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

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Date:	Tue, 13 Oct 2015 18:53:28 -0500
From:	Suravee Suthikulanit <suravee.suthikulpanit@....com>
To:	Bjorn Helgaas <bhelgaas@...gle.com>, <rjw@...ysocki.net>
CC:	<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

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.

Please let me know your opinions, or if you have other suggestions.

Thanks again,
Suravee

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