linux-kernel - Re: [PATCH v2 01/24] input: Add ff-memless-next module

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Message-ID: <20140423172413.GA18181@core.coreip.homeip.net>
Date:	Wed, 23 Apr 2014 10:24:13 -0700
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	Oliver Neukum <oneukum@...e.de>
Cc:	Michal Malý <madcatxster@...oid-pointer.net>,
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
	jkosina@...e.cz, elias.vds@...il.com, anssi.hannula@....fi,
	simon@...gewell.org
Subject: Re: [PATCH v2 01/24] input: Add ff-memless-next module

On Wed, Apr 23, 2014 at 07:06:08PM +0200, Oliver Neukum wrote:
> On Wed, 2014-04-23 at 08:59 -0700, Dmitry Torokhov wrote:
> > On Wed, Apr 23, 2014 at 02:12:59PM +0200, Oliver Neukum wrote:
> > > On Tue, 2014-04-22 at 15:59 +0200, Michal Malý wrote:
> > > > +/* Some devices might have a limit on how many uncombinable effects
> > > > + * can be played at once */
> > > > +static int mlnx_upload_conditional(struct mlnx_device *mlnxdev,
> > > > +                                  const struct ff_effect *effect)
> > > > +{
> > > > +       struct mlnx_effect_command ecmd = {
> > > > +               .cmd = MLNX_UPLOAD_UNCOMB,
> > > > +               .u.uncomb.id = effect->id,
> > > > +               .u.uncomb.effect = effect
> > > > +       };
> > > > +       return mlnxdev->control_effect(mlnxdev->dev, mlnxdev->private,
> > > > &ecmd);
> > > > +}
> > > > +
> > > 
> > > This mean you are building the structure on the stack
> > > 
> > > 1. Are you sure nobody retains a reference?
> > > 2. That is needlessly inefficient
> > 
> > Why is it inefficient?
> 
> The compiler has to include the data structure and then make
> a memcopy to the stack. Instead a pointer to the predined structure
> could be passed.

No, it does not do that. Memory on stack is reserved and zeroed out,
then individual members are filled in as requested. There is no memcopy.

Thanks.

-- 
Dmitry
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