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: Mon, 20 Apr 2020 00:38:17 +0300

On 20-04-19 23:03:57, Heiner Kallweit wrote:
> On 19.04.2020 21:00, Petko Manolov wrote:
> > 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.
> > 
> My understanding is that we have to avoid transferring ownership of
> descriptor to device (by setting DescOwn bit) before opts2 field
> has been written. Using WRITE_ONCE() should be sufficient to prevent
> the compiler from merging or reordering the writes.
> At least that's how I read the process_level() example in
> https://www.kernel.org/doc/Documentation/memory-barriers.txt

WRITE_ONCE() will prevent the compiler from reordering, but not the CPU.  On x86 
this code will run just fine, but not on ARM or PPC.  Based on your description 
above i think dma_wmb() is still needed.

		Petko

> > 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
> >>
> >>
>