Re: [PATCH 17/20] drm: remove checks for same value in set_prop

[Sascha Hauer <s.hauer@pengutronix.de>]

On Wed, Feb 01, 2012 at 11:55:44AM +0000, Chris Wilson wrote:
> On Wed,  1 Feb 2012 11:38:35 +0100, Sascha Hauer <s.hauer@pengutronix.de> wrote:
> > The drivers currently check in set_property whether the property is
> > unchanged. move this check into the core and do not bother the drivers
> > with checking for unchanged properties.
> 
> This patch seems to have functional side-effects beyond the description.
> 
> For example,
> > diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> > index db3b461..0024b59 100644
> > --- a/drivers/gpu/drm/i915/intel_dp.c
> > +++ b/drivers/gpu/drm/i915/intel_dp.c
> > @@ -2223,9 +2223,6 @@ intel_dp_set_property(struct drm_connector *connector,
> >  		int i = val;
> >  		bool has_audio;
> >  
> > -		if (i == intel_dp->force_audio)
> > -			return 0;
> > -
> 
> Here we are checking against the current value of the intel_dp, which
> may in theory be modified elsewhere as well, and avoiding the modeswitch
> if it is unnecessary.

In case of force_audio I just checked that the current value is not
changed elsewhere, but I must admit that I haven't checked this for
all other properties. Would the patch be ok if I review for side
effects?

BTW if someone changes the underlying variable of a property outside
of the .set_property callback there are likely problems elsewhere
because without calling drm_connector_property_set_value the values
of the variable and the property backing store will be inconsistent.
The calls to drm_connector_property_set_value in turn are quite easy
reviewable, those are very few.

Sascha

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