diff options
| author | Ingo Molnar <mingo@elte.hu> | 2008-07-29 00:07:55 +0200 |
|---|---|---|
| committer | Ingo Molnar <mingo@elte.hu> | 2008-07-29 00:07:55 +0200 |
| commit | cb28a1bbdb4790378e7366d6c9ee1d2340b84f92 (patch) | |
| tree | 316436f77dac75335fd2c3ef5f109e71606c50d3 /Documentation/unaligned-memory-access.txt | |
| parent | b6d4f7e3ef25beb8c658c97867d98883e69dc544 (diff) | |
| parent | f934fb19ef34730263e6afc01e8ec27a8a71470f (diff) | |
Merge branch 'linus' into core/generic-dma-coherent
Conflicts:
arch/x86/Kconfig
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Diffstat (limited to 'Documentation/unaligned-memory-access.txt')
| -rw-r--r-- | Documentation/unaligned-memory-access.txt | 32 |
1 files changed, 29 insertions, 3 deletions
diff --git a/Documentation/unaligned-memory-access.txt b/Documentation/unaligned-memory-access.txt index b0472ac5226..f866c72291b 100644 --- a/Documentation/unaligned-memory-access.txt +++ b/Documentation/unaligned-memory-access.txt @@ -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 |