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Message-Id: <a804f2e91bc32cc5e17e012ed90972415606db4e.1687075665.git.christophe.jaillet@wanadoo.fr>
Date:   Sun, 18 Jun 2023 10:08:07 +0200
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     Jiri Kosina <jikos@...nel.org>,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        linux-input@...r.kernel.org
Subject: [PATCH] HID: Reorder fields in 'struct hid_field'

Group some variables based on their sizes to reduce hole and avoid padding.
On x86_64, this shrinks the size of 'struct hid_field'
from 136 to 128 bytes.

It saves a few bytes of memory and is more cache-line friendly.

Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
---
Using pahole

Before:
======
struct hid_field {
	unsigned int               physical;             /*     0     4 */
	unsigned int               logical;              /*     4     4 */
	unsigned int               application;          /*     8     4 */

	/* XXX 4 bytes hole, try to pack */

	struct hid_usage *         usage;                /*    16     8 */
	unsigned int               maxusage;             /*    24     4 */
	unsigned int               flags;                /*    28     4 */
	unsigned int               report_offset;        /*    32     4 */
	unsigned int               report_size;          /*    36     4 */
	unsigned int               report_count;         /*    40     4 */
	unsigned int               report_type;          /*    44     4 */
	__s32 *                    value;                /*    48     8 */
	__s32 *                    new_value;            /*    56     8 */
	/* --- cacheline 1 boundary (64 bytes) --- */
	__s32 *                    usages_priorities;    /*    64     8 */
	__s32                      logical_minimum;      /*    72     4 */
	__s32                      logical_maximum;      /*    76     4 */
	__s32                      physical_minimum;     /*    80     4 */
	__s32                      physical_maximum;     /*    84     4 */
	__s32                      unit_exponent;        /*    88     4 */
	unsigned int               unit;                 /*    92     4 */
	bool                       ignored;              /*    96     1 */

	/* XXX 7 bytes hole, try to pack */

	struct hid_report *        report;               /*   104     8 */
	unsigned int               index;                /*   112     4 */

	/* XXX 4 bytes hole, try to pack */

	struct hid_input *         hidinput;             /*   120     8 */
	/* --- cacheline 2 boundary (128 bytes) --- */
	__u16                      dpad;                 /*   128     2 */

	/* XXX 2 bytes hole, try to pack */

	unsigned int               slot_idx;             /*   132     4 */

	/* size: 136, cachelines: 3, members: 25 */
	/* sum members: 119, holes: 4, sum holes: 17 */
	/* last cacheline: 8 bytes */
};


After:
=====
struct hid_field {
	unsigned int               physical;             /*     0     4 */
	unsigned int               logical;              /*     4     4 */
	unsigned int               application;          /*     8     4 */

	/* XXX 4 bytes hole, try to pack */

	struct hid_usage *         usage;                /*    16     8 */
	unsigned int               maxusage;             /*    24     4 */
	unsigned int               flags;                /*    28     4 */
	unsigned int               report_offset;        /*    32     4 */
	unsigned int               report_size;          /*    36     4 */
	unsigned int               report_count;         /*    40     4 */
	unsigned int               report_type;          /*    44     4 */
	__s32 *                    value;                /*    48     8 */
	__s32 *                    new_value;            /*    56     8 */
	/* --- cacheline 1 boundary (64 bytes) --- */
	__s32 *                    usages_priorities;    /*    64     8 */
	__s32                      logical_minimum;      /*    72     4 */
	__s32                      logical_maximum;      /*    76     4 */
	__s32                      physical_minimum;     /*    80     4 */
	__s32                      physical_maximum;     /*    84     4 */
	__s32                      unit_exponent;        /*    88     4 */
	unsigned int               unit;                 /*    92     4 */
	struct hid_report *        report;               /*    96     8 */
	unsigned int               index;                /*   104     4 */
	bool                       ignored;              /*   108     1 */

	/* XXX 3 bytes hole, try to pack */

	struct hid_input *         hidinput;             /*   112     8 */
	__u16                      dpad;                 /*   120     2 */

	/* XXX 2 bytes hole, try to pack */

	unsigned int               slot_idx;             /*   124     4 */

	/* size: 128, cachelines: 2, members: 25 */
	/* sum members: 119, holes: 3, sum holes: 9 */
};
---
 include/linux/hid.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

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 */
-- 
2.34.1

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