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

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Date:   Mon, 19 Jun 2023 20:20:08 +0200
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     Dan Carpenter <dan.carpenter@...aro.org>
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'

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 */
};

CJ