Re: SM8250 USB-C DP alt mode: DP AUX times out in reversed (CC2) cable orientation

From: BurningHoryd <sjunhyuk1@gmail.com>
To: Neil Armstrong <neil.armstrong@linaro.org>,
	Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Cc: linux-arm-msm@vger.kernel.org, linux-phy@lists.infradead.org,
	dri-devel@lists.freedesktop.org, freedreno@lists.freedesktop.org,
	Vinod Koul <vkoul@kernel.org>,
	Abhinav Kumar <quic_abhinavk@quicinc.com>,
	Heikki Krogerus <heikki.krogerus@linux.intel.com>
Subject: Re: SM8250 USB-C DP alt mode: DP AUX times out in reversed (CC2) cable orientation
Date: Mon,  7 Sep 2026 12:26:43 -0400	[thread overview]
Message-ID: <20260907162643.36585-1-sjunhyuk1@gmail.com> (raw)
In-Reply-To: <f0fc4211-2040-4115-9847-6e5cc3fe8161@linaro.org>

On 9/7/26 08:40, Neil Armstrong wrote:
> This means the nb7vpq904m is _not_ responsible for the AUX swapping, other
> boards have the wcd9395 or the fsa4480 in the middle to use the AUX line
> for the USB-C Alternate Audio support, and does the swap.
>
> Could you check in the vendor tree if there's any reference to those ?

Checked Retroid's downstream Android DT for this board. There is an
FSA4480, plus a GPIO AUX switch, and my mainline port models neither.

1. fsa4480@43, on the same I2C controller as the redriver:

     i2c@884000 {
         fsa4480@43 {
             compatible = "qcom,fsa4480-i2c";
             reg = <0x43>;
         };
         redriver@1c {                 /* nb7vpq904m equivalent */
             compatible = "onnn,redriver";
             status = "disabled";
         };
     };

   The only node that references it is the ASoC machine driver
   (qcom,kona-asoc-snd, via "fsa4480-i2c-handle") - so downstream it is
   used for the USB-C analog-audio / MBHC SBU routing, not by the DP
   driver. The codec is a wcd9385, no USBSS block, so no wcd9395 here.

2. The downstream DP driver does its own AUX orientation over GPIO:

     &dp_display {
         qcom,dp-gpio-aux-switch;
         qcom,aux-sel-gpio    = <&tlmm 8  0>;
         qcom,aux-en-gpio     = <&tlmm 9  0>;
         qcom,usbplug-cc-gpio = <&tlmm 65 0>;
     };

   TLMM 9 there is the same pin my mainline DT currently hands to the
   nb7vpq904m as "enable-gpios", so that assignment is likely wrong, and
   TLMM 8 (the orientation select) is not wired up in my port at all.

3. The redriver (onnn,redriver @ 0x1c, same I2C address as the
   nb7vpq904m on my board) is left status = "disabled" downstream.

So the SBU / DP-AUX orientation switch on this board looks like a
discrete GPIO-controlled analog mux (TLMM 8 select, TLMM 9 enable), and
my port is missing it - I modeled the I2C redriver as the
orientation-switch instead, which is why poking its AUX_CC_REG did
nothing and only the DP_CC2 (pair-swapping, 2-lane) op-mode ever got an
AUX transaction through in reverse.

Does this look like a job for gpio-sbu-mux
(drivers/usb/typec/mux/gpio-sbu-mux.c) - a node with
select-gpios = <&tlmm 8>, enable-gpios = <&tlmm 9>, orientation-switch +
mode-switch, wired to the connector - with the nb7vpq904m kept as
retimer-switch only (or dropped, since downstream disables it)? Or is
the FSA4480 in the DP AUX path as well, making fsa,fsa4480 the right
orientation-switch to add?

I'll wire up gpio-sbu-mux and come back with reverse/normal traces
unless that's the wrong direction.

Thanks,
BurningHoryd

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