summaryrefslogtreecommitdiff
path: root/Documentation/unaligned-memory-access.txt
diff options
context:
space:
mode:
authorLinus Torvalds <torvalds@linux-foundation.org>2017-07-15 22:58:58 +0300
committerLinus Torvalds <torvalds@linux-foundation.org>2017-07-15 22:58:58 +0300
commit486088bc4689f826b80aa317b45ac9e42e8b25ee (patch)
treeadf5847a6119d24da990d9e336f005c4a316e6be /Documentation/unaligned-memory-access.txt
parent52f6c588c77b76d548201470c2a28263a41b462b (diff)
parent43e5f7e1fa66531777c49791014c3124ea9208d8 (diff)
downloadlinux-486088bc4689f826b80aa317b45ac9e42e8b25ee.tar.xz
Merge tag 'standardize-docs' of git://git.lwn.net/linux
Pull documentation format standardization from Jonathan Corbet: "This series converts a number of top-level documents to the RST format without incorporating them into the Sphinx tree. The hope is to bring some uniformity to kernel documentation and, perhaps more importantly, have our existing docs serve as an example of the desired formatting for those that will be added later. Mauro has gone through and fixed up a lot of top-level documentation files to make them conform to the RST format, but without moving or renaming them in any way. This will help when we incorporate the ones we want to keep into the Sphinx doctree, but the real purpose is to bring a bit of uniformity to our documentation and let the top-level docs serve as examples for those writing new ones" * tag 'standardize-docs' of git://git.lwn.net/linux: (84 commits) docs: kprobes.txt: Fix whitespacing tee.txt: standardize document format cgroup-v2.txt: standardize document format dell_rbu.txt: standardize document format zorro.txt: standardize document format xz.txt: standardize document format xillybus.txt: standardize document format vfio.txt: standardize document format vfio-mediated-device.txt: standardize document format unaligned-memory-access.txt: standardize document format this_cpu_ops.txt: standardize document format svga.txt: standardize document format static-keys.txt: standardize document format smsc_ece1099.txt: standardize document format SM501.txt: standardize document format siphash.txt: standardize document format sgi-ioc4.txt: standardize document format SAK.txt: standardize document format rpmsg.txt: standardize document format robust-futexes.txt: standardize document format ...
Diffstat (limited to 'Documentation/unaligned-memory-access.txt')
-rw-r--r--Documentation/unaligned-memory-access.txt57
1 files changed, 30 insertions, 27 deletions
diff --git a/Documentation/unaligned-memory-access.txt b/Documentation/unaligned-memory-access.txt
index 3f76c0c37920..51b4ff031586 100644
--- a/Documentation/unaligned-memory-access.txt
+++ b/Documentation/unaligned-memory-access.txt
@@ -1,6 +1,15 @@
+=========================
UNALIGNED MEMORY ACCESSES
=========================
+:Author: Daniel Drake <dsd@gentoo.org>,
+:Author: Johannes Berg <johannes@sipsolutions.net>
+
+:With help from: Alan Cox, Avuton Olrich, Heikki Orsila, Jan Engelhardt,
+ Kyle McMartin, Kyle Moffett, Randy Dunlap, Robert Hancock, Uli Kunitz,
+ Vadim Lobanov
+
+
Linux runs on a wide variety of architectures which have varying behaviour
when it comes to memory access. This document presents some details about
unaligned accesses, why you need to write code that doesn't cause them,
@@ -73,7 +82,7 @@ memory addresses of certain variables, etc.
Fortunately things are not too complex, as in most cases, the compiler
ensures that things will work for you. For example, take the following
-structure:
+structure::
struct foo {
u16 field1;
@@ -106,7 +115,7 @@ On a related topic, with the above considerations in mind you may observe
that you could reorder the fields in the structure in order to place fields
where padding would otherwise be inserted, and hence reduce the overall
resident memory size of structure instances. The optimal layout of the
-above example is:
+above example is::
struct foo {
u32 field2;
@@ -139,21 +148,21 @@ Code that causes unaligned access
With the above in mind, let's move onto a real life example of a function
that can cause an unaligned memory access. The following function taken
from include/linux/etherdevice.h is an optimized routine to compare two
-ethernet MAC addresses for equality.
+ethernet MAC addresses for equality::
-bool ether_addr_equal(const u8 *addr1, const u8 *addr2)
-{
-#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
+ bool ether_addr_equal(const u8 *addr1, const u8 *addr2)
+ {
+ #ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
u32 fold = ((*(const u32 *)addr1) ^ (*(const u32 *)addr2)) |
((*(const u16 *)(addr1 + 4)) ^ (*(const u16 *)(addr2 + 4)));
return fold == 0;
-#else
+ #else
const u16 *a = (const u16 *)addr1;
const u16 *b = (const u16 *)addr2;
return ((a[0] ^ b[0]) | (a[1] ^ b[1]) | (a[2] ^ b[2])) == 0;
-#endif
-}
+ #endif
+ }
In the above function, when the hardware has efficient unaligned access
capability, there is no issue with this code. But when the hardware isn't
@@ -171,7 +180,8 @@ as it is a decent optimization for the cases when you can ensure alignment,
which is true almost all of the time in ethernet networking context.
-Here is another example of some code that could cause unaligned accesses:
+Here is another example of some code that could cause unaligned accesses::
+
void myfunc(u8 *data, u32 value)
{
[...]
@@ -184,6 +194,7 @@ to an address that is not evenly divisible by 4.
In summary, the 2 main scenarios where you may run into unaligned access
problems involve:
+
1. Casting variables to types of different lengths
2. Pointer arithmetic followed by access to at least 2 bytes of data
@@ -195,7 +206,7 @@ The easiest way to avoid unaligned access is to use the get_unaligned() and
put_unaligned() macros provided by the <asm/unaligned.h> header file.
Going back to an earlier example of code that potentially causes unaligned
-access:
+access::
void myfunc(u8 *data, u32 value)
{
@@ -204,7 +215,7 @@ access:
[...]
}
-To avoid the unaligned memory access, you would rewrite it as follows:
+To avoid the unaligned memory access, you would rewrite it as follows::
void myfunc(u8 *data, u32 value)
{
@@ -215,7 +226,7 @@ To avoid the unaligned memory access, you would rewrite it as follows:
}
The get_unaligned() macro works similarly. Assuming 'data' is a pointer to
-memory and you wish to avoid unaligned access, its usage is as follows:
+memory and you wish to avoid unaligned access, its usage is as follows::
u32 value = get_unaligned((u32 *) data);
@@ -245,18 +256,10 @@ For some ethernet hardware that cannot DMA to unaligned addresses like
4*n+2 or non-ethernet hardware, this can be a problem, and it is then
required to copy the incoming frame into an aligned buffer. Because this is
unnecessary on architectures that can do unaligned accesses, the code can be
-made dependent on CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS like so:
-
-#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
- skb = original skb
-#else
- skb = copy skb
-#endif
-
---
-Authors: Daniel Drake <dsd@gentoo.org>,
- Johannes Berg <johannes@sipsolutions.net>
-With help from: Alan Cox, Avuton Olrich, Heikki Orsila, Jan Engelhardt,
-Kyle McMartin, Kyle Moffett, Randy Dunlap, Robert Hancock, Uli Kunitz,
-Vadim Lobanov
+made dependent on CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS like so::
+ #ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
+ skb = original skb
+ #else
+ skb = copy skb
+ #endif