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

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Message-ID: <CAE-0n53JBtB0YAZxMhLVJhbsUNZo4JPyXUiHu=9EQT4Bx+tHPA@mail.gmail.com>
Date: Tue, 20 Aug 2024 10:33:14 -0700
From: Stephen Boyd <swboyd@...omium.org>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: chrome-platform@...ts.linux.dev, linux-kernel@...r.kernel.org, 
	patches@...ts.linux.dev, devicetree@...r.kernel.org, 
	Douglas Anderson <dianders@...omium.org>, Pin-yen Lin <treapking@...omium.org>, 
	Andrzej Hajda <andrzej.hajda@...el.com>, Benson Leung <bleung@...omium.org>, 
	Conor Dooley <conor+dt@...nel.org>, Daniel Vetter <daniel@...ll.ch>, David Airlie <airlied@...il.com>, 
	Dmitry Baryshkov <dmitry.baryshkov@...aro.org>, dri-devel@...ts.freedesktop.org, 
	Guenter Roeck <groeck@...omium.org>, Jernej Skrabec <jernej.skrabec@...il.com>, 
	Jonas Karlman <jonas@...boo.se>, Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>, 
	Laurent Pinchart <Laurent.pinchart@...asonboard.com>, Lee Jones <lee@...nel.org>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
	Neil Armstrong <neil.armstrong@...aro.org>, Prashant Malani <pmalani@...omium.org>, 
	Robert Foss <rfoss@...nel.org>, Rob Herring <robh+dt@...nel.org>, 
	Thomas Zimmermann <tzimmermann@...e.de>, Tzung-Bi Shih <tzungbi@...nel.org>, 
	Alexandre Belloni <alexandre.belloni@...tlin.com>, Daniel Scally <djrscally@...il.com>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	Heikki Krogerus <heikki.krogerus@...ux.intel.com>, Ivan Orlov <ivan.orlov0322@...il.com>, 
	linux-acpi@...r.kernel.org, linux-usb@...r.kernel.org, 
	Mika Westerberg <mika.westerberg@...ux.intel.com>, 
	"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>, Sakari Ailus <sakari.ailus@...ux.intel.com>, 
	Vinod Koul <vkoul@...nel.org>
Subject: Re: [PATCH v3 02/17] drm/bridge: Verify lane assignment is going to
 work during atomic_check

Quoting Andy Shevchenko (2024-08-20 10:29:41)
> 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).
>

Ok, got it. Thanks!