netdev - Re: [PATCH net-next v2 3/3] r8169: use WRITE_ONCE instead of dma_wmb in rtl8169_mark_to

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Date:   Sun, 19 Apr 2020 23:52:28 +0200
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Realtek linux nic maintainers <nic_swsd@...ltek.com>,
        David Miller <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next v2 3/3] r8169: use WRITE_ONCE instead of dma_wmb
 in rtl8169_mark_to_asic

On 19.04.2020 23:38, Petko Manolov wrote:
> 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 <hkallweit1@...il.com>
>>>> ---
>>>>  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.
> 

After reading a little about ARMv8-A and weakly-ordered memory I tend to agree
with you. It's a pity that I don't know of any use of Realtek network chips
on such a weakly-ordered platform (what of course doesn't mean it doesn't exist).
This would really help me in testing and avoid such "but it works on x86" ..

> 
> 		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
>>>>
>>>>
>>