Re: [PATCH v3 0/3] drm/vc4: hdmi: enable scrambling before the TMDS clock starts

[Cristian Ciocaltea]

Hi Lucas,

On 2026/10/26, Lucas Tanure wrote:
> HDMI 2.0 asks the source to set TMDS_Bit_Clock_Ratio in the sink while
> the TMDS clock is stopped and to start the clock at least 1 ms later.
> vc4 writes the bit, and enables the scrambler, at the end of the enable
> sequence, long after the PHY has started, and clears it before the PHY
> stops. A sink that samples the bit when the clock appears never locks
> a 4K60 signal from a Raspberry Pi 5, while laptops that follow the
> order work on the same sink.
>
> Patch 3 is the fix. Patch 1 is needed on BCM2712, where phy_disable()
> was an empty stub upstream, so the PHY kept running after disable; the
> Raspberry Pi tree has carried this since 2024. Patch 2 fixes a
> pre-existing race in the scrambling work cancel that the fix would
> otherwise make easier to hit.
>
> v1 was tested on a Raspberry Pi 5 running the Raspberry Pi 6.18 kernel
> with the same change applied, against the sink that showed the
> problem: plain enable at 4K60, 1080p60 to 4K60, 4K30 to 4K60, a sink
> reset and a sink power cycle all lock with zero character errors. v2
> and v3 carry the same code; the hardware test of the final form is
> still pending and will be reported in this thread. Mainline was not
> booted on the Pi 5. The BCM2711 path is unchanged except for the
> enable order; I have no Pi 4 at hand.

While converting vc4 to the common HDMI 2.0 scrambling infrastructure, I
ran several tests on the Raspberry Pi 5 and didn't notice any (obvious)
issues.

Hence, could you please verify whether your problem reproduces with a
recent mainline kernel?  If it does, could you also give [1] a try and
report back the actual behavior?

[1] https://lore.kernel.org/lkml/20261014-vc4-common-scramb-v3-0-c68d58d959a2@xxxxxxxxxxxxx/

Regards,
Cristian