DisplayPort: handling of HPD events / link training

[Thomas Zimmermann]

Hi

Am 27.02.24 um 23:40 schrieb Dmitry Baryshkov:
> Hello,
>
> We are currently looking at checking and/or possibly redesigning the
> way the MSM DRM driver handles the HPD events and link training.
>
> After a quick glance at the drivers implementing DP support, I noticed
> following main approaches:
> - Perform link training at the atomic_enable time, don't report
> failures (mtk, analogix, zynqmp, tegra, nouveau)
> - Perform link training at the atomic_enable time, report errors using
> link_status property (i915, mhdp8546)
> - Perform link training on the plug event (msm, it8605).
> - Perform link training from the DPMS handler, also calling it from
> the enable callback (AMDGPU, radeon).
>
> It looks like the majority wins and we should move HPD to
> atomic_enable time. Is that assumption correct?

Did you ever receive an answer to this question? I currently investigate 
ast's DP code, which does link training as part of detecting the 
connector state (in detect_ctx). But most other drivers do this in 
atomic_enable. I wonder if ast should follow.

Best regards
Thomas

>
> Also two related questions:
> - Is there a plan to actually make use of the link_status property?
> Intel presented it at FOSDEM 2018, but since that time it was not
> picked up by other drivers.
>
> - Is there any plan to create generic DP link training helpers? After
> glancing through the DP drivers there is a lot of similar code in the
> link training functions, with minor differences here and there. And
> it's those minor differences that bug me. It means that drivers might
> respond differently to similar devices. Or that there might be minor
> bugs here and there.
>

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
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