linux-kernel - Re: [Intel-gfx] [PATCH] drm/i915: Fix lock dropping in intel_tv

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Date:	Mon, 1 Sep 2014 12:45:56 +0300
From:	Ville Syrjälä <ville.syrjala@...ux.intel.com>
To:	Chris Wilson <chris@...is-wilson.co.uk>,
	intel-gfx@...ts.freedesktop.org, David Airlie <airlied@...ux.ie>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	linux-kernel@...r.kernel.org, Tibor Billes <tbilles@....com>
Subject: Re: [Intel-gfx] [PATCH] drm/i915: Fix lock dropping in
 intel_tv_detect()

On Mon, Sep 01, 2014 at 09:50:22AM +0100, Chris Wilson wrote:
> On Mon, Sep 01, 2014 at 11:09:46AM +0300, ville.syrjala@...ux.intel.com wrote:
> > From: Ville Syrjälä <ville.syrjala@...ux.intel.com>
> > 
> > When intel_tv_detect() fails to do load detection it would forget to
> > drop the locks and clean up the acquire context. Fix it up.
> > 
> > This is a regression from:
> >  commit 208bf9fdcd3575aa4a5d48b3e0295f7cdaf6fc44
> >  Author: Ville Syrjälä <ville.syrjala@...ux.intel.com>
> >  Date:   Mon Aug 11 13:15:35 2014 +0300
> > 
> >     drm/i915: Fix locking for intel_enable_pipe_a()
> > 
> > Cc: Tibor Billes <tbilles@....com>
> > Signed-off-by: Ville Syrjälä <ville.syrjala@...ux.intel.com>
> > ---
> >  drivers/gpu/drm/i915/intel_tv.c | 15 +++++++++------
> >  1 file changed, 9 insertions(+), 6 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/intel_tv.c b/drivers/gpu/drm/i915/intel_tv.c
> > index 32186a6..abbf0ea 100644
> > --- a/drivers/gpu/drm/i915/intel_tv.c
> > +++ b/drivers/gpu/drm/i915/intel_tv.c
> > @@ -1311,7 +1311,8 @@ intel_tv_detect(struct drm_connector *connector, bool force)
> >  {
> >  	struct drm_display_mode mode;
> >  	struct intel_tv *intel_tv = intel_attached_tv(connector);
> > -	int type;
> > +	enum drm_connector_status status = connector->status;
> > +	int type = -1;
> >  
> >  	DRM_DEBUG_KMS("[CONNECTOR:%d:%s] force=%d\n",
> >  		      connector->base.id, connector->name,
> > @@ -1328,21 +1329,23 @@ intel_tv_detect(struct drm_connector *connector, bool force)
> >  		if (intel_get_load_detect_pipe(connector, &mode, &tmp, &ctx)) {
> >  			type = intel_tv_detect_type(intel_tv, connector);
> >  			intel_release_load_detect_pipe(connector, &tmp);
> > +			status = type < 0 ?
> > +				connector_status_disconnected :
> > +				connector_status_connected;
> >  		} else
> > -			return connector_status_unknown;
> > +			status = connector_status_unknown;
> >  
> >  		drm_modeset_drop_locks(&ctx);
> >  		drm_modeset_acquire_fini(&ctx);
> > -	} else
> > -		return connector->status;
> > +	}
> >  
> >  	if (type < 0)
> > -		return connector_status_disconnected;
> > +		return status;
> >  
> >  	intel_tv->type = type;
> >  	intel_tv_find_better_format(connector);
> >  
> > -	return connector_status_connected;
> > +	return status;
> 
> Hmm, this makes the code unclear. if type != -1, status should always be
> connected.
> 
> I think:
> 
> if (status != connector_status_connected)
>   return status;
> 
> if (WARN_ON(type < 0))
>    return connector_status_disconnected;
> 
> intel_tv->type = type;
> find_better_format();
> return connector_status_connected;
> 
> And leave type unset to encourage gcc.
> 
> 
> 
> diff --git a/drivers/gpu/drm/i915/intel_tv.c b/drivers/gpu/drm/i915/intel_tv.c
> index 32186a6..a109b7b 100644
> --- a/drivers/gpu/drm/i915/intel_tv.c
> +++ b/drivers/gpu/drm/i915/intel_tv.c
> @@ -1311,6 +1311,7 @@ intel_tv_detect(struct drm_connector *connector, bool force)
>  {
>         struct drm_display_mode mode;
>         struct intel_tv *intel_tv = intel_attached_tv(connector);
> +       enum drm_connector_status status = connector->status;
>         int type;
>  
>         DRM_DEBUG_KMS("[CONNECTOR:%d:%s] force=%d\n",
> @@ -1327,17 +1328,20 @@ intel_tv_detect(struct drm_connector *connector, bool force)
>  
>                 if (intel_get_load_detect_pipe(connector, &mode, &tmp, &ctx)) {
>                         type = intel_tv_detect_type(intel_tv, connector);
> +                       status = type < 0 ?
> +                               connector_status_disconnected :
> +                               connector_status_connected;
> +
>                         intel_release_load_detect_pipe(connector, &tmp);
>                 } else
> -                       return connector_status_unknown;
> +                       status = connector_status_unknown;
>  
>                 drm_modeset_drop_locks(&ctx);
>                 drm_modeset_acquire_fini(&ctx);
> -       } else
> -               return connector->status;
> +       }
>  
> -       if (type < 0)
> -               return connector_status_disconnected;
> +       if (status != connector_status_connected)
> +               return status;
>  
>         intel_tv->type = type;
>         intel_tv_find_better_format(connector);

With this approach we're going to have to keep the !force else branch
to avoid populating intel_tv->type with stack garbage. But I suppose
the resulting code might still be a bit clearer.

-- 
Ville Syrjälä
Intel OTC
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