Re: [dpdk-dev] [PATCH v2 1/8] eal: introduce DMA memory barriers

[Thomas Monjalon <thomas@monjalon.net>]

16/01/2018 10:10, Jianbo Liu:
> The 01/16/2018 10:49, Andrew Rybchenko wrote:
> > On 01/16/2018 04:10 AM, Yongseok Koh wrote:
> > >This commit introduces rte_dma_wmb() and rte_dma_rmb(), in order to
> > >guarantee the ordering of coherent shared memory between the CPU and a DMA
> > >capable device.
> > >
> > >Signed-off-by: Yongseok Koh <yskoh@mellanox.com>
> > >---
> > >  lib/librte_eal/common/include/generic/rte_atomic.h | 18 ++++++++++++++++++
> > >  1 file changed, 18 insertions(+)
> > >
> > >diff --git a/lib/librte_eal/common/include/generic/rte_atomic.h b/lib/librte_eal/common/include/generic/rte_atomic.h
> > >index 16af5ca57..2e0503ce6 100644
> > >--- a/lib/librte_eal/common/include/generic/rte_atomic.h
> > >+++ b/lib/librte_eal/common/include/generic/rte_atomic.h
> > >@@ -98,6 +98,24 @@ static inline void rte_io_wmb(void);
> > >   */
> > >  static inline void rte_io_rmb(void);
> > >+/**
> > >+ * Write memory barrier for coherent memory between lcore and IO device
> > >+ *
> > >+ * Guarantees that the STORE operations on coherent memory that
> > >+ * precede the rte_dma_wmb() call are visible to I/O device before the
> > >+ * STORE operations that follow it.
> > >+ */
> > >+static inline void rte_dma_wmb(void);
> > >+
> > >+/**
> > >+ * Read memory barrier for coherent memory between lcore and IO device
> > >+ *
> > >+ * Guarantees that the LOAD operations on coherent memory updated by
> > >+ * IO device that precede the rte_dma_rmb() call are visible to CPU
> > >+ * before the LOAD operations that follow it.
> > >+ */
> > >+static inline void rte_dma_rmb(void);
> > >+
> > >  #endif /* __DOXYGEN__ */
> > >  /**
> >
> > I'm not an ARMv8 expert so, my comments could be a bit ignorant.
> > I'd like to understand the difference between io and added here dma
> > barriers.
> > The difference should be clearly explained. Otherwise we'll constantly hit
> > on incorrect choice of barrier type.
> > Also I don't understand why "dma" name is chosen taking into account
> > that definition is bound to coherent memory between lcore and IO device.
> 
> A good explanation can be found here.
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=1077fa36f23e259858caf6f269a47393a5aff523

I agree that something more is needed to explain when to use rte_io_*.
The only difference between rte_io_* and rte_dma_* is "on coherent memory".