linux-kernel - Re: [PATCH 2/2] drm/msm/dp: Implement oob_hotplug

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Message-ID: <YgWS9iZ+1uJBd9Lj@ripper>
Date:   Thu, 10 Feb 2022 14:34:30 -0800
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        Rob Clark <robdclark@...il.com>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>, Sean Paul <sean@...rly.run>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Stephen Boyd <swboyd@...omium.org>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        intel-gfx@...ts.freedesktop.org, linux-arm-msm@...r.kernel.org,
        freedreno@...ts.freedesktop.org, linux-usb@...r.kernel.org
Subject: Re: [PATCH 2/2] drm/msm/dp: Implement oob_hotplug_event()

On Mon 07 Feb 23:40 PST 2022, Greg Kroah-Hartman wrote:

> On Mon, Feb 07, 2022 at 08:43:28PM -0800, Bjorn Andersson wrote:
> > The Qualcomm DisplayPort driver contains traces of the necessary
> > plumbing to hook up USB HPD, in the form of the dp_hpd module and the
> > dp_usbpd_cb struct. Use this as basis for implementing the
> > oob_hotplug_event() callback, by amending the dp_hpd module with the
> > missing logic.
> > 
> > Overall the solution is similar to what's done downstream, but upstream
> > all the code to disect the HPD notification lives on the calling side of
> > drm_connector_oob_hotplug_event().
> > 
> > drm_connector_oob_hotplug_event() performs the lookup of the
> > drm_connector based on fwnode, hence the need to assign the fwnode in
> > dp_drm_connector_init().
> > 
> > Signed-off-by: Bjorn Andersson <bjorn.andersson@...aro.org>
> > ---
> >  drivers/gpu/drm/msm/dp/dp_display.c |  8 ++++++++
> >  drivers/gpu/drm/msm/dp/dp_display.h |  2 ++
> >  drivers/gpu/drm/msm/dp/dp_drm.c     | 10 ++++++++++
> >  drivers/gpu/drm/msm/dp/dp_hpd.c     | 19 +++++++++++++++++++
> >  drivers/gpu/drm/msm/dp/dp_hpd.h     |  4 ++++
> >  5 files changed, 43 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/msm/dp/dp_display.c b/drivers/gpu/drm/msm/dp/dp_display.c
> > index 7cc4d21f2091..124a2f794382 100644
> > --- a/drivers/gpu/drm/msm/dp/dp_display.c
> > +++ b/drivers/gpu/drm/msm/dp/dp_display.c
> > @@ -414,6 +414,13 @@ static int dp_display_usbpd_configure_cb(struct device *dev)
> >  	return dp_display_process_hpd_high(dp);
> >  }
> >  
> > +void dp_display_oob_hotplug_event(struct msm_dp *dp_display, bool hpd_state)
> > +{
> > +	struct dp_display_private *dp = container_of(dp_display, struct dp_display_private, dp_display);
> > +
> > +	dp->usbpd->oob_event(dp->usbpd, hpd_state);
> > +}
> > +
> >  static int dp_display_usbpd_disconnect_cb(struct device *dev)
> >  {
> >  	struct dp_display_private *dp = dev_get_dp_display_private(dev);
> > @@ -1251,6 +1258,7 @@ static int dp_display_probe(struct platform_device *pdev)
> >  	dp->pdev = pdev;
> >  	dp->name = "drm_dp";
> >  	dp->dp_display.connector_type = desc->connector_type;
> > +	dp->dp_display.dev = &pdev->dev;
> 
> You did not properly reference count this pointer you just saved.  What
> is to keep that pointer from going away without you knowing about it?
> 

The "dp" object only lives while &pdev->dev is alive, both logically and
as its devres allocated on &pdev-dev. So for this reference I don't see
that we should refcount it.

> And you already have a pointer to pdev, why save another one here?
> 

The Qualcomm DisplayPort driver has per-c-file private context structs
and "dp" is one such object. So I simply can't dereference it and get to
pdev from the other c-file in the same driver...

But I only need it in dp_drm.c to during initialization to get a
reference to the associated fwnode, so it seems that I can rework this
and pass the pointer as a parameter to dp_drm_connector_init().

That looks to be cleaner as well.

Thanks,
Bjorn