[Intel-wired-lan] [next-queue] fm10k: explain why we need __packed on some TLV structures

[Alexander Duyck]

On 11/03/2015 11:32 AM, Jacob Keller wrote:
> Add an explanatory comment about the __packed attribute on these
> structures due to TLV data layout requirements.
>
> Signed-off-by: Jacob Keller <jacob.e.keller at intel.com>
> ---
>   drivers/net/ethernet/intel/fm10k/fm10k_pf.h | 5 +++++
>   1 file changed, 5 insertions(+)
>
> diff --git a/drivers/net/ethernet/intel/fm10k/fm10k_pf.h b/drivers/net/ethernet/intel/fm10k/fm10k_pf.h
> index a8fc512a2416..661a4062b756 100644
> --- a/drivers/net/ethernet/intel/fm10k/fm10k_pf.h
> +++ b/drivers/net/ethernet/intel/fm10k/fm10k_pf.h
> @@ -74,6 +74,11 @@ enum fm10k_pf_tlv_attr_id_v1 {
>   #define FM10K_MSG_UPDATE_PVID_PVID_SHIFT	16
>   #define FM10K_MSG_UPDATE_PVID_PVID_SIZE		16
>   
> +/* The following data structures are overlayed specifically to TLV mailbox
> + * messages, and must not have gaps between their values. They must line up
> + * correctly to the TLV definition.
> + */
> +
>   struct fm10k_mac_update {
>   	__le32	mac_lower;
>   	__le16	mac_upper;

Really I don't think this tells the story of why the packed attribute is 
needed.  From what I can tell there aren't any holes in the structures 
themselves.  It looks like the issue is the padding on the end.  From 
what I can tell only the only message that really needs the packed 
attribute it is the 1588 message because it uses __le64 values and as 
such the actual size will vary by 4 bytes between 64b and 32b systems.

- Alex