linux-kernel - [PATCH] xhci: Shrink struct xhci_interval

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Date:   Fri,  3 Nov 2023 07:51:46 +0100
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     rostedt@...dmis.org, gregkh@...uxfoundation.org,
        keescook@...omium.org, willy@...radead.org,
        senozhatsky@...omium.org
Cc:     list@...l.com, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>
Subject: [PATCH] xhci: Shrink struct xhci_interval_bw

The struct xhci_interval_bw can be reordered to shrink it from 40 to 32
bytes.
This makes sense because it is used in struct xhci_interval_bw_table in an
array of XHCI_MAX_INTERVAL (i.e. 16) values. So this struct is shrunk by
128 bytes and is more cacheline friendly.

On x86_64 we have:

struct xhci_interval_bw {
	unsigned int               num_packets;          /*     0     4 */

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

	struct list_head           endpoints;            /*     8    16 */
	unsigned int               overhead[3];          /*    24    12 */

	/* size: 40, cachelines: 1, members: 3 */
	/* sum members: 32, holes: 1, sum holes: 4 */
	/* padding: 4 */
	/* last cacheline: 40 bytes */
};

After the change:
struct xhci_interval_bw {
	struct list_head           endpoints;            /*     0    16 */
	unsigned int               num_packets;          /*    16     4 */
	unsigned int               overhead[3];          /*    20    12 */

	/* size: 32, cachelines: 1, members: 3 */
	/* last cacheline: 32 bytes */
};

Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
---
 drivers/usb/host/xhci.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/usb/host/xhci.h b/drivers/usb/host/xhci.h
index 3ea5c092bba7..0430646a3a05 100644
--- a/drivers/usb/host/xhci.h
+++ b/drivers/usb/host/xhci.h
@@ -956,13 +956,13 @@ enum xhci_overhead_type {
 };
 
 struct xhci_interval_bw {
-	unsigned int		num_packets;
 	/* Sorted by max packet size.
 	 * Head of the list is the greatest max packet size.
 	 */
 	struct list_head	endpoints;
 	/* How many endpoints of each speed are present. */
 	unsigned int		overhead[3];
+	unsigned int		num_packets;
 };
 
 #define	XHCI_MAX_INTERVAL	16
-- 
2.34.1