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

On Tue, Sep 26, 2017 at 09:33:54PM -0700, David Miller wrote:

From: Mika Westerberg <mika.westerberg@linux.intel.com>
Date: Mon, 25 Sep 2017 14:07:24 +0300

quoted

+struct tb_property_entry {
+	u32 key_hi;
+	u32 key_lo;
+	u16 length;
+	u8 reserved;
+	u8 type;
+	u32 value;
+} __packed;
+
+struct tb_property_rootdir_entry {
+	u32 magic;
+	u32 length;
+	struct tb_property_entry entries[];
+} __packed;
+
+struct tb_property_dir_entry {
+	u32 uuid[4];
+	struct tb_property_entry entries[];
+} __packed;

There is no apparent need for __packed here, and __packed should be
avoided unless absolutely necessary as it pessimizes the code
significantly on some architectures.

Please remove __packed from these datastructures unless you can
prove it is absolutely needed and, in such case, please document
in a comment why that requirement exists.  Because from the layout
of these types, everything will be packed in just fine without
__packed.

I will thanks.

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?

Thanks!