linux-kernel - Re: [PATCH v3 2/3] drm/bridge: parade-ps8640: Use regmap APIs

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Message-ID: <CAE-0n539tJLLWHdL65ZU_1qOzA-RsEqGqVi-19VLHz_W5dT6VA@mail.gmail.com>
Date:   Tue, 14 Sep 2021 19:50:10 -0700
From:   Stephen Boyd <swboyd@...omium.org>
To:     Doug Anderson <dianders@...omium.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Philip Chen <philipchen@...omium.org>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Jonas Karlman <jonas@...boo.se>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Robert Foss <robert.foss@...aro.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH v3 2/3] drm/bridge: parade-ps8640: Use regmap APIs

Quoting Doug Anderson (2021-09-14 19:17:03)
> Hi,
>
> On Tue, Sep 14, 2021 at 5:29 PM Stephen Boyd <swboyd@...omium.org> wrote:
> >
> > Quoting Philip Chen (2021-09-14 16:28:44)
> > > diff --git a/drivers/gpu/drm/bridge/parade-ps8640.c b/drivers/gpu/drm/bridge/parade-ps8640.c
> > > index e340af381e05..8d3e7a147170 100644
> > > --- a/drivers/gpu/drm/bridge/parade-ps8640.c
> > > +++ b/drivers/gpu/drm/bridge/parade-ps8640.c
> > > @@ -368,6 +396,12 @@ static int ps8640_probe(struct i2c_client *client)
> > >
> > >         ps_bridge->page[PAGE0_DP_CNTL] = client;
> > >
> > > +       ps_bridge->regmap[PAGE0_DP_CNTL] = devm_regmap_init_i2c(client, ps8640_regmap_config);
> > > +       if (IS_ERR(ps_bridge->regmap[PAGE0_DP_CNTL])) {
> > > +               return dev_err_probe(dev, PTR_ERR(ps_bridge->regmap[PAGE0_DP_CNTL]),
> > > +                                    "Error initting page 0 regmap\n");
> >
> > This one also doesn't return -EPROBE_DEFER? The dev_err_probe() should
> > really only be used on "get" style APIs that can defer.
>
> Any reason why you say that dev_err_probe() should only be used on
> "get" style APIs that can defer? Even if an API can't return
> -EPROBE_DEFER, using dev_err_probe() still (IMO) makes the code
> cleaner and should be used for any error cases like this during probe.
> Why?
>
> * It shows the error code in a standard way for you.
> * It returns the error code you passed it so you can make your error
> return "one line" instead of 2.

I'd rather see any sort of error message in getter APIs be pushed into
the callee so that we reduce the text size of the kernel by having one
message instead of hundreds/thousands about "failure to get something".
As far as I can tell this API is designed to skip printing anything when
EPROBE_DEFER is returned, and only print something when it isn't that
particular error code. The other benefit of this API is it sets the
deferred reason in debugfs which is nice to know why some device failed
to probe. Of course now with fw_devlink that almost never triggers so
the feature is becoming useless.

>
> Is there some bad thing about dev_err_probe() that makes it
> problematic to use? If not then the above advantages should be a net
> win, right?
>

I view it as an anti-pattern. We should strive for driver probe to be
fairly simple so that it's basically getting resources and registering
with frameworks. The error messages in probe may help when you're trying
to get the driver to work and the resource APIs don't make any sense but
after that it's basically debug messages hiding as error messages.
They're never supposed to happen in practice, because the code is
tested, right?