linux-kernel - Re: [PATCH v2 2/2] parisc: define stronger ordering for the default readX()

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Message-ID: <97fc18a7-b321-0039-0413-d461abc2097b@bell.net>
Date:   Tue, 17 Apr 2018 18:53:52 -0400
From:   John David Anglin <dave.anglin@...l.net>
To:     Sinan Kaya <okaya@...eaurora.org>,
        James Bottomley <James.Bottomley@...senPartnership.com>,
        linux-parisc@...r.kernel.org, arnd@...db.de, timur@...eaurora.org,
        sulrich@...eaurora.org
Cc:     linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, Helge Deller <deller@....de>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] parisc: define stronger ordering for the default
 readX()

On 2018-04-17 2:28 PM, Sinan Kaya wrote:
> The correct terminology here would be to use observability. Yes, it can be
> cached in whatever part of the system for some amount of time as long as
> PCI device sees it in the correct order.
>
> Let's do this exercise.
> 1. OS writes to memory for some descriptor update
> 2. OS writes to the device via writel to hit a doorbell
> 3. Device comes and fetches the memory contents for the descriptor
>
> writel() of PA-RISC needs to ensure that 3. cannot bypass 1. This is typically
> done by a write barrier embedded into the writel() on relaxed architectures.
The sequence point after the argument evaluation for writel prevents the 
compiler from reordering
1 and 2.  Accesses to I/O space are strongly ordered on PA-RISC, so 1 
must occur before 2 (Page G-1
of the PA-RISC 2.0 Architecture).  Thus, the current code is okay.

Dave

-- 
John David Anglin  dave.anglin@...l.net