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Date: Sat, 27 Apr 2013 12:06:20 -0700
Message-ID: <CAJ-VmokUFtFjQk0T87E57kEhp40f1v4XgJL-iN35Bzo4h-McDQ@mail.gmail.com> (sfid-20130427_210629_745002_8A7BEB2B)
Subject: Re: [PATCH v2] ath9k: collect statistics about Rx-Dup and Rx-STBC packets
From: Adrian Chadd <adrian@freebsd.org>
To: Oleksij Rempel <linux@rempel-privat.de>,
	Felix Fietkau <nbd@nbd.name>
Cc: ath9k-devel@lists.ath9k.org, linux-wireless@vger.kernel.org
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On 27 April 2013 11:53, Oleksij Rempel <linux@rempel-privat.de> wrote:

>> (And then go and re-align things inside that struct so you don't waste
>> space.)
>
>
> hmm.. what do you mean here?

Structure alignment? Well, you typically want to have everything be
dword aligned (32 bits) or word (16 bits) aligned. Otherwise the
compiler may insert extra padding between fields in order to meet
alignment requirements on platforms that need it (MIPS, older ARM) or
platforms that perform slower (newer ARM.)

Eg:

u32 a;
u16 b;
u8 c;
u8 d;

.. that's fine - the u32 is dword aligned, the u16 is word aligned,
the u8's don't need aligning.

But, considder:

u32 a;
u8 b;
u16 c;
u8 d;

.. u32 is dword aligned, u8 b is fine as it's a a byte and doesn't
need aligning, but 'u16 c' isn't dword aligned! So the compiler will
insert a byte padding between 'b' and 'c'.

same deal with:

u32 a;
u16 b;
u32 c;

.. 'a' is fine; 'b' is fine, but 'c' starts at a word boundary, not a
dword boundary.

Hence why things like IP/TCP headers and such look the way they do. :-)

Now, i don't know what 'bool' is, whether it's a byte, word or dword.
That "is_mybeacon" field should probably be just another flag in
rx_status, then just extend 'rs_flags' to 16 bits and include it. That
way the alignment is easy to see - all the fields in rx_status and the
htc rx_status structs have explicit sizes. :-)

Felix, what do you think?


Adrian