linux-kernel - [PATCH v4] devres: Align data[] to ARCH_KMALLOC

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Message-Id: <20181031152547.9066-1-abrodkin@synopsys.com>
Date:   Wed, 31 Oct 2018 18:25:47 +0300
From:   Alexey Brodkin <alexey.brodkin@...opsys.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-snps-arc@...ts.infradead.org, linux-arch@...r.kernel.org,
        Alexey Brodkin <alexey.brodkin@...opsys.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        David Laight <David.Laight@...LAB.COM>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vineet Gupta <vineet.gupta1@...opsys.com>,
        Will Deacon <will.deacon@....com>, Greg KH <greg@...ah.com>,
        stable@...r.kernel.org
Subject: [PATCH v4] devres: Align data[] to ARCH_KMALLOC_MINALIGN

Initially we bumped into problem with 32-bit aligned atomic64_t
on ARC, see [1]. And then during quite lengthly discussion Peter Z.
mentioned ARCH_KMALLOC_MINALIGN which IMHO makes perfect sense.
If allocation is done by plain kmalloc() obtained buffer will be
ARCH_KMALLOC_MINALIGN aligned and then why buffer obtained via
devm_kmalloc() should have any other alignment?

This way we at least get the same behavior for both types of
allocation.

[1] http://lists.infradead.org/pipermail/linux-snps-arc/2018-July/004009.html
[2] http://lists.infradead.org/pipermail/linux-snps-arc/2018-July/004036.html

Signed-off-by: Alexey Brodkin <abrodkin@...opsys.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Geert Uytterhoeven <geert@...ux-m68k.org>
Cc: David Laight <David.Laight@...LAB.COM>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Vineet Gupta <vgupta@...opsys.com>
Cc: Will Deacon <will.deacon@....com>
Cc: Greg KH <greg@...ah.com>
Cc: <stable@...r.kernel.org> # 4.8+
---

Changes v3 -> v4:

 * Use ARCH_KMALLOC_MINALIGN for alignment instead of "8" [Peter]

Changes v2 -> v3:

 * Align explicitly to 8 bytes [David]
 * Rephrased in-line comment [David]
 * Added more techinical details to commit message [Greg]
 * Mention more alignment options in commit message [Geert]

Changes v1 -> v2:

 * Reworded commit message
 * Inserted comment right in source [Thomas]

 drivers/base/devres.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/base/devres.c b/drivers/base/devres.c
index 4aaf00d2098b..e038e2b3b7ea 100644
--- a/drivers/base/devres.c
+++ b/drivers/base/devres.c
@@ -26,8 +26,14 @@ struct devres_node {
 
 struct devres {
 	struct devres_node		node;
-	/* -- 3 pointers */
-	unsigned long long		data[];	/* guarantee ull alignment */
+	/*
+	 * Some archs want to perform DMA into kmalloc caches
+	 * and need a guaranteed alignment larger than
+	 * the alignment of a 64-bit integer.
+	 * Thus we use ARCH_KMALLOC_MINALIGN here and get exactly the same
+	 * buffer alignment as if it was allocated by plain kmalloc().
+	 */
+	u8 __aligned(ARCH_KMALLOC_MINALIGN) data[];
 };
 
 struct devres_group {
-- 
2.17.2