linux-kernel - Re: [PATCH] drm/bridge: ti-sn65dsi83: use dev_err

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Message-ID: <20240819133840.26045152@booty>
Date: Mon, 19 Aug 2024 13:38:40 +0200
From: Luca Ceresoli <luca.ceresoli@...tlin.com>
To: Andi Shyti <andi.shyti@...ux.intel.com>
Cc: Andrzej Hajda <andrzej.hajda@...el.com>, Neil Armstrong
 <neil.armstrong@...aro.org>, Robert Foss <rfoss@...nel.org>, Laurent
 Pinchart <Laurent.pinchart@...asonboard.com>, Jonas Karlman
 <jonas@...boo.se>, Jernej Skrabec <jernej.skrabec@...il.com>, Maarten
 Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard
 <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, David Airlie
 <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>, Thomas Petazzoni
 <thomas.petazzoni@...tlin.com>, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/bridge: ti-sn65dsi83: use dev_err_probe when
 failing to get panel bridge

Hello Andi,

thanks for your additional comments.

On Tue, 13 Aug 2024 16:32:50 +0200
Andi Shyti <andi.shyti@...ux.intel.com> wrote:

> Hi Luca,
> 
> On Tue, Aug 13, 2024 at 10:16:43AM +0200, Luca Ceresoli wrote:
> > On Thu, 8 Aug 2024 11:35:23 +0100
> > Andi Shyti <andi.shyti@...ux.intel.com> wrote:  
> > > On Thu, Aug 08, 2024 at 12:26:14PM +0200, Luca Ceresoli wrote:  
> > > > When devm_drm_of_get_bridge() fails, the probe fails silently. Use
> > > > dev_err_probe() instead to log an error or report the deferral reason,
> > > > whichever is applicable.
> > > > 
> > > > Signed-off-by: Luca Ceresoli <luca.ceresoli@...tlin.com>
> > > > ---
> > > >  drivers/gpu/drm/bridge/ti-sn65dsi83.c | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git a/drivers/gpu/drm/bridge/ti-sn65dsi83.c b/drivers/gpu/drm/bridge/ti-sn65dsi83.c
> > > > index 57a7ed13f996..60b9f14d769a 100644
> > > > --- a/drivers/gpu/drm/bridge/ti-sn65dsi83.c
> > > > +++ b/drivers/gpu/drm/bridge/ti-sn65dsi83.c
> > > > @@ -606,7 +606,7 @@ static int sn65dsi83_parse_dt(struct sn65dsi83 *ctx, enum sn65dsi83_model model)
> > > >  
> > > >  	panel_bridge = devm_drm_of_get_bridge(dev, dev->of_node, 2, 0);
> > > >  	if (IS_ERR(panel_bridge))
> > > > -		return PTR_ERR(panel_bridge);
> > > > +		return dev_err_probe(dev, PTR_ERR(panel_bridge), "Failed to get panel bridge\n");    
> > > 
> > > patch looks good, but the message is a bit misleading. You are
> > > not failing to get the panel bridge, but you are failing to find
> > > a panel bridge in a DT node. Right?  
> > 
> > As I can see from both the documentation and the code,
> > devm_drm_of_get_bridge() is really returning a pointer to a panel
> > bridge, potentially allocating and adding it in case it was not present
> > before. Navigating the device tree is only a part of what it does.
> > 
> > Do you think I am missing something?  
> 
> No, maybe it's me being a bit pedantic. In the sense that we are
> not really failing to get the panel, but most probably the panel
> is not installed.

The panels I'm used to, which I believe to be the most common in
embedded systems just have no way of being detected, so the operating
system cannot detect a "panel not installed" condition.

However I went back to the code and realized your initial remark ("you
are failing to find a panel bridge in a DT node") is more correct than
I initially thought. Indeed there are two failure reasons for
devm_drm_of_get_bridge() to fail: DT lookup and panel bridge creation
failures. The latter however can be due to -ENOMEM (unlikely) or
(panel->connector_type == DRM_MODE_CONNECTOR_Unknown), which in turn
can be due to either a panel driver error or again a DT error in case
the driver gets the panel type from DT, as panel-simple.c does.

That said, the role of devm_drm_of_get_bridge() is to provide a panel
bridge object. If it fails, that means it is unable to provide such an
object for whatever reason. Reasons currently include DT issues (the
most likely), driver bug and -ENOMEM. There could be more reasons in
future versions of the implementation.

I'm afraid I'm unable to express all the above logic in a single commit
title line. However, should you have a better commit title or message
to suggest, I'm still open to improvements. I value good commit
messages.

> I'm not strong on this comment, though, so that
> feel free to add:
> 
> Reviewed-by: Andi Shyti <andi.shyti@...ux.intel.com>

Otherwise, I'm sending v2 with your review tag by the end of the week.

Luca

-- 
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com