[PATCH] driver core/platform: remove unused implicit padding in platform_object

Yann Droneaud
2014-05-30T20:02:47Z

Up to 7 bytes are wasted at the end of struct platform_object
in the form of padding after name field: unfortunately this
padding is not used when allocating the memory to hold the
name.

This patch converts name array from name[1] to C99 flexible
array name[] (equivalent to name[0]) so that no padding is
required by the presence of this field. Memory allocation
is updated to take care of allocating an additional byte for
the NUL terminating character.

Built on Fedora 20, using GCC 4.8, for ARM, i386, SPARC64 and
x86_64 architectures, the data structure layout can be reported
with following command:

$ pahole drivers/base/platform.o \
--recursive \
--class_name device,pdev_archdata,platform_device,platform_object

Please find below some comparisons of structure layout for arm,
i386, sparc64 and x86_64 architecture before and after the patch.

--- obj-arm/drivers/base/platform.o.pahole.v3.15-rc7-79-gfe45736f4134 2014-05-30 10:32:06.290960701 +0200
+++ obj-arm/drivers/base/platform.o.pahole.v3.15-rc7-80-g2cdb06858d71 2014-05-30 11:26:20.851988347 +0200
@@ -81,10 +81,9 @@
/* XXX last struct has 4 bytes of padding */

/* --- cacheline 6 boundary (384 bytes) was 8 bytes ago --- */
- char name[1]; /* 392 1 */
+ char name[0]; /* 392 0 */

- /* size: 400, cachelines: 7, members: 2 */
- /* padding: 7 */
+ /* size: 392, cachelines: 7, members: 2 */
/* paddings: 1, sum paddings: 4 */
- /* last cacheline: 16 bytes */
+ /* last cacheline: 8 bytes */
};

--- obj-i386/drivers/base/platform.o.pahole.v3.15-rc7-79-gfe45736f4134 2014-05-30 10:32:06.305960691 +0200
+++ obj-i386/drivers/base/platform.o.pahole.v3.15-rc7-80-g2cdb06858d71 2014-05-30 11:26:20.875988332 +0200
@@ -73,9 +73,8 @@
struct platform_object {
struct platform_device pdev; /* 0 396 */
/* --- cacheline 6 boundary (384 bytes) was 12 bytes ago --- */
- char name[1]; /* 396 1 */
+ char name[0]; /* 396 0 */

- /* size: 400, cachelines: 7, members: 2 */
- /* padding: 3 */
- /* last cacheline: 16 bytes */
+ /* size: 396, cachelines: 7, members: 2 */
+ /* last cacheline: 12 bytes */
};

--- obj-sparc64/drivers/base/platform.o.pahole.v3.15-rc7-79-gfe45736f4134 2014-05-30 10:32:06.406960625 +0200
+++ obj-sparc64/drivers/base/platform.o.pahole.v3.15-rc7-80-g2cdb06858d71 2014-05-30 11:26:20.971988269 +0200
@@ -94,9 +94,8 @@
struct platform_object {
struct platform_device pdev; /* 0 2208 */
/* --- cacheline 34 boundary (2176 bytes) was 32 bytes ago --- */
- char name[1]; /* 2208 1 */
+ char name[0]; /* 2208 0 */

- /* size: 2216, cachelines: 35, members: 2 */
- /* padding: 7 */
- /* last cacheline: 40 bytes */
+ /* size: 2208, cachelines: 35, members: 2 */
+ /* last cacheline: 32 bytes */
};

--- obj-x86_64/drivers/base/platform.o.pahole.v3.15-rc7-79-gfe45736f4134 2014-05-30 10:32:06.432960608 +0200
+++ obj-x86_64/drivers/base/platform.o.pahole.v3.15-rc7-80-g2cdb06858d71 2014-05-30 11:26:21.000988250 +0200
@@ -84,9 +84,8 @@
struct platform_object {
struct platform_device pdev; /* 0 720 */
/* --- cacheline 11 boundary (704 bytes) was 16 bytes ago --- */
- char name[1]; /* 720 1 */
+ char name[0]; /* 720 0 */

- /* size: 728, cachelines: 12, members: 2 */
- /* padding: 7 */
- /* last cacheline: 24 bytes */
+ /* size: 720, cachelines: 12, members: 2 */
+ /* last cacheline: 16 bytes */
};

Changes from v5 [1]:
- dropped dma_mask allocation changes and only kept padding
removal changes (name array length set to 0).

Changes from v4 [2]:
[by Emil Goode <emilgoode@gmail.com>:]
- Split v4 of the patch into two separate patches.
- Generated new object file size and data structure layout info.
- Updated the changelog message.

Changes from v3 [3]:
- fixed commit message so that git am doesn't fail.

Changes from v2 [4]:
- move 'dma_mask' to platform_object so that it's always
allocated and won't leak on release; remove all previously
added support functions.
- use C99 flexible array member for 'name' to remove padding
at the end of platform_object.

Changes from v1 [5]:
- remove unneeded kfree() from error path
- add reference to author/commit adding allocation of dmamask

Changes from v0 [6]:
- small rewrite to squeeze the patch to a bare minimal

[1] http://lkml.kernel.org/r/1401122483-31603-2-git-send-email-emilgoode@gmail.com
http://lkml.kernel.org/r/1401122483-31603-1-git-send-email-emilgoode@gmail.com
http://lkml.kernel.org/r/1401122483-31603-3-git-send-email-emilgoode@gmail.com

[2] http://lkml.kernel.org/r/1390817152-30898-1-git-send-email-ydroneaud@opteya.com
https://patchwork.kernel.org/patch/3541871/

[3] http://lkml.kernel.org/r/1390771138-28348-1-git-send-email-ydroneaud@opteya.com
https://patchwork.kernel.org/patch/3540081/

[4] http://lkml.kernel.org/r/1389683909-17495-1-git-send-email-ydroneaud@opteya.com
https://patchwork.kernel.org/patch/3484411/

[5] http://lkml.kernel.org/r/1389649085-7365-1-git-send-email-ydroneaud@opteya.com
https://patchwork.kernel.org/patch/3480961/

[6] http://lkml.kernel.org/r/1386886207-2735-1-git-send-email-ydroneaud@opteya.com

Cc: Emil Goode <emilgoode@gmail.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Shawn Guo <shawn.guo@freescale.com>
Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Olof Johansson <olof@lixom.net>
Cc: Uwe Kleine-KÃ¶nig <u.kleine-koenig@pengutronix.de>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Yann Droneaud <ydroneaud@opteya.com>
---
drivers/base/platform.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/base/platform.c b/drivers/base/platform.c
index 5b47210889e0..b126c9363f53 100644
--- a/drivers/base/platform.c
+++ b/drivers/base/platform.c
@@ -162,7 +162,7 @@ EXPORT_SYMBOL_GPL(platform_add_devices);

struct platform_object {
struct platform_device pdev;
- char name[1];
+ char name[];
};

/**
@@ -203,7 +203,7 @@ struct platform_device *platform_device_alloc(const char *name, int id)
{
struct platform_object *pa;

- pa = kzalloc(sizeof(struct platform_object) + strlen(name), GFP_KERNEL);
+ pa = kzalloc(sizeof(*pa) + strlen(name) + 1, GFP_KERNEL);
if (pa) {
strcpy(pa->name, name);
pa->pdev.name = pa->name;
--
1.9.3