linux-kernel - Re: [PATCH 3/7] phy: qcom-qmp-combo: Introduce orientation variable

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Message-ID: <20230504151633.GH870858@hu-bjorande-lv.qualcomm.com>
Date:   Thu, 4 May 2023 08:16:33 -0700
From:   Bjorn Andersson <quic_bjorande@...cinc.com>
To:     Johan Hovold <johan@...nel.org>
CC:     Vinod Koul <vkoul@...nel.org>,
        Kishon Vijay Abraham I <kishon@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        <linux-arm-msm@...r.kernel.org>, <linux-phy@...ts.infradead.org>,
        <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/7] phy: qcom-qmp-combo: Introduce orientation variable

On Thu, May 04, 2023 at 03:44:53PM +0200, Johan Hovold wrote:
> On Wed, May 03, 2023 at 08:29:07PM -0700, Bjorn Andersson wrote:
> > On Tue, May 02, 2023 at 01:48:16PM +0200, Johan Hovold wrote:
> > > On Mon, Apr 24, 2023 at 08:40:06PM -0700, Bjorn Andersson wrote:
> 
> > > >  static void qmp_v3_dp_aux_init(struct qmp_combo *qmp);
> > > > @@ -1955,29 +1962,23 @@ static void qmp_v3_configure_dp_tx(struct qmp_combo *qmp)
> > > >  static bool qmp_combo_configure_dp_mode(struct qmp_combo *qmp)
> > > >  {
> > > >  	u32 val;
> > > > -	bool reverse = false;
> > > > +	bool reverse = qmp->orientation == TYPEC_ORIENTATION_REVERSE;
> 
> > > It also looks like these callbacks end up being called without holding
> > > the qmp->phy_mutex via phy->power_on(). Perhaps there is no risk for a
> > > concurrent switch notification and dp phy power-on but it's not that
> > > obvious.
> 
> > It seems we're arriving here from hpd_event_thread(), while
> > phy_power_on() and phy_power_off() will be called in some other context.
> > I've not been able to convince myself if DP driver ensures ordering, or
> > if we have an existing race here...
> 
> > Unless you insist, I would prefer to follow up with an additional patch
> > once we've landed this series. The fix will depend on the phy_mutex
> > shuffling patch anyways...
> 
> Sure.
> 
> But perhaps you can just move the orientation == qmp->orientation check
> under the mutex in qmp_combo_typec_switch_set() for now (in case I
> forgot to point that out earlier).
> 

qmp_combo_probe() and qmp_combo_typec_switch_set() are the only writers
to qmp->orientation, so that check can't race with any updates and hence
doesn't need to be protected.

Reading the code again, qmp_combo_configure_dp_mode() is invoked from
phy_power_on(), not the hpd_event_thread(), as I claimed yesterday.

But we shouldn't do qmp_combo_dp_power_on() in parallel with the
reinitialization following a switch in orientation, qmp->orientation
might change, but we definitely would have two contexts reconfiguring
the hardware simultaneously - perhaps this was the cause for the 10%
crashes I hit when trying to extend this to handle typec_mux as well...

I will grab the phy_mux in qmp_combo_configure_dp_mode() as well, thanks
for "insisting" :)

Regards,
Bjorn