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charset=us-ascii Content-Disposition: inline In-Reply-To: <3bd2992b-e0da-0ee6-ae82-03d75e8fa706@gmail.com> Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org 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. 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 > >> > >> >