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

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Date:	Wed, 3 Jun 2015 22:49:53 +0200
From:	Antonio Ospite <ao2@....it>
To:	"Tirdea, Irina" <irina.tirdea@...el.com>
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

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.

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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