.cgi?id=1205259
>
>>       ret = clk_prepare_enable(vc4_hdmi->hsm_clock);
>>       if (ret)
>>           return ret;
>>