linux-kernel - Re: [PATCH 2/2] dma: add Qualcomm Technologies HIDMA channel driver

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Date:	Tue, 3 Nov 2015 16:07:34 -0500
From:	Sinan Kaya <okaya@...eaurora.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	dmaengine@...r.kernel.org, timur@...eaurora.org,
	cov@...eaurora.org, jcm@...hat.com,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Vinod Koul <vinod.koul@...el.com>,
	Dan Williams <dan.j.williams@...el.com>,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] dma: add Qualcomm Technologies HIDMA channel driver



On 11/3/2015 5:43 AM, Arnd Bergmann wrote:
> Ok, got it.
>
> Best add an explanation like the above in the interrupt handler,
> to prevent this from accidentally getting 'cleaned up' to use
> readl(), or copied into a driver that uses PCI ordering rules
> where it is actually wrong.
>

I'm adding this disclaimer into the ISR routine.

	/*
	 * Fine tuned for this HW...
	 *
	 * This ISR has been designed for this particular hardware. Relaxed read
	 * and write accessors are used for performance reasons due to interrupt
	 * delivery guarantees. Do not copy this code blindly and expect
	 * that to work.
	 */


> I think it should be done like this:
>
> - anything that is not performance critical, use normal readl/writel
> - in the fast path, add a comment to each readl_relaxed()/writel_relaxed()
>    that is safe in this driver but that would not be safe in a PCI
>    device
> - For the ones that would be safe on PCI as weel, use
>    readl_relaxed()/writel_relaxed() without a comment on each one,
>    but clarify somewhere that these are all intentional.

Makes sense.

-- 
Sinan Kaya
Qualcomm Technologies, Inc. on behalf of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a 
Linux Foundation Collaborative Project
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