Re: [PATCH] introduce HAVE_EFFICIENT_UNALIGNED_ACCESS Kconfig symbol

From: Stefan Richter
Date: Tue Apr 08 2008 - 11:41:51 EST


Johannes Berg wrote:
> --- everything.orig/Documentation/unaligned-memory-access.txt 2008-04-04 23:42:50.000000000 +0200
> +++ everything/Documentation/unaligned-memory-access.txt 2008-04-04 23:52:15.000000000 +0200
> @@ -218,9 +218,35 @@ If use of such macros is not convenient,
> 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

. On

> +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

. So

> +DMA to an address that is can be expressed as 4*n + 2. One notable exception

which can

> +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 depend on CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS like so:

made dependent on

> +
> +#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
> + skb = original skb
> +#else
> + skb = copy skb
> +#endif
> +
> --
> -Author: Daniel Drake <dsd@xxxxxxxxxx>
> +Authors: Daniel Drake <dsd@xxxxxxxxxx>,
> + Johannes Berg <johannes@xxxxxxxxxxxxxxxx>
> 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
>
> --- everything.orig/arch/powerpc/Kconfig 2008-04-04 23:42:53.000000000 +0200
> +++ everything/arch/powerpc/Kconfig 2008-04-04 23:52:15.000000000 +0200
> @@ -103,6 +103,7 @@ config PPC
> select HAVE_OPROFILE
> select HAVE_KPROBES
> select HAVE_KRETPROBES
> + select HAVE_EFFICIENT_UNALIGNED_ACCESS
>
> config EARLY_PRINTK
> bool
> --- everything.orig/arch/x86/Kconfig 2008-04-04 23:42:50.000000000 +0200
> +++ everything/arch/x86/Kconfig 2008-04-04 23:52:15.000000000 +0200
> @@ -23,6 +23,7 @@ config X86
> select HAVE_KPROBES
> select HAVE_KRETPROBES
> select HAVE_KVM if ((X86_32 && !X86_VOYAGER && !X86_VISWS && !X86_NUMAQ) || X86_64)
> + select HAVE_EFFICIENT_UNALIGNED_ACCESS
>
>
> config GENERIC_LOCKBREAK
> --- everything.orig/arch/Kconfig 2008-04-04 23:42:50.000000000 +0200
> +++ everything/arch/Kconfig 2008-04-04 23:52:15.000000000 +0200
> @@ -36,3 +36,22 @@ config HAVE_KPROBES
>
> config HAVE_KRETPROBES
> def_bool n
> +
> +config HAVE_EFFICIENT_UNALIGNED_ACCESS
> + def_bool n
> + help
> + Some architectures are unable to perform unaligned accesses
> + without the use of get_unaligned/put_unaligned. Others are
> + unable to perform such accesses efficiently (e.g. trap on

Wouldn't a comment be more appropriate instead of a help text? This
option shouldn't be a visible make XYZconfig prompt.

I have no comment on the option itself; it's not my domain.
--
Stefan Richter
-=====-==--- -=-- -=---
http://arcgraph.de/sr/
--
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