From 5846581e35637771952602eecc1e20ece5ced011 Mon Sep 17 00:00:00 2001 From: Will Deacon Date: Mon, 14 May 2018 15:55:26 -0700 Subject: [PATCH] locking/memory-barriers.txt: Fix broken DMA vs. MMIO ordering example The section of memory-barriers.txt that describes the dma_Xmb() barriers has an incorrect example claiming that a wmb() is required after writing to coherent memory in order for those writes to be visible to a device before a subsequent MMIO access using writel() can reach the device. In fact, this ordering guarantee is provided (at significant cost on some architectures such as arm and power) by writel, so the wmb() is not necessary. writel_relaxed exists for cases where this ordering is not required. Fix the example and update the text to make this clearer. Reported-by: Sinan Kaya Signed-off-by: Will Deacon Signed-off-by: Paul E. McKenney Cc: Andrew Morton Cc: Arnd Bergmann Cc: Benjamin Herrenschmidt Cc: Jason Gunthorpe Cc: Jonathan Corbet Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: akiyks@gmail.com Cc: boqun.feng@gmail.com Cc: dhowells@redhat.com Cc: j.alglave@ucl.ac.uk Cc: linux-arch@vger.kernel.org Cc: luc.maranget@inria.fr Cc: npiggin@gmail.com Cc: parri.andrea@gmail.com Cc: stern@rowland.harvard.edu Link: http://lkml.kernel.org/r/1526338533-6044-1-git-send-email-paulmck@linux.vnet.ibm.com Signed-off-by: Ingo Molnar --- Documentation/memory-barriers.txt | 15 ++++++++------- 1 file changed, 8 insertions(+), 7 deletions(-) diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index 6dafc8085acc..34c1970908a5 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -1920,9 +1920,6 @@ There are some more advanced barrier functions: /* 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); } @@ -1930,11 +1927,15 @@ There are some more advanced barrier functions: The dma_rmb() allows us guarantee the device has released ownership before we read the data from the descriptor, and the 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. + can see it now has ownership. Note that, when using writel(), a prior + wmb() is not needed to guarantee that the cache coherent memory writes + have completed before writing to the MMIO region. The cheaper + writel_relaxed() does not provide this guarantee and must not be used + here. - See Documentation/DMA-API.txt for more information on consistent memory. + See the subsection "Kernel I/O barrier effects" for more information on + relaxed I/O accessors and the Documentation/DMA-API.txt file for more + information on consistent memory. MMIO WRITE BARRIER