Re: [PATCH] net: atlantic: fix ring buffer alignment

Simon Horman
2023-06-16T13:10:15Z

On Fri, Jun 16, 2023 at 11:26:32AM +0200, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
>
> clang warns about conflicting packing annotations:
>
> drivers/net/ethernet/aquantia/atlantic/aq_ring.h:72:2: error: field within 'struct aq_ring_buff_s' is less aligned than 'union aq_ring_buff_s::(anonymous at drivers/net/ethernet/aquantia/atlantic/aq_ring.h:72:2)' and is usually due to 'struct aq_ring_buff_s' being packed, which can lead to unaligned accesses [-Werror,-Wunaligned-access]

FWIIW, I was able to reproduce this (warning) with clang-16 on ARM (32bit).
And I agree with the approach you have taken here.

> This was originally intended to ensure the structure fits exactly into
> 32 bytes on 64-bit architectures, but apparently never did, and instead
> just produced misaligned pointers as well as forcing byte-wise access
> on hardware without unaligned load/store instructions.
>
> Update the comment to more closely reflect the layout and remove the
> broken __packed annotation.
>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
> .../net/ethernet/aquantia/atlantic/aq_ring.h | 26 +++++++++++--------
> 1 file changed, 15 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/net/ethernet/aquantia/atlantic/aq_ring.h b/drivers/net/ethernet/aquantia/atlantic/aq_ring.h
> index 0a6c34438c1d0..a9cc5a1c4c479 100644
> --- a/drivers/net/ethernet/aquantia/atlantic/aq_ring.h
> +++ b/drivers/net/ethernet/aquantia/atlantic/aq_ring.h
> @@ -26,19 +26,23 @@ struct aq_rxpage {
> unsigned int pg_off;
> };
>
> -/* TxC SOP DX EOP
> - * +----------+----------+----------+-----------
> - * 8bytes|len l3,l4 | pa | pa | pa
> - * +----------+----------+----------+-----------
> - * 4/8bytes|len pkt |len pkt | | skb
> - * +----------+----------+----------+-----------
> - * 4/8bytes|is_gso |len,flags |len |len,is_eop
> - * +----------+----------+----------+-----------
> +/* TxC SOP DX EOP RX
> + * +----------+----------+----------+----------+-------
> + * 8bytes|len l3,l4 | pa | pa | pa | hash
> + * +----------+----------+----------+----------+-------
> + * 4/8bytes|len pkt |len pkt | | skb | page
> + * +----------+----------+----------+----------+-------
> + * 4/8bytes|is_gso |len,flags |len |len,is_eop| daddr
> + * +----------+----------+----------+----------+-------
> + * 4/8bytes| | | | | order,pgoff
> + * +----------+----------+----------+----------+-------
> + * 2bytes | | | | | vlan_rx_tag
> + * +----------+----------+----------+----------+-------
> + * 8bytes + flags
> + * +----------+----------+----------+----------+-------

Perhaps it just me. But I do have trouble reconciling the description
above with the structure below. As such, my suggest would be to simply
delete it.

> *
> - * This aq_ring_buff_s doesn't have endianness dependency.
> - * It is __packed for cache line optimizations.
> */
> -struct __packed aq_ring_buff_s {
> +struct aq_ring_buff_s {
> union {
> /* RX/TX */
> dma_addr_t pa;