linux-kernel - Re: [PATCH v3 02/17] drm/bridge: Verify lane assignment is going to work during atomic

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Message-ID: <ZsTShZnD-NbZqGio@smile.fi.intel.com>
Date: Tue, 20 Aug 2024 20:29:41 +0300
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Stephen Boyd <swboyd@...omium.org>
Cc: chrome-platform@...ts.linux.dev, linux-kernel@...r.kernel.org,
	patches@...ts.linux.dev, devicetree@...r.kernel.org,
	Douglas Anderson <dianders@...omium.org>,
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	Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
	dri-devel@...ts.freedesktop.org,
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	Jernej Skrabec <jernej.skrabec@...il.com>,
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	Vinod Koul <vkoul@...nel.org>
Subject: Re: [PATCH v3 02/17] drm/bridge: Verify lane assignment is going to
 work during atomic_check

On Tue, Aug 20, 2024 at 10:24:47AM -0700, Stephen Boyd wrote:
> Quoting Andy Shevchenko (2024-08-20 10:17:46)
> > On Tue, Aug 20, 2024 at 10:12:55AM -0700, Stephen Boyd wrote:
> > > Quoting Andy Shevchenko (2024-08-20 03:09:29)
> > > > On Mon, Aug 19, 2024 at 03:38:16PM -0700, Stephen Boyd wrote:

...

> > > > > +     /*
> > > > > +      * Ensure this bridge is aware that the next bridge wants to
> > > > > +      * reassign lanes.
> > > > > +      */
> > > > > +     for (i = 0; i < num_input_lanes; i++)
> > > > > +             if (i != input_lanes[i].logical && !num_output_lanes)
> > > > > +                     return -ENOTSUPP;
> > > >
> > > > Besides missing {} this code is internal to the Linux kernel. Is it okay?
> > >
> > > ENOTSUPP is used by select_bus_fmt_recursive() so I simply followed that
> > > style.
> >
> > Okay, just be aware of that side effect of that code, also checkpatch may
> > complain (however it might be false positive).
> 
> Yes checkpatch complained but didn't enlighten me. Please tell me the
> side effect as I'm unaware!

I already told you above, if this code ever appears in user space it will be
printed as a number and very much confuse the user!

That's why usage of this code either has to be documented or be subsystem
_known_ practice (GPIO library comes to my mind as it uses it internally,\
but filters for user space).

-- 
With Best Regards,
Andy Shevchenko