netdev - Re: [PATCH v2 02/16] thunderbolt: Add support for XDomain properties

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Date:   Wed, 27 Sep 2017 09:22:16 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     mika.westerberg@...ux.intel.com
Cc:     gregkh@...uxfoundation.org, andreas.noever@...il.com,
        michael.jamet@...el.com, yehezkel.bernat@...el.com,
        amir.jer.levy@...el.com, Mario.Limonciello@...l.com,
        lukas@...ner.de, andriy.shevchenko@...ux.intel.com, andrew@...n.ch,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH v2 02/16] thunderbolt: Add support for XDomain
 properties

From: Mika Westerberg <mika.westerberg@...ux.intel.com>
Date: Wed, 27 Sep 2017 14:32:41 +0300

> Just for my education, is there some rule which tells when __packed is
> to be used? For example the above structures are all 32-bit aligned but
> how about something like:
> 
> struct foo {
> 	u32 value1;
> 	u8 value2;
> };
> 
> If the on-wire format requires such structures I assume __packed
> is needed here?

Usually header elements are 32-bit aligned in a protocol, so it wouldn't
be specified like that.

The only legitimate case I've seen is where things are purposefully
misaligned within the header, like this:

struct foo {
	u16	x;
	u64	y;
	u16	z;
};

Where the 'y' element is 2-byte aligned.

Fortunately, those situations are extremely rare.