linux-kernel - Re: [PATCH v3 5/6] phy: qcom: qmp-combo: register a typec mux to change the QMPPHY

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Message-ID: <2zk4ef3ezdfwpyhepnzpf37xxbci32l4sovsyfiymcucga22yz@zs7darysskd2>
Date: Tue, 3 Jun 2025 13:32:47 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Neil Armstrong <neil.armstrong@...aro.org>
Cc: Konrad Dybcio <konrad.dybcio@....qualcomm.com>,
        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
Subject: Re: [PATCH v3 5/6] phy: qcom: qmp-combo: register a typec mux to
 change the QMPPHY_MODE

On Mon, Jun 02, 2025 at 10:51:42AM +0200, Neil Armstrong wrote:
> On 28/05/2025 10:58, Dmitry Baryshkov wrote:
> > On Wed, May 28, 2025 at 12:22:01AM +0200, Konrad Dybcio wrote:
> > > On 5/27/25 11:55 PM, Dmitry Baryshkov wrote:
> > > > On Tue, May 27, 2025 at 10:40:07PM +0200, Konrad Dybcio wrote:
> > > > > From: Neil Armstrong <neil.armstrong@...aro.org>
> > > > > 
> > > > > 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.
> > > > > 
> > > > > The Type-C Mode/SVID only changes on plug/unplug, and USB SAFE states
> > > > > will be set in between of USB-Only, Combo and DisplayPort Only so
> > > > > this will leave enough time to the DRM DisplayPort controller to
> > > > > turn of the DisplayPort PHY.
> > > > > 
> > > > > Signed-off-by: Neil Armstrong <neil.armstrong@...aro.org>
> > > > > [konrad: renaming, rewording, bug fixes]
> > > > > Signed-off-by: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
> > > > > ---
> > > 
> > > [...]
> > > 
> > > > > +	} else {
> > > > > +		/* Fall back to USB3+DP mode if we're not sure it's strictly USB3-only */
> > > > 
> > > > Why? if the SID is not DP, then there can't be a DP stream.
> > > > 
> > > > > +		if (state->mode == TYPEC_MODE_USB3 || state->mode == TYPEC_STATE_USB)
> > > > > +			new_mode = QMPPHY_MODE_USB3_ONLY;
> > > > > +		else
> > > > > +			new_mode = QMPPHY_MODE_USB3DP;
> > > 
> > > To be honest I don't really know.. Neil chose to do that, but I don't
> > > think there's a strict requirement.. Should we default to 4ln-USB3?
> > 
> > Yes, QMPPHY_MODE_USB3_ONLY. Nit: there is no 4ln-USB3 (it is a special
> > mode). We handle 2ln-USB3 only.
> > 
> > > 
> > > [...]
> > > 
> > > > Consider the following scenario: connect DP dongle, use modetest to
> > > > setup DP stream, disconnect dongle, connect USB3 device. What would be
> > > > the actual state of the PHY? Modetest is still running, so DP stream is
> > > > not going to be shut down from the driver.
> > > > 
> > > > I think we might need a generic notifier from the PHY to the user,
> > > > telling that the PHY is going away (or just that the PHY is changing the
> > > > state). Then it would be usable by both the DP and USB drivers to let
> > > > them know that they should toggle the state.
> > > 
> > > 
> > > If modetest won't stop running even though there was a DP unplug
> > > (and therefore presumably a destruction of the display), I don't
> > > think things are designed very well
> > 
> > They are, but differently. Display settings are always controlled by
> > DRM clients (typically, a userspace compositor). They can decide to
> > send data to unconnected display, they can decide to ignore HPD events,
> > etc. Even if userspace responds to the event, there always will be some
> > delay. I choose modetest, because it's a particularly good example of a
> > delay going to the infinity.
> > 
> 
> DP link state is handled separately from the DRM state, if you look at the
> MSM/DP, you get the following calls on an hdp event:
> dp_bridge_hpd_notify
> hpd_event_thread

And this part will hopefully go away soon... Which means that DP link
will stay on until atomic_disable().

> dp_hpd_unplug_handle
> dp_ctrl_off_link
> phy_power_off
> dp_display_host_phy_exit
> phy_exit
> 
> independently of any DRM state change, DRM will be notified at the end of
> a disconnect with dp_display_notify_disconnect().
> 
> So even with a modeset running, phy will disabled following an UCSI disconnect
> event.
> 
> Neil

-- 
With best wishes
Dmitry