From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752618AbeC0NLV (ORCPT ); Tue, 27 Mar 2018 09:11:21 -0400 Received: from foss.arm.com ([217.140.101.70]:54806 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752060AbeC0NLS (ORCPT ); Tue, 27 Mar 2018 09:11:18 -0400 From: Will Deacon To: linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org Cc: okaya@codeaurora.org, Will Deacon , Benjamin Herrenschmidt , Arnd Bergmann , Jason Gunthorpe , "Paul E. McKenney" , Peter Zijlstra , Ingo Molnar , Jonathan Corbet Subject: [PATCH] docs/memory-barriers.txt: Fix broken DMA vs MMIO ordering example Date: Tue, 27 Mar 2018 14:11:27 +0100 Message-Id: <1522156287-15169-1-git-send-email-will.deacon@arm.com> X-Mailer: git-send-email 2.1.4 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. Cc: Benjamin Herrenschmidt Cc: Arnd Bergmann Cc: Jason Gunthorpe Cc: "Paul E. McKenney" Cc: Peter Zijlstra Cc: Ingo Molnar Cc: Jonathan Corbet Reported-by: Sinan Kaya Signed-off-by: Will Deacon --- Documentation/memory-barriers.txt | 17 +++++++++-------- 1 file changed, 9 insertions(+), 8 deletions(-) diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index a863009849a3..3247547d1c36 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -1909,9 +1909,6 @@ 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); } @@ -1919,11 +1916,15 @@ 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. - - See Documentation/DMA-API.txt for more information on consistent memory. + 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 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 -- 2.1.4