Subject: Re: [PATCH net-next v2 3/3] r8169: use WRITE_ONCE instead of dma_wmb in rtl8169_mark_to_asic

From: Petko Manolov <[email-masked]>
Date: Sun, 19 Apr 2020 22:00:29 +0300

On 20-04-19 20:16:21, Heiner Kallweit wrote:
> We want to ensure that desc->opts1 is written as last descriptor field.
> This doesn't require a full compiler barrier, WRITE_ONCE provides the
> ordering guarantee we need.
> 
> Signed-off-by: Heiner Kallweit <[email-masked]>
> ---
>  drivers/net/ethernet/realtek/r8169_main.c | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/net/ethernet/realtek/r8169_main.c b/drivers/net/ethernet/realtek/r8169_main.c
> index 2fc65aca3..3e4ed2528 100644
> --- a/drivers/net/ethernet/realtek/r8169_main.c
> +++ b/drivers/net/ethernet/realtek/r8169_main.c
> @@ -3892,11 +3892,9 @@ static inline void rtl8169_mark_to_asic(struct RxDesc *desc)
>  {
>  	u32 eor = le32_to_cpu(desc->opts1) & RingEnd;
>  
> -	desc->opts2 = 0;
> -	/* Force memory writes to complete before releasing descriptor */
> -	dma_wmb();

If dma_wmb() was really ever needed here you should leave it even after you 
order these writes with WRITE_ONCE().  If not, then good riddance.

Just saying, i am not familiar with the hardware nor with the driver. :)

		Petko

> -
> -	desc->opts1 = cpu_to_le32(DescOwn | eor | R8169_RX_BUF_SIZE);
> +	/* Ensure ordering of writes */
> +	WRITE_ONCE(desc->opts2, 0);
> +	WRITE_ONCE(desc->opts1, cpu_to_le32(DescOwn | eor | R8169_RX_BUF_SIZE));
>  }
>  
>  static struct page *rtl8169_alloc_rx_data(struct rtl8169_private *tp,
> @@ -3919,7 +3917,7 @@ static struct page *rtl8169_alloc_rx_data(struct rtl8169_private *tp,
>  		return NULL;
>  	}
>  
> -	desc->addr = cpu_to_le64(mapping);
> +	WRITE_ONCE(desc->addr, cpu_to_le64(mapping));
>  	rtl8169_mark_to_asic(desc);
>  
>  	return data;
> -- 
> 2.26.1
> 
>