linux-kernel - Re: [PATCH v4 4/6] infiniband: cxgb4: Eliminate duplicate barriers on weakly-ordered archs

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Date:   Thu, 22 Mar 2018 19:44:51 +0000
From:   Casey Leedom <leedom@...lsio.com>
To:     SWise OGC <swise@...ngridcomputing.com>,
        Sinan Kaya <okaya@...eaurora.org>,
        'kbuild test robot' <lkp@...el.com>
CC:     "kbuild-all@...org" <kbuild-all@...org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "timur@...eaurora.org" <timur@...eaurora.org>,
        "sulrich@...eaurora.org" <sulrich@...eaurora.org>,
        "linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Steve Wise <swise@...lsio.com>,
        "'Doug Ledford'" <dledford@...hat.com>,
        'Jason Gunthorpe' <jgg@...pe.ca>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Michael Werner <werner@...lsio.com>
Subject: Re: [PATCH v4 4/6] infiniband: cxgb4: Eliminate duplicate barriers on
 weakly-ordered archs

| From: Steve Wise <swise@...ngridcomputing.com>
| Sent: Thursday, March 22, 2018 9:28 AM
|
| | From: Sinan Kaya <okaya@...eaurora.org>
| | Date: Thursday, March 22, 2018 7:52 AM
| |
| | Isn't this a PowerPC problem? Why penalize other architectures?
| 
| I worry it breaks PPC.

And all other architectures.  Aparraently there isn't a formal API
description for writel_relaxed() and Co., nor __raw_writel(), etc.  What I
think we need is a formal semantic definition of exactly what these APIs is
supposed to do and then we can make sure that they all do that.  Till we
have a consistent definition/implementation, trying to use these APIs in
multi-platform code will be a problem.

For instance, and this is merely an example not a prescription of what I'm
talking about:

    writel():
      -- Ensures correct byte ordering.
      -- Ensures no compiler reordering.
      -- Ensures instruction level synchronization with respect
      --   to previous and succeeding reads and writes.

    writel_relaxed():
      -- Ensures correct byte ordering.
      -- Ensures no compiler reordering.
      -- Ensures instruction level synchronization with respect
      --   to previous writes.

    __raw_writel():
      -- Ensures correct byte ordering.
      -- Ensures no compiler reordering.

| I appreciate you doing this.

As do I!  I'm just worried that because the API semantic definitions don't
seem to be formalized for writel_relaxed() and Co., we're in danger of
getting wildly different results on one platform or another based on
differring implementation semantics.

Casey