drm/i915: panel: invert brightness via quirk

[pandora-kernel.git] / Documentation / unaligned-memory-access.txt

diff --git a/Documentation/unaligned-memory-access.txt b/Documentation/unaligned-memory-access.txt
index 6223eac..f866c72 100644 (file)
--- a/Documentation/unaligned-memory-access.txt
+++ b/Documentation/unaligned-memory-access.txt
@@ -57,7 +57,7 @@ here; a summary of the common scenarios is presented below:
    unaligned access to be corrected.
  - Some architectures are not capable of unaligned memory access, but will
    silently perform a different memory access to the one that was requested,
-   resulting a a subtle code bug that is hard to detect!
+   resulting in a subtle code bug that is hard to detect!
 
 It should be obvious from the above that if your code causes unaligned
 memory accesses to happen, your code will not work correctly on certain
@@ -209,7 +209,7 @@ memory and you wish to avoid unaligned access, its usage is as follows:
 
        u32 value = get_unaligned((u32 *) data);
 
-These macros work work for memory accesses of any length (not just 32 bits as
+These macros work for memory accesses of any length (not just 32 bits as
 in the examples above). Be aware that when compared to standard access of
 aligned memory, using these macros to access unaligned memory can be costly in
 terms of performance.
@@ -218,9 +218,35 @@ If use of such macros is not convenient, another option is to use memcpy(),
 where the source or destination (or both) are of type u8* or unsigned char*.
 Due to the byte-wise nature of this operation, unaligned accesses are avoided.
 
+
+Alignment vs. Networking
+========================
+
+On architectures that require aligned loads, networking requires that the IP
+header is aligned on a four-byte boundary to optimise the IP stack. For
+regular ethernet hardware, the constant NET_IP_ALIGN is used. On most
+architectures this constant has the value 2 because the normal ethernet
+header is 14 bytes long, so in order to get proper alignment one needs to
+DMA to an address which can be expressed as 4*n + 2. One notable exception
+here is powerpc which defines NET_IP_ALIGN to 0 because DMA to unaligned
+addresses can be very expensive and dwarf the cost of unaligned loads.
+
+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
+
 --
-Author: Daniel Drake <dsd@gentoo.org>
+Authors: Daniel Drake <dsd@gentoo.org>,
+         Johannes Berg <johannes@sipsolutions.net>
 With help from: Alan Cox, Avuton Olrich, Heikki Orsila, Jan Engelhardt,
-Johannes Berg, Kyle McMartin, Kyle Moffett, Randy Dunlap, Robert Hancock,
-Uli Kunitz, Vadim Lobanov
+Kyle McMartin, Kyle Moffett, Randy Dunlap, Robert Hancock, Uli Kunitz,
+Vadim Lobanov