linux-kernel - RE: [PATCH 2/9] input: goodix: fix variable length array warning

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Date:	Fri, 5 Jun 2015 16:34:38 +0000
From:	"Tirdea, Irina" <irina.tirdea@...el.com>
To:	'Antonio Ospite' <ao2@....it>
CC:	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Bastien Nocera <hadess@...ess.net>,
	"linux-input@...r.kernel.org" <linux-input@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 2/9] input: goodix: fix variable length array warning



> -----Original Message-----
> From: linux-input-owner@...r.kernel.org [mailto:linux-input-owner@...r.kernel.org] On Behalf Of Antonio Ospite
> Sent: 03 June, 2015 23:50
> To: Tirdea, Irina
> Cc: Dmitry Torokhov; Bastien Nocera; linux-input@...r.kernel.org; devicetree@...r.kernel.org; linux-kernel@...r.kernel.org
> Subject: Re: [PATCH 2/9] input: goodix: fix variable length array warning
> 
> On Wed, 3 Jun 2015 10:26:47 +0000
> "Tirdea, Irina" <irina.tirdea@...el.com> wrote:
> 
> > > -----Original Message-----
> > > From: Antonio Ospite [mailto:ao2@....it]
> > > Sent: 28 May, 2015 18:58
> > > To: Tirdea, Irina
> > > Cc: Dmitry Torokhov; Bastien Nocera; linux-input@...r.kernel.org; devicetree@...r.kernel.org; linux-kernel@...r.kernel.org
> > > Subject: Re: [PATCH 2/9] input: goodix: fix variable length array warning
> > >
> > > On Thu, 28 May 2015 15:47:38 +0300
> > > Irina Tirdea <irina.tirdea@...el.com> wrote:
> > >
> > > > Fix sparse warning:
> > > > drivers/input/touchscreen/goodix.c:182:26: warning:
> > > > Variable length array is used.
> > > >
> > > > Replace the variable length array with fixed length.
> > > >
> > > > Signed-off-by: Irina Tirdea <irina.tirdea@...el.com>
> > > > ---
> > > >  drivers/input/touchscreen/goodix.c | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/drivers/input/touchscreen/goodix.c b/drivers/input/touchscreen/goodix.c
> > > > index c2e785c..dac1b3c 100644
> > > > --- a/drivers/input/touchscreen/goodix.c
> > > > +++ b/drivers/input/touchscreen/goodix.c
> > > > @@ -147,7 +147,7 @@ static void goodix_ts_report_touch(struct goodix_ts_data *ts, u8 *coor_data)
> > > >   */
> > > >  static void goodix_process_events(struct goodix_ts_data *ts)
> > > >  {
> > > > -	u8  point_data[1 + GOODIX_CONTACT_SIZE * ts->max_touch_num];
> > > > +	u8  point_data[1 + GOODIX_CONTACT_SIZE * GOODIX_MAX_CONTACTS];
> > >
> > > Hi,
> > >
> >
> > Hi Antonio,
> >
> > > this fixes the warning from sparse, but also changes the semantics of
> > > the code: ts->max_touch_num is less that GOODIX_MAX_CONTACTS for 5
> > > touches devices and in this case we'll end up using more memory than is
> > > necessary.
> > >
> >
> > I wasn't sure if it is better to save the 5 bytes or fix the warning,
> > so I sent this to get some more input.
> > Thanks for the feedback, I will  drop this patch.
> >
> 
> Use kmalloc() or, alternatively, add at least a comment telling why you
> think that sacrificing a few bytes —only for some devices— has
> advantages over dynamic allocation.
> 

You are right, kmalloc will solve both problems - the sparse warning and allocating
more bytes than necessary. Don't know why I did not think of that.
Will use that in v2.

Thanks,
Irina

> I am not necessarily against the static allocation change, I was just
> pointing out the issue.
> 
> Thanks,
>    Antonio
> 
> --
> Antonio Ospite
> http://ao2.it
> 
> A: Because it messes up the order in which people normally read text.
>    See http://en.wikipedia.org/wiki/Posting_style
> Q: Why is top-posting such a bad thing?
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