[PATCH 7/8] drm/msm/dp: Retry Link Training 2 with lower pattern

[Konrad Dybcio]

On 12/3/24 3:07 PM, Dmitry Baryshkov wrote:
> On Tue, Dec 03, 2024 at 04:13:22PM +0800, Xiangxu Yin wrote:
>>
>>
>> On 11/29/2024 9:53 PM, Dmitry Baryshkov wrote:
>>> On Fri, 29 Nov 2024 at 09:59, Xiangxu Yin <quic_xiangxuy at quicinc.com> wrote:
>>>>
>>>> Add a mechanism to retry Link Training 2 by lowering the pattern level
>>>> when the link training #2 first attempt fails. This approach enhances
>>>> compatibility, particularly addressing issues caused by certain hub
>>>> configurations.
>>>
>>> Please reference corresponding part of the standard, describing this lowering.
>>>
>> Per DisplayPort 1.4a specification Section 3.5.1.2 and Table 3-10, while the standard doesn't explicitly define a TPS downgrade mechanism, it does specify:
> 
> Anything in DP 2.1?
> 
>> - All devices shall support TPS1 and TPS2
>> - HDR2-capable devices shall support TPS3
>> - HDR3-capable devices shall support TPS4
>> While these capabilities are explicitly defined DPCD for sink devices, source device capabilities are less strictly defined, with the minimum requirement being support for TPS1 and TPS2.
>> In QCS615 DP phy is only supporting to HBR2, we observed a critical interoperability scenario with a DP->HDMI bridge. When link training at TPS4 consistently failed, downgrading to the next lower training pattern successfully established the link and display output successfully.
> 
> Any other driver doing such TPS lowering? Or maybe we should be
> selecting TPS3 for HBR2-only devices?

Bump, this patch looks interesting and I'd like to see it revisited if
it's correct

Konrad