linux-kernel - Re: [PATCH V3 2/4] ACPI/scan: Clean up acpi_check

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Date:	Mon, 19 Oct 2015 21:17:35 -0500
From:	Bjorn Helgaas <helgaas@...nel.org>
To:	Suravee Suthikulanit <suravee.suthikulpanit@....com>
Cc:	Bjorn Helgaas <bhelgaas@...gle.com>, rjw@...ysocki.net,
	lenb@...nel.org, catalin.marinas@....com, will.deacon@....com,
	hanjun.guo@...aro.org, linux-acpi@...r.kernel.org,
	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	Rob Herring <robh+dt@...nel.org>,
	Jeremy Linton <Jeremy.Linton@....com>,
	Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH V3 2/4] ACPI/scan: Clean up acpi_check_dma

On Tue, Oct 13, 2015 at 06:53:28PM -0500, Suravee Suthikulanit wrote:
> Bjorn / Rafael,
> 
> On 10/13/2015 10:52 AM, Suravee Suthikulpanit wrote:
> >
> >On 09/14/2015 09:34 AM, Bjorn Helgaas wrote:
> >>[..]
> >>I think acpi_check_dma_coherency() is better, but only slightly.  It
> >>still doesn't give a hint about the *sense* of the return value.  I
> >>think it'd be easier to read if there were two functions, e.g.,
> >
> >I have been going back-and-forth between the current version, and the
> >two-function-approach in the past. I can definitely go with this route
> >if you would prefer. Although, if acpi_dma_is_coherent() == 0, it would
> >be ambiguous whether DMA is not supported or non-coherent DMA is
> >supported. Then, we would need to call acpi_dma_is_supported() to find
> >out. So, that's okay with you?
> 
> Thinking about this again, I still think having one API (which can
> tell whether DMA is supported or not, and if so whether it is
> coherent or non-coherent) would be the least confusing and least
> error prone.
> 
> What if we would just have:
> 
>     enum dev_dma_type acpi_get_dev_dma_type(struct acpi_device *adev);
> 
> where:
>     enum dev_dma_type {
>         DEV_DMA_NOT_SUPPORTED,
>         DEV_DMA_NON_COHERENT,
>         DEV_DMA_COHERENT,
>     };
> 
> This would probably mean that we should modify
> drivers/base/property.c to replace:
>     bool device_dma_is_coherent()
> to:
>     enum dev_dma_type device_get_dma_type()
> 
> We used to discuss the enum approach in the past
> (https://lkml.org/lkml/2015/8/25/868). But we only considered at the
> ACPI level at the time. Actually, this should also reflect in the
> property.c.
> 
> At this point, only drivers/crypto/ccp/ccp-platform.c and
> drivers/net/ethernet/amd/xgbe/xgbe-main.c are calling the
> device_dma_is_coherent(). So, it should be easy to change this API.

OK, I'm fine with either the enum or Rafael's 0/1/-ENOTSUPP idea.

Bjorn
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