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

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Message-ID: <ZIxfK1MVRL+1wDvq@corigine.com>
Date: Fri, 16 Jun 2023 15:10:03 +0200
From: Simon Horman <simon.horman@...igine.com>
To: Arnd Bergmann <arnd@...nel.org>
Cc: Igor Russkikh <irusskikh@...vell.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Arnd Bergmann <arnd@...db.de>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Tom Rix <trix@...hat.com>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH] net: atlantic: fix ring buffer alignment

On Fri, Jun 16, 2023 at 11:26:32AM +0200, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.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@...db.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;