path: root/Documentation
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authorLinus Torvalds <torvalds@linux-foundation.org>2014-12-12 16:11:12 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2014-12-12 16:11:12 -0800
commitf96fe225677b3efb74346ebd56fafe3997b02afa (patch)
tree75afbbfd687d678587de244b075a6f836c206425 /Documentation
parentMerge git://git.kernel.org/pub/scm/linux/kernel/git/davem/ide (diff)
parentMerge branch 'gem' (diff)
Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net
Pull another networking update from David Miller: "Small follow-up to the main merge pull from the other day: 1) Alexander Duyck's DMA memory barrier patch set. 2) cxgb4 driver fixes from Karen Xie. 3) Add missing export of fixed_phy_register() to modules, from Mark Salter. 4) DSA bug fixes from Florian Fainelli" * git://git.kernel.org/pub/scm/linux/kernel/git/davem/net: (24 commits) net/macb: add TX multiqueue support for gem linux/interrupt.h: remove the definition of unused tasklet_hi_enable jme: replace calls to redundant function net: ethernet: davicom: Allow to select DM9000 for nios2 net: ethernet: smsc: Allow to select SMC91X for nios2 cxgb4: Add support for QSA modules libcxgbi: fix freeing skb prematurely cxgb4i: use set_wr_txq() to set tx queues cxgb4i: handle non-pdu-aligned rx data cxgb4i: additional types of negative advice cxgb4/cxgb4i: set the max. pdu length in firmware cxgb4i: fix credit check for tx_data_wr cxgb4i: fix tx immediate data credit check net: phy: export fixed_phy_register() fib_trie: Fix trie balancing issue if new node pushes down existing node vlan: Add ability to always enable TSO/UFO r8169:update rtl8168g pcie ephy parameter net: dsa: bcm_sf2: force link for all fixed PHY devices fm10k/igb/ixgbe: Use dma_rmb on Rx descriptor reads r8169: Use dma_rmb() and dma_wmb() for DescOwn checks ...
Diffstat (limited to 'Documentation')
1 files changed, 42 insertions, 0 deletions
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 7ee2ae6d5451..70a09f8a0383 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -1633,6 +1633,48 @@ There are some more advanced barrier functions:
operations" subsection for information on where to use these.
+ (*) dma_wmb();
+ (*) dma_rmb();
+ These are for use with consistent memory to guarantee the ordering
+ of writes or reads of shared memory accessible to both the CPU and a
+ DMA capable device.
+ For example, consider a device driver that shares memory with a device
+ and uses a descriptor status value to indicate if the descriptor belongs
+ to the device or the CPU, and a doorbell to notify it when new
+ descriptors are available:
+ if (desc->status != DEVICE_OWN) {
+ /* do not read data until we own descriptor */
+ dma_rmb();
+ /* read/modify data */
+ read_data = desc->data;
+ desc->data = write_data;
+ /* flush modifications before status update */
+ dma_wmb();
+ /* assign ownership */
+ desc->status = DEVICE_OWN;
+ /* force memory to sync before notifying device via MMIO */
+ wmb();
+ /* notify device of new descriptors */
+ writel(DESC_NOTIFY, doorbell);
+ }
+ The dma_rmb() allows us guarantee the device has released ownership
+ before we read the data from the descriptor, and he dma_wmb() allows
+ us to guarantee the data is written to the descriptor before the device
+ can see it now has ownership. The wmb() is needed to guarantee that the
+ cache coherent memory writes have completed before attempting a write to
+ the cache incoherent MMIO region.
+ See Documentation/DMA-API.txt for more information on consistent memory.