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

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Date:   Tue, 17 Apr 2018 10:13:28 -0400
From:   Sinan Kaya <okaya@...eaurora.org>
To:     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()

Hi James,

> 
> Perhaps if you gave an example of the actual problem you're trying to
> fix we could assess if it affects parisc.

Let me clarify myself here. Maybe, there is a better solution.

		/* assign ownership */
		desc->status = DEVICE_OWN;

		/* notify device of new descriptors */
		writel(DESC_NOTIFY, doorbell);

The difference between writel() and writel_relax() is writel() guarantees
memory transactions to be flushed to the device before the register write.
writel_relaxed() does not provide any guarantees about the memory and IO
operations.

Similarly, readl() provides following code to observe the DMA result while
readl_relaxed() does not provide this guarantee.

Ideally, you want to embed rmb() and wmb() into the writel() and readl()
to provide this guarantee.

PA-RISC doesn't seem to support neither one of the barrier types. If you are
familiar with the architecture, maybe you could guide us here.

Is __raw_writeX() enough to provide this guarantee for this architecture?

Sinan

-- 
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.