linux-kernel - Re: [PATCH] HID: Reorder fields in 'struct hid

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Date:   Tue, 20 Jun 2023 08:09:03 +0300
From:   Dan Carpenter <dan.carpenter@...aro.org>
To:     Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc:     Jiri Kosina <jikos@...nel.org>,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
        linux-input@...r.kernel.org
Subject: Re: [PATCH] HID: Reorder fields in 'struct hid_field'

On Mon, Jun 19, 2023 at 08:20:08PM +0200, Christophe JAILLET wrote:
> Le 19/06/2023 à 07:18, Dan Carpenter a écrit :
> > On Sun, Jun 18, 2023 at 10:08:07AM +0200, Christophe JAILLET wrote:
> > > diff --git a/include/linux/hid.h b/include/linux/hid.h
> > > index 39e21e3815ad..5be5e671c263 100644
> > > --- a/include/linux/hid.h
> > > +++ b/include/linux/hid.h
> > > @@ -480,9 +480,9 @@ struct hid_field {
> > >   	__s32     physical_maximum;
> > >   	__s32     unit_exponent;
> > >   	unsigned  unit;
> > > -	bool      ignored;		/* this field is ignored in this event */
> > >   	struct hid_report *report;	/* associated report */
> > >   	unsigned index;			/* index into report->field[] */
> > > +	bool      ignored;		/* this field is ignored in this event */
> > >   	/* hidinput data */
> > >   	struct hid_input *hidinput;	/* associated input structure */
> > >   	__u16 dpad;			/* dpad input code */
> > 
> > You could move the dpad next to the ignored to save another 4 bytes.
> > I think it is still grouped logically that way but I don't really know
> > what dpad is so I might be wrong.
> > 
> >    	struct hid_report *report;	/* associated report */
> >    	unsigned index;			/* index into report->field[] */
> > 	bool      ignored;		/* this field is ignored in this event */
> >   	/* hidinput data */
> >    	__u16 dpad;			/* dpad input code */
> >    	struct hid_input *hidinput;	/* associated input structure */
> > 
> > regards,
> > dan carpenter
> > 
> > 
> 
> In fact, not really,
> It would fill a hole better, but would generate some padding at the end of
> the struct:
> 
> 	bool                       ignored;              /*   108     1 */
> 
> 	/* XXX 1 byte hole, try to pack */
> 
> 	__u16                      dpad;                 /*   110     2 */
> 	struct hid_input *         hidinput;             /*   112     8 */
> 	unsigned int               slot_idx;             /*   120     4 */
> 
> 	/* size: 128, cachelines: 2, members: 25 */
> 	/* sum members: 119, holes: 2, sum holes: 5 */
> 	/* padding: 4 */
> };

Ah.  Right.  I should have looked at that more closely.

regards,
dan carpenter