linux-kernel - Re: [PATCH 2/2] drm/bridge: ti-sn65dsi83: protect device resources on unplug

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Message-ID: <20250908154901.64f1a639@booty>
Date: Mon, 8 Sep 2025 15:49:01 +0200
From: Luca Ceresoli <luca.ceresoli@...tlin.com>
To: Maxime Ripard <mripard@...nel.org>
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>, Thomas Zimmermann
 <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, Simona Vetter
 <simona@...ll.ch>, Hui Pu <Hui.Pu@...ealthcare.com>, Thomas Petazzoni
 <thomas.petazzoni@...tlin.com>, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org, Dmitry Baryshkov <lumag@...nel.org>
Subject: Re: [PATCH 2/2] drm/bridge: ti-sn65dsi83: protect device resources
 on unplug

Hello Maxime,

On Wed, 20 Aug 2025 13:13:02 +0200
Luca Ceresoli <luca.ceresoli@...tlin.com> wrote:

> > > +	/*
> > > +	 * sn65dsi83_atomic_disable() should release some resources, but it
> > > +	 * cannot if we call drm_bridge_unplug() before it can
> > > +	 * drm_bridge_enter(). If that happens, let's release those
> > > +	 * resources now.
> > > +	 */
> > > +	if (ctx->disable_resources_needed) {
> > > +		if (!ctx->irq)
> > > +			sn65dsi83_monitor_stop(ctx);
> > > +
> > > +		gpiod_set_value_cansleep(ctx->enable_gpio, 0);
> > > +		usleep_range(10000, 11000);
> > > +
> > > +		regulator_disable(ctx->vcc);
> > > +	}    
> > 
> > I'm not sure you need this. Wouldn't registering a devm action do the
> > same thing?  
> 
> Good idea, thanks. I'll give it a try.

I'm catching up with this series after being busy a few weeks...

I looked at this, but contrary my initial impression I think it would
not be an improvement.

The reason is at least one of these cleanup actions (namely the
regulator_disable()) must be done only if there is a matching enable,
which is in atomic_pre_enable. This is why I introduced a flag in the
first place.

I'm not sure which usage of devres you had in mind, but I see two
options.

Option 1: in probe, add a devres action to call a function like:

sn65dsi83_cleanups()
{
	if (ctx->disable_resources_needed) {
		/* the same cleanups */
	}    
}

But that is just a more indirect way of doing the same thing, and
relies on the same flag.

Option 2: have a function to unconditionally do the cleanups:

sn65dsi83_cleanups()
{
	/* the same cleanups (no if) */
}

And then:
 * in atomic_pre_enable, instead of setting the flag
   add a devres action to call sn65dsi83_cleanups()
 * in atomic_disable, instead of clearing the flag
   remove the devres action

Even this option looks like more complicated and less readable code
to do the same thing.

Do you have in mind a better option that I haven't figured out?

If you don't, I think this part of the patch should stay as is.

Luca

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