Re: 5.6 DP-MST regression: 1 of 2 monitors on TB3 (DP-MST) dock no longer light up

[Hans de Goede <hdegoede@redhat.com>]

Hi Lyude,

On 3/7/20 12:54 AM, Lyude Paul wrote:
> On Wed, 2020-02-26 at 18:52 +0100, Hans de Goede wrote:
>> Hi,
>>
>> On 2/26/20 5:05 PM, Alex Deucher wrote:
>>> On Wed, Feb 26, 2020 at 10:43 AM Hans de Goede <hdegoede@redhat.com>
>>> wrote:
>>>> Hi,
>>>>
>>>> On 2/26/20 4:29 PM, Alex Deucher wrote:
>>>>> On Wed, Feb 26, 2020 at 10:16 AM Hans de Goede <hdegoede@redhat.com>
>>>>> wrote:
>>>>>> Hi Lyude and everyone else,
>>>>>>
>>>>>> Lyude I'm mailing you about this because you have done a lot of
>>>>>> work on DP MST, but if this rings a bell to anyone else feel
>>>>>> free to weigh in on this.
>>>>>
>>>>> Might be a duplicate of:
>>>>> https://gitlab.freedesktop.org/drm/amd/issues/1052
>>>>
>>>> Looks like you are right, reverting the commit which the bisect
>>>> from that issue points to:
>>>>
>>>> cd82d82cbc04 ("drm/dp_mst: Add branch bandwidth validation to MST atomic
>>>> check")
>>>>
>>>> Fixes the issue for me. I will add a comment to the issue.
>>>>
>>>> Note I'm using integrated Intel gfx, so that means that this issue
>>>> definitely is not amdgpu specific.
>>>>
>>>
>>> I'm not too familiar with the mst code, but I wonder if we were
>>> exceeding the bandwidth limits in some setups and it just happened to
>>> work, but now that we enforcing them, they don't which is correct, but
>>> a regression from some users' perspective?
>>
>> I seriously doubt that is the case according to:
>> https://support.lenovo.com/nl/en/solutions/pd029622
>>
>> The gen 2 tb3 dock can handle 2 external
>> displays at 3840*2160@60Hz together with the internal
>> panel being on and both my external displays run at
>> 1920x1080@60 so I'm consuming less then half of the
>> maximum bandwidth.
> 
> OK-so I wasn't actually able to reproduce this issue with my setup (I've got a
> X1 Carbon 7th generation, but I don't have the 2nd generation dock - only the
> first generation dock) but I'm certain I've actually fixed it now, since I
> realized we did not have a very good understanding of how PBN limitations are
> advertised with MST. I rewrote the bandwidth checks again, and in the process
> also found a much more subtle regression that got introduced in 5.6, which
> would sometimes cause MST probing to appear to just stop in it's tracks with
> no messages.
> 
> I cc'd both patch series to you, so I'd recommend applying them both onto your
> kernel and seeing if that fixes your issues. If it still doesn't, then get me
> some kernel logs with:
> 
> drm.debug=0x116 log_buf_len=50M
> 
> And I'll take a closer look. I'm pretty confident this should fix everything
> though :)

I can confirm that the v2 series you posted fixes the problem of only of the 2
FHD monitors on my Lenovo TB3 gen 2 dock lighting up, thank you!

Regards,

Hans

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