netdev - Re: [PATCH] net: nixge: Add __packed attribute to DMA descriptor struct

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Date:   Tue, 19 Jun 2018 10:13:55 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Moritz Fischer <mdf@...nel.org>, davem@...emloft.net
Cc:     keescook@...omium.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: nixge: Add __packed attribute to DMA descriptor
 struct

On 06/19/2018 09:54 AM, Moritz Fischer wrote:
> Add __packed attribute to DMA descriptor structure  in order to
> make sure that the DMA engine's alignemnt requirements are met.
> 
> Fixes commit 492caffa8a1a ("net: ethernet: nixge: Add support for
> National Instruments XGE netdev")
> Signed-off-by: Moritz Fischer <mdf@...nel.org>
> ---
> 
> Hi David,
> 
> this addresses an issue where padding occured breaking the alignment
> in the array the descriptors are allocated in coherent memory.
> This was discovered when we tried to bring up the driver via a PCIe
> bridge on x86.

How could padding be inserted given than all of the structure members
are naturally aligned (all u32 type). Compiler bug?

Also

> 
> Thanks,
> 
> Moritz
> 
> ---
>  drivers/net/ethernet/ni/nixge.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/ni/nixge.c b/drivers/net/ethernet/ni/nixge.c
> index 09f674ec0f9e..fea0e994324b 100644
> --- a/drivers/net/ethernet/ni/nixge.c
> +++ b/drivers/net/ethernet/ni/nixge.c
> @@ -122,7 +122,7 @@ struct nixge_hw_dma_bd {
>  	u32 sw_id_offset;
>  	u32 reserved5;
>  	u32 reserved6;
> -};
> +} __packed;
>  
>  struct nixge_tx_skb {
>  	struct sk_buff *skb;
> 


-- 
Florian