linux-kernel - Re: [PATCH v8 1/4] drm/msm/dp: Add eDP support via aux

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Message-ID: <CAD=FV=UOALugpmgJK8dHxK=b_GO+c12d3xmGE3HiqJK+yznViA@mail.gmail.com>
Date:   Thu, 21 Apr 2022 09:07:44 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     "Sankeerth Billakanti (QUIC)" <quic_sbillaka@...cinc.com>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        freedreno <freedreno@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, Rob Clark <robdclark@...il.com>,
        Sean Paul <seanpaul@...omium.org>,
        Stephen Boyd <swboyd@...omium.org>,
        quic_kalyant <quic_kalyant@...cinc.com>,
        "Abhinav Kumar (QUIC)" <quic_abhinavk@...cinc.com>,
        "Kuogee Hsieh (QUIC)" <quic_khsieh@...cinc.com>,
        "bjorn.andersson@...aro.org" <bjorn.andersson@...aro.org>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        "dmitry.baryshkov@...aro.org" <dmitry.baryshkov@...aro.org>,
        quic_vproddut <quic_vproddut@...cinc.com>,
        "Aravind Venkateswaran (QUIC)" <quic_aravindh@...cinc.com>,
        Steev Klimaszewski <steev@...i.org>
Subject: Re: [PATCH v8 1/4] drm/msm/dp: Add eDP support via aux_bus

Hi,

On Thu, Apr 21, 2022 at 9:00 AM Sankeerth Billakanti (QUIC)
<quic_sbillaka@...cinc.com> wrote:
>
> >> +       }
> >> +
> >> +       /*
> >> +        * External bridges are mandatory for eDP interfaces: one has to
> >> +        * provide at least an eDP panel (which gets wrapped into panel-
> >bridge).
> >> +        *
> >> +        * For DisplayPort interfaces external bridges are optional, so
> >> +        * silently ignore an error if one is not present (-ENODEV).
> >> +        */
> >> +       rc = dp_parser_find_next_bridge(dp_priv->parser);
> >> +       if (rc && dp->is_edp) {
> >> +               DRM_ERROR("eDP: cannot find the next bridge, rc = %d\n", rc);
> >> +               goto edp_error;
> >> +       } else if (rc && rc != -ENODEV) {
> >> +               DRM_ERROR("DP: cannot find the next bridge, rc = %d\n", rc);
> >> +               goto error;
> >> +       }
> >
> >The above wouldn't be my favorite way of doing this. Instead, I would have
> >written:
> >
> >  if (rc) {
> >    DRM_ERROR("Cannot find the next bridge, rc = %d\n", rc);
> >    goto err;
> >  }
> >  ...
> >
> >err:
> >  if (dp->is_edp) {
> >    disable_irq(...);
> >    dp_display_host_phy_exit(...);
> >    dp_display_host_deinit(...);
> >  }
> >  return rc;
> >
>
> If rc is ENODEV for DP, then we need to return 0. Shall I add like below ?
>
> err:
>   if (dp->is_edp) {
>     disable_irq(...);
>     dp_display_host_phy_exit(...);
>     dp_display_host_deinit(...);
>   } else
>         If (rc == -ENODEV)
>             rc = 0;
>   return rc;

I wouldn't. Then you're essentially going to "err" for a case that you
don't consider an error. I would instead have just handled it right
away.

rc = dp_parser_find_next_bridge(dp_priv->parser);
if (!dp->is_edp && rc == -ENODEV)
  return 0;

This also is better IMO because it means you aren't assuming that
`dp_priv->parser->next_bridge` is "valid" (or at least NULL) after
dp_parser_find_next_bridge() returned an error.

-Doug