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[2003:ea:8f29:6000:39dc:7003:657c:dd6e]) by smtp.googlemail.com with ESMTPSA id s17sm17494348wmc.48.2020.04.19.14.52.34 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 19 Apr 2020 14:52:34 -0700 (PDT) Subject: Re: [PATCH net-next v2 3/3] r8169: use WRITE_ONCE instead of dma_wmb in rtl8169_mark_to_asic To: Realtek linux nic maintainers , David Miller , "netdev@vger.kernel.org" References: <612105ff-f21d-40c4-2b02-0ac0c12615fb@gmail.com> <20200419190029.GA37084@carbon> <3bd2992b-e0da-0ee6-ae82-03d75e8fa706@gmail.com> <20200419213817.GA39723@carbon> From: Heiner Kallweit Message-ID: <19c7d811-5e41-011b-b35f-c4bbe7d890ee@gmail.com> Date: Sun, 19 Apr 2020 23:52:28 +0200 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0 MIME-Version: 1.0 In-Reply-To: <20200419213817.GA39723@carbon> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org 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 >>>> --- >>>> 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 >>>> >>>> >>