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

["Maíra Canal" <mcanal@igalia.com>]

Hi Cristian,

On 26/10/26 22:23, Cristian Ciocaltea wrote:
> 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?
> 

I suspect you didn't hit this on mainline because vc6_hdmi_phy_disable()
is a stub, so the TMDS clock never stops between disable and enable.

Best regards,
- Maíra

> [1] https://lore.kernel.org/lkml/20261014-vc4-common-scramb-v3-0-c68d58d959a2@collabora.com/
> 
> Regards,
> Cristian