linux-kernel - Re: [PATCH v3 0/6] arm64: qcom: allow up to 4 lanes for the Type-C DisplayPort Altmode

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Message-ID: <aa17d7d4-b77d-4a0a-88c3-86255dfbc29d@oss.qualcomm.com>
Date: Wed, 28 May 2025 18:56:00 +0200
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
Cc: Konrad Dybcio <konradybcio@...nel.org>, Vinod Koul <vkoul@...nel.org>,
        Kishon Vijay Abraham I <kishon@...nel.org>,
        Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Conor Dooley
 <conor+dt@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        linux-arm-msm@...r.kernel.org, linux-phy@...ts.infradead.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Krzysztof Kozlowski <krzk@...nel.org>
Subject: Re: [PATCH v3 0/6] arm64: qcom: allow up to 4 lanes for the Type-C
 DisplayPort Altmode

On 5/28/25 1:22 PM, Dmitry Baryshkov wrote:
> On Wed, May 28, 2025 at 01:13:26PM +0200, Konrad Dybcio wrote:
>> On 5/28/25 11:00 AM, Dmitry Baryshkov wrote:
>>> On Wed, May 28, 2025 at 12:24:02AM +0200, Konrad Dybcio wrote:
>>>> On 5/27/25 11:10 PM, Dmitry Baryshkov wrote:
>>>>> On Tue, May 27, 2025 at 10:40:02PM +0200, Konrad Dybcio wrote:
>>>>>> Register a typec mux in order to change the PHY mode on the Type-C
>>>>>> mux events depending on the mode and the svid when in Altmode setup.
>>>>>>
>>>>>> The DisplayPort phy should be left enabled if is still powered on
>>>>>> by the DRM DisplayPort controller, so bail out until the DisplayPort
>>>>>> PHY is not powered off.
>>>>>
>>>>> This series doesn't seem to solve the USB side of a problem. When the
>>>>> PHY is being switch to the 4-lane mode, USB controller looses PIPE
>>>>> clock, so it needs to be switched to the USB-2 mode.
>>>>
>>>> I didn't find issues with that on X13s.. Not sure if it's related, but
>>>> on the SL7, after plugging in a 4ln DP connection, I need to plug in
>>>> the USB thumb drive twice for the first time (only in that sequence)
>>>
>>> Might be.
>>>
>>>> But there's nothing interesting in dmesg and the phy seems to be
>>>> programmed with the same values on both the initial and the subsequent
>>>> working plug-in
>>>
>>> Please try using a DP dongle with USB 2 passthrough (there are some).
>>> Or just emulate this by enabling DP PHY / DP chain for plugged in USB3
>>> devices. Would you be able to see the USB device on a bus?
>>
>> I only have a dongle with both display and usb that does 2ln dp
>> (I tested 4ln dp on a type-c display that I don't think has a hub)
>>
>> USB3 - yes, USB2 - no (but it works after a replug)
>>
>> Are you talking about essentially doing qcom,select-utmi-as-pipe-clk
>> at runtime?
> 
> I think so.

So after quite some time playing with that, I noticed that the USB2
device was never actually kicked off the bus.. and works totally fine
after connecting the display output (2ln DP)

I was looking at dmesg, checking for discovery/disconnect messages,
but the device was simply never disconnected (which makes sense given
repurposing USB3 TX/RX lanes doesn't affect the D+/D- of USB2)

I also read some docs and learnt that what we call
qcom,select-utmi-as-pipe-clk is suppossed to be a debug feature
and is unnecessary to set on USB2.0-only controllers

The USB controller programming guide though doesn't talk about DP,
but I'd expect that we may need to set that override for 4lnDP+USB2
use cases (which I don't have a dongle for)

Konrad